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Stretch-dependent slow force response in isolated rabbit myocardium is Na+ dependent
Abstract
Stretch induces functional and trophic effects in mammalian myocardium via various signal transduction pathways. We tested stretch signal transduction on immediate and slow force response (SFR) in rabbit myocardium.
Experiments were performed in isolated right ventricular muscles from adult rabbit hearts (37 degrees C, 1 Hz stimulation rate, bicarbonate-buffer). Muscles were rapidly stretched from 88% of optimal length (L88) to near optimal length (L98) for functional analysis. The resulting immediate and slow increases in twitch force (first phase and SFR, respectively) were assessed at reduced [Na+]o or without and with blockade of stretch activated ion channels (SACs), angiotensin-II (AT1) receptors, endothelin-A (ET(A)) receptors, Na+/H+-exchange (NHE1), reverse mode Na+/Ca2+-exchange (NCX), or Na+/K+-ATPase. The effects of stretch on sarcoplasmic reticulum Ca2+-load were characterized using rapid cooling contractures (RCCs). Intracellular pH was measured in BCECF-AM loaded muscles, and action potential duration (APD) was assessed using floating electrodes.
On average, force increased to 216+/-8% of the pre-stretch value during the immediate phase, followed by a further increase to 273+/-10% during the SFR (n=81). RCCs significantly increased during SFR, whereas pH and APD did not change. Neither inhibition of SACs, AT1, or ET(A) receptors affected the stretch-dependent immediate phase nor SFR. In contrast, SFR was reduced by NHE inhibition and almost completely abolished by reduced [Na+]o or inhibition of reverse-mode NCX, whereas increased SFR was seen after raising [Na+]i by Na+/K+-ATPase inhibition.
The data demonstrate the existence of a delayed, Na+- and Ca2+-dependent but pH and APD independent SFR to stretch in rabbit myocardium. This inotropic response appears to be independent of autocrine/paracrine AT1 or ET(A) receptor activation, but mediated through stretch-induced activation of NHE and reverse mode NCX.
... We previously showed suppression of SFR in the right ventricular 274 trabeculae of MCT rats and established the role of Ca-TnC interaction in this phenomenon 275 [20]. To date, it is known that SFR is manifested at all structural levels of myocardial tissue: 276 from a single cardiomyocyte to a whole chamber [17,18,[21][22][23][24][25] and it provides ~20-30% 277 additional contractility relative to the state immediately after stretch. The exact mecha-278 nism(s) of extra calcium that provides the development of SFR remains unknown [17,26]. ...
... We previously showed suppression of SFR in the right ventricular trabeculae of MCT rats and established the role of Ca-TnC interaction in this phenomenon [20]. To date, it is known that SFR is manifested at all structural levels of myocardial tissue: from a single cardiomyocyte to a whole chamber [17,18,[21][22][23][24][25] and it provides~20-30% additional contractility relative to the state immediately after stretch. The exact mechanism(s) of extra calcium that provides the development of SFR remains unknown [17,26]. ...
... This study showed that if sequential activation of the atrium and ventricle is turned on (the correct pattern of activation), the pressure in the ventricle begins to change slowly and significantly while under synchronous activation of the atrium and ventricle, this slow pressure response in the ventricle is inverted and its extent is decreased. This change matches the character and extent of SFR that characterizes the ventricular myocardium of many warm-blooded and human species [17,19,20,23,25,28]. It is hypothesized that the above-described maladaptive changes in the SFR of right ventricular myocardium can be compensated (at least in part) by an elevated atrial SFR that provides adequate pre-distension of the ventricular chamber and ensures sufficient ejection [7,29]. ...
There is a lack of data about the contractile behavior of the right atrial myocardium in chronic pulmonary heart disease. We thoroughly characterized the contractility and Ca transient of isolated right atrial strips of healthy rats (CONT) and rats with the experimental model of monocrotaline-induced pulmonary hypertension (MCT) in steady state at different preloads (isometric force-length), during slow force response to stretch (SFR), and during post-rest potentiation after a period of absence of electrical stimulation (PRP). The preload-dependent changes in the isometric twitch and Ca transient did not differ between CONT and MCT rats while the kinetics of the twitch and Ca transient were noticeably slowed down in the MCT rats. The magnitude of SFR was significantly elevated in the MCT right atrial strips and this was accompanied by the significantly higher elevation of the Ca transient relative amplitude at the end of SFR. The slow changes in the contractility and Ca transient in the PRP protocol did not differ between CONT and MCT. In conclusion, the alterations in the contractility and Ca transient of the right atrial myocardium of monocrotaline-treated rats with pulmonary hypertension mostly concern the elevation in SFR. We hypothesize that this positive inotropic effect in the atrial myocardium may (partly) compensate the systolic deficiency of the right ventricular failing myocardium.
... ETR and ATR activation are not without controversy or apparent species differences. ATR and ETR blockade have no effect on the SFR in rabbit, 54 human ventricular tissue 3,49 nor rat tissue. 94 The SFR in ferret tissue was also found to be immune to AT blockade yet significant SFR blunting occurred in the presence of an ET blocker. ...
... In addition to investigations of the signalling pathways that target the calcium handling of the SR, a number of studies have examined whether a functional SR is actually necessary for the SFR. 4,12,49,54,195 Such studies typically reason that if the SR is involved, then the SR load and/or its fractional release of calcium should increase. However, results regarding SR loading during the SFR are controversial. ...
... However, results regarding SR loading during the SFR are controversial. While SR loading has been observed throughout the SFR in rabbit 54,195 and failing human myocardium, 49 it has not been observed in wild-type rat 58 or mouse 113,185 myocardia. Transgenic mice with human NOX2 do exhibit SR loading during Ang II-induced inotropy, 113 highlighting the possibility that species may be differentially ROS dependent. ...
When exposed to an abrupt stretch, cardiac muscle exhibits biphasic active force enhancement. The initial, instantaneous, force enhancement is well explained by the Frank‐Starling mechanism. However, the cellular mechanisms associated with the second, slower, phase remain contentious. This review explores hypotheses regarding this ‘slow force response’ with the intention of clarifying some apparent contradictions in the literature. The review is partitioned into three sections. The first section considers pathways that modify the intracellular calcium handling to address the role of the sarcoplasmic reticulum in the mechanism underlying the slow force response. The second section focuses on extracellular calcium fluxes and explores the identity and contribution of the stretch‐activated, non‐specific, cation channels as well as signalling cascades associated with G‐protein coupled receptors. The final section introduces promising candidates for the mechanosensor(s) responsible for detecting the stretch perturbation.
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... While the SR is the major source of intra-cellular Ca 2+ released on each beat, in rabbit papillary muscle inhibition of SR Ca 2+ release (by ryanodine) (Bluhm and Lew, 1995;Kentish et al., 1992) or combined block of Ca 2+ release and uptake (by cyclopiazonic acid) (Bluhm and Lew, 1995) has no effect on the relative magnitude of the SFR (Fig. 4A) (although SFR time course is delayed by block of Ca 2+ uptake, indicating that the SR is partly involved in removal of the extra Ca 2+ , which is in line with the increase in total SR Ca 2+ content observed during the SFR). It has been shown in rabbit isolated ventricular muscle that the SFR is reduced by block of the Na + /H + exchanger (by HOE 642 (Luers et al., 2005;von Lewinski et al., 2003) or 5-(N-methyl-N-isobutyl)-amiloride (Neves et al., 2013)). The SFR is almost completely abolished by reduced extracellular Na + concentration (von Lewinski et al., 2003) or inhibition of the Na + /Ca 2+ exchanger (by KB-R 7943) (Fig. 4B) (Luers et al., 2005;Neves et al., 2013;von Lewinski et al., 2003), while it is increased by raising intra-cellular Na + concentration or blocking the Na + /K + pump (by strophanthidin) (von Lewinski et al., 2003). ...
... It has been shown in rabbit isolated ventricular muscle that the SFR is reduced by block of the Na + /H + exchanger (by HOE 642 (Luers et al., 2005;von Lewinski et al., 2003) or 5-(N-methyl-N-isobutyl)-amiloride (Neves et al., 2013)). The SFR is almost completely abolished by reduced extracellular Na + concentration (von Lewinski et al., 2003) or inhibition of the Na + /Ca 2+ exchanger (by KB-R 7943) (Fig. 4B) (Luers et al., 2005;Neves et al., 2013;von Lewinski et al., 2003), while it is increased by raising intra-cellular Na + concentration or blocking the Na + /K + pump (by strophanthidin) (von Lewinski et al., 2003). In contrast, block of SAC NS (by gadolinium), angiotensin-II receptor 1 (by CV 11974), or endothelin-A receptors (byBQ123) were shown to have no effect in one study (von Lewinski et al., 2003), while block of angiotensin-II receptor 1 (by ZD-7155) and protein kinase C (by chelerythrine) (Neves et al., 2013) or protein kinase G (by Rp-8-Br-PET-cGMPSRp-8-Br-PET-cGMPS) (Castro-Ferreira et al., 2014) reduced the response in other reports. ...
... It has been shown in rabbit isolated ventricular muscle that the SFR is reduced by block of the Na + /H + exchanger (by HOE 642 (Luers et al., 2005;von Lewinski et al., 2003) or 5-(N-methyl-N-isobutyl)-amiloride (Neves et al., 2013)). The SFR is almost completely abolished by reduced extracellular Na + concentration (von Lewinski et al., 2003) or inhibition of the Na + /Ca 2+ exchanger (by KB-R 7943) (Fig. 4B) (Luers et al., 2005;Neves et al., 2013;von Lewinski et al., 2003), while it is increased by raising intra-cellular Na + concentration or blocking the Na + /K + pump (by strophanthidin) (von Lewinski et al., 2003). In contrast, block of SAC NS (by gadolinium), angiotensin-II receptor 1 (by CV 11974), or endothelin-A receptors (byBQ123) were shown to have no effect in one study (von Lewinski et al., 2003), while block of angiotensin-II receptor 1 (by ZD-7155) and protein kinase C (by chelerythrine) (Neves et al., 2013) or protein kinase G (by Rp-8-Br-PET-cGMPSRp-8-Br-PET-cGMPS) (Castro-Ferreira et al., 2014) reduced the response in other reports. ...
Cardiac auto-regulation involves integrated regulatory loops linking electrics and mechanics in the heart. Whereas mechanical activity is usually seen as ‘the endpoint’ of cardiac auto-regulation, it is important to appreciate that the heart would not function without feed-back from the mechanical environment to cardiac electrical (mechano-electric coupling, MEC) and mechanical (mechano-mechanical coupling, MMC) activity. MEC and MMC contribute to beat-by-beat adaption of cardiac output to physiological demand, and they are involved in various pathological settings, potentially aggravating cardiac dysfunction. Experimental and computational studies using rabbit as a model species have been integral to the development of our current understanding of MEC and MMC. In this paper we review this work, focusing on physiological and pathological implications for cardiac function.
... These include the release of autocrine/paracrine factors such as angiotensin II (Ang II) [6,8,9], influx of Ca 2+ and/or Na + via stretch-activated nonselective cation channels [7,10] and enhanced Ca 2+ spark rate due to stretch-mediated production of nitric oxide [11]. There is good evidence that the SFR is linked to increased sarcolemmal (SL) Na + entry which reduces net Ca 2+ efflux via Na + /Ca 2+ -exchange (NCX) [6,8,10,[12][13][14][15]. Decreasing [16] or increasing [7] resting muscle length in isolated muscle preparations can decrease or increase resting Ca 2+ , respectively, suggesting that SL Ca 2+ regulation is directly involved in the SFR. ...
... Fig. 7 shows a schematic diagram of how the SFR may be modulated by the acid extrusion systems. Both NHE and Na + -independent Cl − -HCO 3 − exchanger (CBE) have been implicated in the SFR [6,10,12,14,40]. The importance of NHE in the SFR is supported by siRNA NHE-1 down regulation inhibiting the SFR [41]. ...
... One pathway that has been proposed is an Ang II-mediated increase in NHE activity [6] and reactive oxygen species [45] (for a review, see [46]). Although we saw a significant inotropic response to Ang II, losartan did not block the SFR, which implies that this pathway is not a significant contributor to the SFR in this preparation (see also [13,14,47]). Furthermore, some groups [12,48] have reported cellular alkalinization during the SFR (albeit at a much longer time period after the initial stretch than investigated in our study). ...
When cardiac muscle is stretched, there is an initial inotropic response that coincides with the stretch followed by a slower increase in twitch force that develops over several minutes (the "slow force response", or SFR). Unlike the initial response to stretch, the SFR is produced by an increase in Ca(2+) transient amplitude, but the cellular mechanisms that give rise to the increased transients are still debated. We have examined the relationship between the SFR, intracellular [Ca(2+)] and the inotropic state of right ventricular trabeculae from rat hearts at 37°C. The magnitude of the SFR varied with [Ca(2+)]o and stimulation frequency, so that the SFR was greatest for conditions associated with a reduced SR Ca(2+) content. The SFR was not blocked by the AT1 receptor blocker losartan, but was reduced by SN-6, an inhibitor of reverse mode Na(+)/Ca(2+)-exchange (NCX). The Na(+)/H(+)-exchange (NHE) inhibitor HOE642 had no effect in HCO3(-) -buffered solutions, but blocked 50% of the SFR in HCO3(-)-free solution. Inhibition of HCO3(-) transport by DIDS increased the SFR and made it sensitive to HOE642. The addition of cross-bridge cycle inhibitors (20mM BDM or 20μM blebbistatin) to the superfusate reduced the SFR as monitored by changes in Ca(2+). In HCO3(-)-free conditions, the SFR was associated with a slow acidification that was inhibited by BDM, and by stopping electrical stimulation. These results can be explained by stretch increasing metabolic demand and stimulating Na(+) entry via both NHE and the Na(+)/ HCO3(-) transporters. This mechanism provides a novel link between inotropic state and stretch, as well as a way for the cell to compensate for increased acid load. The feedback mechanism between force and Ca(2+) transient amplitude that we describe is also limited by the degree of SR Ca(2+) load.
... This rapid increase in contractile force is due to an increase in the myofilament calcium sensitivity and was originally described in the ventricle (Allen and Kurihara, 1982;Hibberd and Jewell, 1982) although the same mechanism has been shown to occur in the atria (Tavi et al., 1998). A second, slower increase in contractility also occurs in response to myocardial stretch and is known as the slow force response (Parmley and Chuck, 1973;von Lewinski et al., 2004). Much of the initial work characterizing this phenomenon focused on the ventricle and showed it was brought about by an increase in the calcium transient (Alvarez et al., 1999), whereby myocyte stretch activates the sodium-proton exchanger increasing intracellular sodium providing a gradient to increase calcium influx via reverse NCX (Alvarez et al., 1999;von Lewinski et al., 2003;Kockskamper et al., 2008). ...
... A second, slower increase in contractility also occurs in response to myocardial stretch and is known as the slow force response (Parmley and Chuck, 1973;von Lewinski et al., 2004). Much of the initial work characterizing this phenomenon focused on the ventricle and showed it was brought about by an increase in the calcium transient (Alvarez et al., 1999), whereby myocyte stretch activates the sodium-proton exchanger increasing intracellular sodium providing a gradient to increase calcium influx via reverse NCX (Alvarez et al., 1999;von Lewinski et al., 2003;Kockskamper et al., 2008). In ventricular myocytes, this is associated with an increase in the frequency of sparks (Iribe and Kohl, 2008;Iribe et al., 2009) with one potential mechanism being via enhanced ROS production leading to increased open probability of RyRs (Prosser et al., 2013). ...
Atrial fibrillation (AF) is commonly associated with heart failure. A bidirectional relationship exists between the two—AF exacerbates heart failure causing a significant increase in heart failure symptoms, admissions to hospital and cardiovascular death, while pathological remodeling of the atria as a result of heart failure increases the risk of AF. A comprehensive understanding of the pathophysiology of AF is essential if we are to break this vicious circle. In this review, the latest evidence will be presented showing a fundamental role for calcium in both the induction and maintenance of AF. After outlining atrial electrophysiology and calcium handling, the role of calcium-dependent afterdepolarizations and atrial repolarization alternans in triggering AF will be considered. The atrial response to rapid stimulation will be discussed, including the short-term protection from calcium overload in the form of calcium signaling silencing and the eventual progression to diastolic calcium leak causing afterdepolarizations and the development of an electrical substrate that perpetuates AF. The role of calcium in the bidirectional relationship between heart failure and AF will then be covered. The effects of heart failure on atrial calcium handling that promote AF will be reviewed, including effects on both atrial myocytes and the pulmonary veins, before the aspects of AF which exacerbate heart failure are discussed. Finally, the limitations of human and animal studies will be explored allowing contextualization of what are sometimes discordant results.
... Mechanical stretch increases sodium and calcium entry into myocytes [1]. Several mechanisms have been implicated in this phenomenon, such as increased activity of the Na+/H+ and reverse mode of Na+/Ca2+ exchangers [2][3][4], sodium influx through stretch-activated channels [1,5], or the mechanosensitivity of the voltage-gated sodium channels (Nav1.5) [6,7]. ...
... As mentioned previously, stretch increases cytosolic Na+ and Ca2+ [1-3, 20, 47]. The increase in Na+ and Ca2+ inflow into the cells [2,3] also modifies the redox state. Both the stretchinduced increase in intracellular Ca2+ and the increased production of ROS enhance INaL [8,11,14]. ...
Purpose:
Mechanical stretch increases sodium and calcium entry into myocytes and activates the late sodium current. GS967, a triazolopyridine derivative, is a sodium channel blocker with preferential effects on the late sodium current. The present study evaluates whether GS967 inhibits or modulates the arrhythmogenic electrophysiological effects of myocardial stretch.
Methods:
Atrial and ventricular refractoriness and ventricular fibrillation modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts (n = 28) using epicardial multiple electrodes and high-resolution mapping techniques under control conditions and during the perfusion of GS967 at different concentrations (0.03, 0.1, and 0.3 μM).
Results:
On comparing ventricular refractoriness, conduction velocity and wavelength obtained before stretch had no significant changes under each GS967 concentration while atrial refractoriness increased under GS967 0.3 μM. Under GS967, the stretch-induced changes were attenuated, and no significant differences were observed between before and during stretch. GS967 0.3 μM diminished the normal stretch-induced changes resulting in longer (less shortened) atrial refractoriness (138 ± 26 ms vs 95 ± 9 ms; p < 0.01), ventricular refractoriness (155 ± 18 ms vs 124 ± 16 ms; p < 0.01) and increments in spectral concentration (23 ± 5% vs 17 ± 2%; p < 0.01), the fifth percentile of ventricular activation intervals (46 ± 8 ms vs 31 ± 3 ms; p < 0.05), and wavelength of ventricular fibrillation (2.5 ±0.5 cm vs 1.7 ± 0.3 cm; p < 0.05) during stretch. The stretch-induced increments in dominant frequency during ventricular fibrillation (control = 38%, 0.03 μM = 33%, 0.1 μM = 33%, 0.3 μM = 14%; p < 0.01) and the stretch-induced increments in arrhythmia complexity index (control = 62%, 0.03μM = 41%, 0.1 μM = 32%, 0.3 μM = 16%; p < 0.05) progressively decreased on increasing the GS967 concentration.
Conclusions:
GS967 attenuates stretch-induced changes in cardiac electrophysiology.
... Mechanical stretch induces an increase in Na + influx into myocytes and activates the Na + /H + and reverse mode of the Na + /Ca 2+ exchangers [2,9,10,24], giving rise to changes in cardiac electrophysiological properties. The electrophysiological effects of acute myocardial stretching in isolated heart preparations include a shortening of the refractory periods, a decrease in the duration of the action potentials, a decrease of the wavelength of the activation process, depolarization of the resting potential, and the generation of after depolarizations [6,7,[25][26][27][28]. ...
... The electrophysiological effects of acute myocardial stretching in isolated heart preparations include a shortening of the refractory periods, a decrease in the duration of the action potentials, a decrease of the wavelength of the activation process, depolarization of the resting potential, and the generation of after depolarizations [6,7,[25][26][27][28]. The stretch-induced increase in Na + inflow has been related to a number of mechanisms, including stretchactivated channels that exhibit nonselective permeability to various cations including Na + [29,30], mechanical activation of the Na + /H + exchanger [24], or indirect Na + /H + exchanger activation through autocrine/paracrine mechanisms [2]. An increase in peak Na + current due to an increased number of active channels has also been described [31] as an expression of mechanosensitivity of the voltage-sensitive sodium channel Nav1.5. ...
Mechanical stretch is an arrhythmogenic factor found in situations of cardiac overload or dyssynchronic contraction. Ranolazine is an antianginal agent that inhibits the late Na (+) current and has been shown to exert a protective effect against arrhythmias. The present study aims to determine whether ranolazine modifies the electrophysiological responses induced by acute mechanical stretch.
The ventricular fibrillation modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts using epicardial multiple electrodes under control conditions (n = 9) or during perfusion of the late Na (+) current blocker ranolazine 5 μM (n = 9). Spectral and mapping techniques were used to establish the ventricular fibrillation dominant frequency, the spectral concentration and the complexity of myocardial activation in three situations: baseline, stretch and post-stretch.
Ranolazine attenuated the increase in ventricular fibrillation dominant frequency produced by stretch (23.0 vs 40.4 %) (control: baseline =13.6 ± 2.6 Hz, stretch = 19.1 ± 3.1 Hz, p < 0.0001; ranolazine: baseline = 11.4 ± 1.8 Hz, stretch =14.0 ± 2.4 Hz, p < 0.05 vs baseline, p < 0.001 vs control). During stretch, ventricular fibrillation was less complex in the ranolazine than in the control series, as evaluated by the lesser percentage of complex maps and the greater spectral concentration of ventricular fibrillation. These changes were associated to an increase in the fifth percentile of VV intervals during ventricular fibrillation (50 ± 8 vs 38 ± 5 ms, p < 0.01) and in the wavelength of the activation (2.4 ± 0.3 vs 1.9 ± 0.2 cm, p < 0.001) under ranolazine.
The late inward Na (+) current inhibitor ranolazine attenuates the electrophysiological effects responsible for the acceleration and increase in complexity of ventricular fibrillation produced by myocardial stretch.
... Currently, there are a number of conflicting reports regarding the potential role of SACs in the development of the SFR. For example, the application of gadolinium (Gd 3þ ), a non-specific blocker of SACs, failed to block the SFR in rabbit or failing human myocardium (Lewinski et al., 2004(Lewinski et al., , 2003. On the other hand, the SFR was reduced in isolated myocytes and intact papillary muscles from rat hearts in the presence of streptomycin (Calaghan, 2004). ...
... Yew et al. (1998) reported the positive inotropic action of PGF2a, although in the absence of any change to the amplitude of the Ca 2þ transients (Yew et al., 1998). Interestingly, these authors concluded that the PGF2a effect was mediated via activation of the sarcolemmal Na þ -H þ exchanger, also implicated in the SFR by us (Shen et al., 2013), and others (Lewinski et al., 2003;Luers et al., 2005;Vargas et al., 2013). ...
... Recently, increased effort has been made to identify the underlying mechanisms responsible for the SFR in the mammalian heart. The least controversial proposal is that stretch results in stimulation of the Na + /H + exchanger (NHE), facilitating Na + entry into the cell and that this increase in intracellular [Na + ] ([Na + ] i ) favours the reverse mode of the Na + /Ca 2+ exchanger (NCX) (Alvarez et al., 1999;Perez et al., 2001;von Lewinski et al., 2003;Calaghan and White, 2004;Luers et al., 2005;Niederer and Smith, 2007). In this scenario, the rise in [Na + ] i due to NHE activation is followed by an increase in [Ca 2+ ] i via the NCX, resulting in the SFR (Perez et al., 2001;von Lewinski et al., 2003;Luers et al., 2005). ...
... The least controversial proposal is that stretch results in stimulation of the Na + /H + exchanger (NHE), facilitating Na + entry into the cell and that this increase in intracellular [Na + ] ([Na + ] i ) favours the reverse mode of the Na + /Ca 2+ exchanger (NCX) (Alvarez et al., 1999;Perez et al., 2001;von Lewinski et al., 2003;Calaghan and White, 2004;Luers et al., 2005;Niederer and Smith, 2007). In this scenario, the rise in [Na + ] i due to NHE activation is followed by an increase in [Ca 2+ ] i via the NCX, resulting in the SFR (Perez et al., 2001;von Lewinski et al., 2003;Luers et al., 2005). Nonselective cationic mechanosensitive ion channels (MSC NS ) may also conduct Na + or Ca 2+ into the myocyte and thus may be able to mediate stretch-dependent inotropy and the SFR (Calaghan and White, 2004;Niederer and Smith, 2007;Ward et al., 2008). ...
Mammalian myocardial studies reveal a biphasic increase in the force of contraction due to stretch. The first rapid response, known as the Frank-Starling response, occurs within one heartbeat of stretch. A second positive inotropic response occurs over the minutes following the initial stretch and is known as the slow force response (SFR). The SFR has been observed in mammalian isolated whole hearts, muscle preparations and individual myocytes. We present the first direct study into the SFR in the heart of a non-mammalian vertebrate, the rainbow trout (Oncorhynchus mykiss). We stretched ventricular trabecular muscle preparations from 88% to 98% of their optimal length and individual ventricular myocytes by 7% of their slack sarcomere length (SL). Stretch caused an immediate increase in force in both preparations, indicative of the Frank-Starling response. However, we found no significant effect of prolonged stretch on the force of contraction in either the ventricular trabecular preparations or the single myocytes. This indicates that rainbow trout ventricular myocardium does not exhibit a SFR and that, in contrast to mammals, the piscine Frank-Starling response may not be associated with the SFR. We speculate that this is due to the fish myocardium modulating cardiac output via changes in stroke volume to a larger extent than heart rate.
... Apart from an instantaneous rise in twitch amplitude there was an additional~25% increase that develops for several tens of seconds (Fig 3(A)). A slow response very similar to that shown in Fig 3 was observed in cardiac muscle samples from human [52] and other mammalians [53]. Just after stretch, calculated twitch amplitude increased, while the amplitude of Ca 2+ transient slightly decreased. ...
A model of myocardial electromechanics is suggested. It combines modified and simplified versions of previously published models of cardiac electrophysiology, excitation-contraction coupling, and mechanics. The mechano-calcium and mechano-electrical feedbacks, including the strain-dependence of the propagation velocity of the action potential, are also accounted for. The model reproduces changes in the twitch amplitude and Ca2+-transients upon changes in muscle strain including the slow response. The model also reproduces the Bowditch effect and changes in the twitch amplitude and duration upon changes in the interstimulus interval, including accelerated relaxation at high stimulation frequency. Special efforts were taken to reduce the stiffness of the differential equations of the model. As a result, the equations can be integrated numerically with a relatively high time step making the model suitable for multiscale simulation of the human heart and allowing one to study the impact of myocardial mechanics on arrhythmias.
... Ca 2+ influx through TRPC3/6 may directly stimulate SFR, while it is also conceivable that TRPC3/6 activates ERK1/2 (Yao et al., 2009;Chiluiza et al., 2013) upstream of NHE-1, thus inducing an inward Na + current that in turn triggers Ca 2+ entry via NCX (Poburko et al., 2007(Poburko et al., , 2008Louhivuori et al., 2010) (Figure 3, blue arrows). While many of these studies have suggested the involvement of AT 1 R as an upstream component of SFR signaling pathways, there are several opposing reports describing that Ang II and AT 1 R are not involved in the process because SFR was not suppressed by Ang II receptor blockers (ARBs) (Calaghan and White, 2001;von Lewinski et al., 2003;Shen et al., 2013). Despite the inverse agonistic effect of ARBs (Sato et al., 2016), mechanical stretch may differentially activate AT 1 R even in the presence of ARBs which should normally suppress the ligand-stimulated signaling pathway. ...
The myocardium has an intrinsic ability to sense and respond to mechanical load in order to adapt to physiological demands. Primary examples are the augmentation of myocardial contractility in response to increased ventricular filling caused by either increased venous return (Frank–Starling law) or aortic resistance to ejection (the Anrep effect). Sustained mechanical overload, however, can induce pathological hypertrophy and dysfunction, resulting in heart failure and arrhythmias. It has been proposed that angiotensin II type 1 receptor (AT1R) and apelin receptor (APJ) are primary upstream actors in this acute myocardial autoregulation as well as the chronic maladaptive signaling program. These receptors are thought to have mechanosensing capacity through activation of intracellular signaling via G proteins and/or the multifunctional transducer protein, β-arrestin. Importantly, ligand and mechanical stimuli can selectively activate different downstream signaling pathways to promote inotropic, cardioprotective or cardiotoxic signaling. Studies to understand how AT1R and APJ integrate ligand and mechanical stimuli to bias downstream signaling are an important and novel area for the discovery of new therapeutics for heart failure. In this review, we provide an up-to-date understanding of AT1R and APJ signaling pathways activated by ligand versus mechanical stimuli, and their effects on inotropy and adaptive/maladaptive hypertrophy. We also discuss the possibility of targeting these signaling pathways for the development of novel heart failure therapeutics.
... The net result of acutely applied PGF2α on cardiac muscle preparations is a steady increase in the amplitude of the Ca 2+ transients and twitch force (Shen et al. 2016;Yew et al. 1998). Yew et al. (1998) proposed that the positively inotropic effect of PGF2α was mediated via activation of the sarcolemmal Na + -H + exchanger (Yew et al. 1998), which is also implicated in the SFR (Cingolani et al. 2003;Luers et al. 2005;Shen et al. 2013;Vargas et al. 2013;von Lewinski et al. 2003). ...
The mechanical response of the heart to myocardial stretch has been understood since the work of muscle physiologists more than 100 years ago, whereby an increase in ventricular chamber filling during diastole increases the subsequent force of contraction. The stretch-induced increase in contraction is biphasic. There is an abrupt increase in the force that coincides with the stretch (the rapid response), which is then followed by a slower response that develops over several minutes (the slow force response, or SFR). The SFR is associated with a progressive increase in the magnitude of the Ca²⁺ transient, the event that initiates myocyte cross-bridge cycling and force development. However, the mechanisms underlying the stretch-dependent increase in the Ca²⁺ transient are still debated. This review outlines recent literature on the SFR and summarizes the different stretch-activated Ca²⁺ entry pathways. The SFR might result from a combination of several different cellular mechanisms initiated in response to activation of different cellular stretch sensors.
... The slow change in contractility after sudden stretch (slow force response, SFR) is an important regulator of pump function of the heart as this provides a long-term adaptation of the heart to the changed environmental conditions, the phenomenon known as the Anrep effect [1,2]. This phenomenon is found in the healthy mammalian and human myocardium and observed in single cardiac cells and multicellular tissues as well as in the whole organ [3][4][5][6][7]. Impaired mechano-signaling to stretch in diseased myocardium, e.g., in heart failure, may lead to a lack of manifestation of SFR [8][9][10][11]. ...
The slow force response (SFR) to stretch is an important adaptive mechanism of the heart. The SFR may result in ~ 20–30% extra force but it is substantially attenuated in heart failure. We investigated the relation of SFR magnitude with Ca²⁺ transient decay in healthy (CONT) and monocrotaline-treated rats with heart failure (MCT). Right ventricular trabeculae were stretched from 85 to 95% of optimal length and held stretched for 10 min at 30 °C and 1 Hz. Isometric twitches and Ca²⁺ transients were collected on 2, 4, 6, 8, 10 min after stretch. The changes in peak tension and Ca²⁺ transient decay characteristics during SFR were evaluated as a percentage of the value measured immediately after stretch. The amount of Ca²⁺ utilized by TnC was indirectly evaluated using the methods of Ca²⁺ transient “bump” and “difference curve.” The muscles of CONT rats produced positive SFR and they showed prominent functional relation between SFR magnitude and the magnitude (amplitude, integral intensity) of Ca²⁺ transient “bump” and “difference curve.” The myocardium of MCT rats showed negative SFR to stretch (force decreased in time) which was not correlated well with the characteristics of Ca²⁺ transient decay, evaluated by the methods of “bump” and “difference curve.” We conclude that the intracellular mechanisms of Ca²⁺ balancing during stretch-induced slow adaptation of myocardial contractility are disrupted in failing rat myocardium. The potential significance of our findings is that the deficiency of slow force response in diseased myocardium may be diminished under augmented kinetics of Ca–TnC interaction.
... A number of mechanisms have been described in relation to the inotropic responses to myocardial stretch [10,[45][46][47][48]. These responses are characterized by an increase in amplitude of the transient Ca 2+ currents, which among other mechanisms has been associated to a rise in NO production [9]. ...
Electromechanical coupling studies have described the intervention of nitric oxide and S-nitrosylation processes in Ca²⁺ release induced by stretch, with heterogeneous findings. On the other hand, ion channel function activated by stretch is influenced by nitric oxide, and concentration-dependent biphasic effects upon several cellular functions have been described. The present study uses isolated and perfused rabbit hearts to investigate the changes in mechanoelectric feedback produced by two different concentrations of the nitric oxide carrier S-nitrosoglutathione. Epicardial multielectrodes were used to record myocardial activation at baseline and during and after left ventricular free wall stretch using an intraventricular device. Three experimental series were studied: (a) control (n = 10); (b) S-nitrosoglutathione 10 µM (n = 11); and (c) S-nitrosoglutathione 50 µM (n = 11). The changes in ventricular fibrillation (VF) pattern induced by stretch were analyzed and compared. S-nitrosoglutathione 10 µM did not modify VF at baseline, but attenuated acceleration of the arrhythmia (15.6 ± 1.7 vs. 21.3 ± 3.8 Hz; p < 0.0001) and reduction of percentile 5 of the activation intervals (42 ± 3 vs. 38 ± 4 ms; p < 0.05) induced by stretch. In contrast, at baseline using the 50 µM concentration, percentile 5 was shortened (38 ± 6 vs. 52 ± 10 ms; p < 0.005) and the complexity index increased (1.77 ± 0.18 vs. 1.27 ± 0.13; p < 0.0001). The greatest complexity indices (1.84 ± 0.17; p < 0.05) were obtained during stretch in this series. S-nitrosoglutathione 10 µM attenuates the effects of mechanoelectric feedback, while at a concentration of 50 µM the drug alters the baseline VF pattern and accentuates the increase in complexity of the arrhythmia induced by myocardial stretch.
... Although the five primary mechano-sensors in the model have each been verified as immediately responsive to mechanical strain, it is unclear whether the activation of several other "stretch receptors" is direct or indirect. For example, there is broad agreement that NHE mediates stretchdependent signals [26], but it remains controversial whether the role of NHE is dependent on both AT1R and ET1R [42,48], on ET1R alone [49], or on neither [50,51]. Likewise, activation of gp130 and autocrine release of Ang II and ET-1 have all been implicated as contributors to stretch-induced signaling [25,27,28], but the direct cause of each of these effects remains unknown. ...
Author summary
Common stresses such as high blood pressure or heart attack can lead to heart failure, which afflicts over 25 million people worldwide. These stresses cause cardiomyocytes to grow and remodel, which may initially be beneficial but ultimately worsen heart function. Current heart failure drugs such as beta-blockers counteract biochemical cues prompting cardiomyocyte growth, yet mechanical cues to cardiomyocytes such as stretch are just as important in driving cardiac dysfunction. However, no pharmacological treatments have yet been approved that specifically target mechano-signaling, in part because it is not clear how cardiomyocytes integrate signals from multiple mechano-responsive sensors and pathways into their decision to grow. To address this challenge, we built a systems-level computational model that represents 125 interactions between 94 stretch-responsive signaling molecules. The model correctly predicts 134 of 172 previous independent experimental observations, and identifies the key regulators of stretch-induced cardiomyocyte remodeling. Although cardiomyocytes have many mechano-signaling pathways that function largely independently, we find that cooperation between them is necessary to cause growth and remodeling. We identify mechanisms by which a recently approved heart failure drug pair affects mechano-signaling, and we further predict additional pairs of drug targets that could be used to help reverse heart failure.
... The stretch-dependent changes in the cardiac contraction force have two components: first, a rapid and larger increase in force, and second, a slow increase in force (Allen et al. 1988;LeGuennec et al. 1991;Saeki et al. 1993;Kurihara and Komukai 1995). Increases in muscle length in the ventricle or atrium are accompanied by increases in Ca 2? transient amplitude (Allen and Kurihara 1982;Cingolani et al. 1998;Alvarez et al. 1999;Tavi et al. 1999;Perez et al. 2001;von Lewinski et al. 2003). Slow force response under stretch is thought to be caused by stimulation of the Na ? ...
Cardiac contraction is controlled by a Ca(2+) signaling sequence that includes L-type Ca(2+) current-gated opening of Ca(2+) release channels (ryanodine receptors) in the sarcoplasmic reticulum (SR). Local Ca(2+) signaling in the atrium differs from that in the ventricle because atrial myocytes lack transverse tubules and have more abundant corbular SR. Myocardium is subjected to a variety of forces with each contraction, such as stretch, shear stress, and afterload, and adapts to those mechanical stresses. These mechanical stimuli increase in heart failure, hypertension, and valvular heart diseases that are clinically implicated in atrial fibrillation and stroke. In the present review, we describe distinct responses of atrial and ventricular myocytes to shear stress and compare them with other mechanical responses in the context of local and global Ca(2+) signaling and ion channel regulation. Recent evidence suggests that shear mechanotransduction in cardiac myocytes involves activation of gap junction hemichannels, purinergic signaling, and generation of mitochondrial reactive oxygen species. Significant alterations in Ca(2+) signaling and ionic currents by shear stress may be implicated in the pathogenesis of cardiac arrhythmia and failure.
... Myocardial Na þ /H þ exchanger activity (NHE1) is increased via complex pathways that either involve reactive oxygen species (ROS) production (Caldiz et al., 2007), or protein kinase C activation via the phospholipase C and inositol 1,4,5-trisphosphate (IP3) cascade (Woo and Lee, 1999). The subsequent increase in cytosolic Na þ reduces the extrusion of Ca 2þ via the Na þ /Ca 2þ exchanger (NCX) resulting in net gain of Ca 2þ and therefore larger Ca 2þ transients and twitch force (Lewinski et al., 2003). Several authors have also suggested the involvement of stretch activated channels (SAC) that conduct Na þ and/or Ca 2þ (Calaghan and White, 2004;Ward et al., 2008). ...
In cardiac myocytes, calcium (Ca²⁺) signalling is tightly controlled in dedicated microdomains. At the dyad, i.e. the narrow cleft between t-tubules and junctional sarcoplasmic reticulum (SR), many signalling pathways combine to control Ca²⁺-induced Ca²⁺ release during contraction. Local Ca²⁺ gradients also exist in regions where SR and mitochondria are in close contact to regulate energetic demands. Loss of microdomain structures, or dysregulation of local Ca²⁺ fluxes in cardiac disease, is often associated with oxidative stress, contractile dysfunction and arrhythmias. Ca²⁺ signalling at these microdomains is highly mechanosensitive. Recent work has demonstrated that increasing mechanical load triggers rapid local Ca²⁺ releases that are not reflected by changes in global Ca²⁺. Key mechanisms involve rapid mechanotransduction with reactive oxygen species or nitric oxide as primary signalling molecules targeting SR or mitochondria microdomains depending on the nature of the mechanical stimulus. This review summarizes the most recent insights in rapid Ca²⁺ microdomain mechanosensitivity and re-evaluates its (patho)physiological significance in the context of historical data on the macroscopic role of Ca²⁺ in acute force adaptation and mechanically-induced arrhythmias. We distinguish between preload and afterload mediated effects on local Ca²⁺ release, and highlight differences between atrial and ventricular myocytes. Finally, we provide an outlook for further investigation in chronic models of abnormal mechanics (eg post-myocardial infarction, atrial fibrillation), to identify the clinical significance of disturbed Ca²⁺ mechanosensitivity for arrhythmogenesis.
... Consistently, N H E -1 h a s b e e n d e s c r i b e d t o r e g u l a t e invadosome-mediated matrix degradation through acidification of the nanospace surrounding invadosome [156,157]. Further, NHE1 has been shown to be activated in response to stretch [158] and osmotic stress in a PKC-dependent manner [159]. Further, membrane stretching increases the magnitude and kinetics of voltage-activated Na + currents [160,161]. ...
Degradation of the extracellular matrix is a critical step of tumor cell invasion. Both protease-dependent and -independent mechanisms have been described as alternate processes in cancer cell motility. Interestingly, some effectors of protease-dependent degradation are focalized at invadosomes and are directly coupled with contractile and adhesive machineries composed of multiple mechanosensitive proteins. This review presents recent findings in protease-dependent mechanisms elucidating the ways the force affects extracellular matrix degradation by targeting protease expression and activity at invadosome. The aim is to highlight mechanosensing and mechanotransduction processes to direct the degradative activity at invadosomes, with the focus on membrane tension, proteases and mechanosensitive ion channels.
... These are (1) the upstream mechanism that is related to the gross concentration of [Ca 2+ ], (2) the central mechanism that is related to the sensitivity of the regulatory protein troponin C (TnC) to the available [Ca 2+ ], and lastly, (3) the downstream mechanism that is related to the interaction between contractile proteins actin and myosin, and actinomyosin cross-bridge cycling. Myocardial stretch improves contractile function acutely by the Frank-Starling mechanism and more slowly due to the slow force response (von Lewinski et al. 2003). Although stretch-induced improvement in contractile function has been linked to increased Na + and K + flux (Bustamante et al. 1991;Calaghan and White 1999;Ward et al. 2008), the acute effect of stretch on enhancing contractility because of enhancing intracellular [Ca 2+ ] is unclear (Kentish and Wrzosek 1998;Calaghan and White 1999;Calaghan et al. 2003;Yeung et al. 2005;Sukharev and Sachs 2012). ...
Stretching the cardiac left ventricle (LV) enhances contractility but its effect on myoplasmic [Ca(2+)] is controversial. We measured LV pressure (LVP) and [Ca(2+)] as a function of intra-LV stretch in guinea pig intact hearts before and after 15 min global stunning ± perfusion with streptomycin (STM), a stretch-activated channel blocker. LV wall [Ca(2+)] was measured by indo-1 fluorescence and LVP by a saline-filled latex balloon inflated in 50 μL steps to stretch the LV. We implemented a mathematical model to interpret cross-bridge dynamics and myofilament Ca(2+) responsiveness from the instantaneous relationship between [Ca(2+)] and LVP ± stretching. We found that: (1) stretch enhanced LVP but not [Ca(2+)] before and after stunning in either control (CON) and STM groups, (2) after stunning [Ca(2+)] increased in both groups although higher in STM versus CON (56% vs. 39%), (3) STM-enhanced LVP after stunning compared to CON (98% vs. 76% of prestunning values), and (4) stretch-induced effects on LVP were independent of [Ca(2+)] before or after stunning in both groups. Mathematical modeling suggested: (1) cooperativity in cross-bridge kinetics and myofilament Ca(2+) handling is reduced after stunning in the unstretched heart, (2) stunning results in depressed myofilament Ca(2+) sensitivity in the presence of attached cross-bridges regardless of stretch, and (3) the initial mechanism responsible for increased contractility during stretch may be enhanced formation of cross-bridges. Thus stretch-induced enhancement of contractility is not due to increased [Ca(2+)], whereas enhanced contractility after stunning in STM versus CON hearts results from improved Ca(2+) handling and/or enhanced actinomyosin cross-bridge cycling.
© 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.
... Interestingly, while myocardial stretch-mediated endogenous ANG II release appears to be a key factor for the development of cardiac hypertrophy (17,32,45), we have shown that AT1 receptor activation (probably due to endogenous ANG II release) is the initial step of the chain of events leading to the SFR where NHE1 activation is crucial (3,39). Other hypertrophic responses, which have been mimicked by stretching of isolated cardiomyocytes (45) and ventricular muscle (3,9,13,16,21,39,57,58), are induced partially through an increase in secretion and synthesis of vasoactive peptides, such as endothelin (ET). In addition, stretch, through AT1 receptor activation, mediates the release of ET-3 in feline papillary muscles, which seems to be responsible for the initial step of the chain of events that underlie the SFR (21). ...
Myocardial stretch is an established signal that leads to hypertrophy. Myocardial stretch induces a first immediate force increase followed by a slow force response (SFR), which is a consequence of increased Ca(2+) transient that follows the NHE1 Na(+)/H(+) exchanger activation. Carbonic anhydrase II (CAII) binds to extreme C-terminus of NHE1 and regulates its transport activity. We aimed to test the role of CAII bound to NHE1 in the SFR. The SFR and changes in intracellular pH (pHi) were evaluated in rat papillary muscle bathed with CO2/HCO3(-) buffer and stretched from 92% to 98% of the muscle maximal force development length for 10 min, in the presence of the CA inhibitor 6-ethoxzolamide (ETZ, 100 μM). SFR control was 120±3% (n=8) of the rapid initial phase and was fully blocked by ETZ, 99±4% (n=6). The SFR corresponded with a maximal increase in pHi of 0.18±0.02 pH units (n=4), and pHi changes were blocked by ETZ (0.04±0.04, n=6), monitored by epifluorescence. NHE1/CAII physical association was examined in the SFR by coimmunoprecipitation, using muscle lysates. CAII immunoprecipitated with an anti-NHE1 antibody and the CAII immunoprecipitated protein levels increased 58±9% (n=6) upon stretch of muscles, assessed by immunoblots. The p90(RSK) kinase inhibitor SL0101-1 (10 μM) blocked the SFR of heart muscles after stretch 102±2% (n=4), and reduced the binding of CAII to NHE1, suggesting that the stretch-induced phosphorylation of NHE1 increases its binding to CAII. CAII/NHE1 interaction constitutes a component of the SFR to heart muscle stretch which potentiates NHE1-mediated H(+) transport in the myocardium.
... However, the mechanism of action of endothelin release in adult myocardium and in a more physiological model remains unclear. The stretch activated slow inotropic (second phase) response mediated by increased intracellular Ca 2+ is still being debated as to whether it occurs dependent or independent of ET-1 [40,41]. Moreover, our previous study showed that neither an ET A receptor antagonist nor a nonselective ET receptor inhibited the increase in developed force after 24-hour of high-load in cultured rabbit trabeculae [13]. ...
Endothelin (ET-1) is a peptide hormone mediating a wide variety of biological processes and is associated with development of cardiac dysfunction. Generally, ET-1 is regarded as a molecular marker released only in correlation with the observation of a hypertrophic response or in conjunction with other hypertrophic stress. Although the cardiac hypertrophic effect of ET-1 is demonstrated, inotropic properties of cardiac muscle during chronic ET-1-induced hypertrophy remain largely unclear. Through the use of a novel in vitro multicellular culture system, changes in contractile force and kinetics of rabbit cardiac trabeculae in response to 1 nM ET-1 for 24 hours can be observed. Compared to the initial force at t = 0 hours, ET-1 treated muscles showed a ~2.5 fold increase in developed force after 24 hours without any effect on time to peak contraction or time to 90% relaxation. ET-1 increased muscle diameter by 12.5 ± 3.2% from the initial size, due to increased cell width compared to non-ET-1 treated muscles. Using specific signaling antagonists, inhibition of NCX, CaMKII, MAPKK, and IP3 could attenuate the effect of ET-1 on increased developed force. However, among these inhibitions only IP3 receptor blocker could not prevent the increase muscle size by ET-1. Interestingly, though calcineurin-NFAT inhibition could not suppress the effect of ET-1 on force development, it did prevent muscle hypertrophy. These findings suggest that ET-1 provokes both inotropic and hypertrophic activations on myocardium in which both activations share the same signaling pathway through MAPK and CaMKII in associated with NCX activity.
... Therefore , the exact molecular mode of action remains unexplained . Of not, a stretch-dependent effect on intracellular Na handling was previously proposed by us mainly through reverse mode NCX [25]. In addition, Lovelock et al. [26] could describe an effect of ranolazine on myofilament Ca sensitivity, thereby improving diastolic function. ...
The late Na current is of pathophysiological importance for the heart. Ranolazine is an innovative anti-ischemic and antianginal agent that inhibits the late Na current, thereby reducing the Na-dependent Ca-overload, which improves diastolic tone and oxygen handling during myocardial ischemia. In addition, ranolazine seems to exert beneficial effects on diastolic cardiac function. Moreover, there are experimental and clinical data about its antiarrhythmic properties. A beneficial atrial selectivity of ranolazine has been suggested that may be helpful for the treatment of atrial fibrillation. The purpose of this review article is to discuss possible future clinical indications based on novel experimental and preclinical results and the significance of the available data.
... It has even been suggested that this amount of applied stretch can alter the permeability properties of the stretch-activated, nonselective cation channels which are presumed to be activated. However, this interpretation assumes that elective cation channels, as opposed to specific alterations in conventional time-and voltage-dependent conductances, are the main targets for applied stretch in excitable membranes (von Lewinski et al., 2003; Lin et al. 2007).Table 7.1 summarizes the effects of stretch and/or shear stress on different types of recombinant voltage gated channels that have been reported so far. Our results, and many of the other findings in this volume, provide meaningful starting points for beginning to attempt to understand the roles of the human cardiac Na + current in triggering or maintaining cardiac rhythm disorders, including some types of genetic mutations known to target cardiac Na + channels (Bennett et al., 1995; Dumaine et al., 1996; Clancy and Rudy 1999; Ruan et al., 2007). ...
In the mammalian heart, electrical activity triggers and strongly modulates the contractions. In addition, under both physiological
and pathophysiological conditions the mechanical activity of the heart may change tissue excitability, the action potential
waveform and/or the pattern of conduction. In some cases, this mechanoelectrical feedback can alter the myocardium such that
extrasystoles or rhythm disturbances are observed. It is thought that this sensitivity to mechanical perturbations is due
to stretch-induced activation or alteration of ion channels which are expressed in the sarcolemma of cardiac myocytes. In
the present manuscript, we describe studies on the effects of membrane stretch on the Na+ channel alpha subunit, Nav1.5 (which is predominant in the adult mammalian heart). Three different approaches have been utilized: (i) recordings of
Na+ current from adult rat ventricular myocytes, (ii) studies of currents due to this Na+ channel transcript expressed in a Xenopus laevis oocyte preparation, and (iii) integration of these findings, following appropriate alterations of the descriptors for this
Na+ current in a mathematical model of the human ventricular action potential. The results demonstrate that in both native mammalian
myocytes and in the heterologous expression system, applied stretch causes the Na+ current to activate at more negative membrane potentials. Stretch also significantly increases the Na+ current density. When these effects are incorporated into a mathematical model of the human ventricular action potential,
myocyte excitability is enhanced, and there is also a significant increase in the maximum rate of rise in the action potential.
Thus, in the mammalian heart the effects of stretch on conventional time- and voltage-dependent intrinsic Na+ currents need to be taken into account when attempting to understand either the basis for, or the consequences of mechanoelectrical
feedback.
... This is followed by a [Na + ] i -dependent Ca 2+ entry via the Na + /Ca 2+ exchanger working in its reverse mode [1, 21, 127]. Furthermore a number of authors describe stretch-induced alterations in action potential duration, cAMP, or NO signaling [16, 156, 157]. These issues are discussed at different levels – whole heart, multicellular preparations, and isolated cardiomyocytes [20, 155]. ...
Mechanosensitivity, i.e. the specific response to mechanical stimulation, is common to a wide variety of cells in many different organisms ranging from bacteria to mammals. Mechanical stress can modulate physiological processes at the molecular, cellular, and systemic level. The primary target for mechanical stimulation is the plasma membrane of the cell, which can respond to variable physical stress with changes of the open probability of mechanosensitive ion channels. Thus, acting on ion channels in the plasma membrane, mechanical stress can elicit a multitude of biochemical processes – both transient and long-lasting – inside a cell. This may ultimately influence the function of tissues and organs in health and disease. Several stretch-induced signaling cascades have been described with multiple levels of crosstalk between the different pathways. Increased sensitivity of the cells to mechanical stress is found under various pathological conditions. A detailed study of the underlying mechanisms may therefore help to identify novel therapeutic targets for a future clinical use.
... It has even been suggested that this amount of applied stretch can alter the permeability properties of the stretch-activated, nonselective cation channels which are presumed to be activated. However, this interpretation assumes that elective cation channels, as opposed to specific alterations in conventional time-and voltage-dependent conductances, are the main targets for applied stretch in excitable membranes (von Lewinski et al., 2003; Lin et al. 2007).Table 7.1 summarizes the effects of stretch and/or shear stress on different types of recombinant voltage gated channels that have been reported so far. Our results, and many of the other findings in this volume, provide meaningful starting points for beginning to attempt to understand the roles of the human cardiac Na + current in triggering or maintaining cardiac rhythm disorders, including some types of genetic mutations known to target cardiac Na + channels (Bennett et al., 1995; Dumaine et al., 1996; Clancy and Rudy 1999; Ruan et al., 2007). ...
The presence of two slowly inactivating mutants of the cardiac sodium channel (hNa(V)1.5), R1623Q and R1626P, associate with sporadic Long-QT3 (LQT3) syndrome, and may contribute to ventricular tachyarrhythmias and/or lethal ventricular disturbances. Cardiac mechanoelectric feedback is considered a factor in such sporadic arrhythmias. Since stretch and shear forces modulate hNa(V)1.5 gating, detailed electrophysiological study of LQT-Na(V)1.5 mutant channel alpha subunit(s) might provide insights. We compared recombinant R1623Q and WT currents in control vs. stretched membrane of cell-attached patches of Xenopus oocytes. Macroscopic current was monitored before, during, and after stretch induced by pipette suction. In either mutant Na(+) channel, peak current at small depolarizations could be more than doubled by stretch. As in WT, R1623Q showed reversible and stretch intensity dependent acceleration of current onset and decay at all voltages, with kinetic coupling between these two processes retained during stretch. These two Na(V)1.5 channel alpha subunits differed in the absolute extent of kinetic acceleration for a given stretch intensity; over a range of intensities, R1623Q inactivation speed increased significantly less than did WT. The LQT3 mutant R1626P also retained its kinetic coupling during stretch. Whereas WT stretch-difference currents (I(Na)(V,t) without stretch minus I(Na)(V,t) with stretch) were mostly inhibitory (equivalent to outward current), they were substantially (R1623Q) or entirely (R1626P) excitatory for the LQT3 mutants. If stretch-modulated Na(V)1.5 current (i.e., brief excitation followed by accelerated current decay) routinely contributes to cardiac mechanoelectric feedback, then during hemodynamic load variations, the abnormal stretch-modulated components of R1623Q and R1626P current could be pro-arrhythmic.
En condiciones fisiológicas, el estiramiento de las fibras miocárdicas ayuda a mantener el gasto cardíaco en cifras óptimas para una adecuada perfusión tisular. Sin embargo, existen patologías como la cardiomiopatía dilatada en las que se dan anomalías mecánicas y electrofisiológicas debido a un estiramiento miocárdico patológico, con la capacidad de generar arritmias cardíacas de tipo fibrilación atrial y ventricular. Ante esto, se ha visto que la ranolazina es capaz de generar un efecto protector ante el estiramiento miocárdico debido a su acción sobre los canales de Na+ tardíos, la corriente rectificadora tardía rápida de K+ (IKr) y el intercambiador Na+/Ca2+ (NCX). Objetivos: informar sobre los efectos producidos por la ranolazina sobre la refractariedad del músculo cardíaco ante el estiramiento miocárdico y cómo esto evitaría la generación de post-despolarizaciones y el desarrollo de arritmias cardíacas. Métodos: se buscaron artículos apropiados en MEDLINE/PubMED y EMBASE, utilizando una estrategia de búsqueda MeSH (Medical Subject Headings), además se incluyeron libros de referencia que tuvieran relación con el sistema cardiovascular. Conclusiones: ante situaciones que produzcan un estiramiento anómalo de las fibras miocárdicas, la ranolazina podría considerarse como una opción terapéutica como agente antiarrítmico, aparte de sus ya conocidos atributos antianginosos, ya que posee la capacidad de incrementar el período refractario efectivo de los miocitos, disminuyendo la generación de potenciales de acción posdespolarización.
Reactive cardiac hypertrophy (CH) is an increase in heart mass in response to hemodynamic overload. Exercise-induced CH emerges as an adaptive response with improved cardiac function, in contrast to pathological CH that represents a risk factor for cardiovascular health.
The Na⁺/H⁺ exchanger (NHE-1) is a membrane transporter that not only regulates intracellular pH but also intracellular Na⁺ concentration. In the scenario of cardiovascular diseases, myocardial NHE-1 is activated by a variety of stimuli, such as neurohumoral factors and mechanical stress, leading to intracellular Na⁺ overload and activation of prohypertrophic cascades. NHE-1 hyperactivity is intimately linked to heart diseases, including ischemia-reperfusion injury, maladaptive CH and heart failure.
In this review, we will present evidence to support that the NHE-1 hyperactivity constitutes a “switch on/off” for the pathological phenotype during CH development. We will also discuss some classical and novel strategies to avoid NHE-1 hyperactivity, and that are therefore worthwhile to improve cardiovascular health.
Mechanical load directly regulates cardiac force. Stretching myocardial tissue results in a biphasic increase in contractility: an immediate increase (Frank-Starling mechanism) followed by a further slow increase (slow force response, SFR). Most experiments published have been performed in ventricular myocardium and very little in human tissue. We therefore highlight stretch dependent slow force responses in human myocardium and compare signal transduction in atrial and ventricular tissue. Although of comparable amplitude underlying signal transduction varies between the two tissue types. In ventricular muscle strips, the SFR is significantly reduced by inhibition of Na+/H+- (NHE) and Na+/Ca2+-exchange (NCX) but not affected by AT- and ET-receptor antagonism. In contrast, SFR in atrial tissue is neither affected by NHE- nor NCX-inhibition but interestingly, inhibition of AT-receptors or pre-incubation with angiotensin II or endothelin-1 attenuate the atrial SFR. In addition, stretch results in a large NHE-dependent [Na+]i increase in ventricle but only a small, NHE-independent [Na+]i increase in atrium. Stretch activated channels are not involved in the SFR in either tissue but contribute to basal force development in atrium but not ventricle. Thus, in human heart both atrial and ventricular myocardium exhibit a stretch-dependent SFR that is likely to serve as adjustment mechanism regulating cardiac output in case of increased preload. In ventricle on the one hand, there is a significant NHE-dependent (but angiotensin II- and endothelin-1- independent) [Na+]i increase that is translated into a [Ca2+]i and force increase via NCX. In atrium, on the other hand, there is an angiotensin II- and endothelin-dependent (but NHE- and NCX-independent) force increase. Increased myofilament Ca2+ sensitivity through MLCK-induced phosphorylation of MLC2 is contributing to the SFR in both atrium and ventricle.
When a cardiac muscle is held in a stretched position, its [Ca²⁺] transient increases slowly over several minutes in a process known as stress-induced slow increase in intracellular Ca²⁺ concentration ([Ca²⁺]i) (SSC). Transient receptor potential canonical (TRPC) 3 forms a non-selective cation channel regulated by the angiotensin II type 1 receptor (AT1R). In this study, we investigated the role of TRPC3 in the SSC. Isolated mouse ventricular myocytes were electrically stimulated and subjected to sustained stretch. An AT1R blocker, a phospholipase C inhibitor, and a TRPC3 inhibitor suppressed the SSC. These inhibitors also abolished the observed SSC-like slow increase in [Ca²⁺]i induced by angiotensin II, instead of stretch. Furthermore, the SSC was not observed in TRPC3 knockout mice. Simulation and immunohistochemical studies suggest that sarcolemmal TRPC3 is responsible for the SSC. These results indicate that sarcolemmal TRPC3, regulated by AT1R, causes the SSC.
Prostaglandins are ubiquitous signaling molecules in the body that produce autocrine/paracrine effects on target cells in response to mechanical or chemical signals. In the heart, long-term exposure to prostaglandin (PG) F2 alpha has been linked to the development of hypertrophy; however, there is no consensus on the acute effect of PGF2 alpha. Our aim was to determine the response to exogenous PGF2 alpha in isolated trabeculae from rat hearts. PGF2 alpha (1 mu M) increased both the Ca2+ transients and the isometric stress in trabeculae, reaching steady state after 10-15 minutes, without altering the time course of Ca2+ transient decay. The precursor of PGF2 alpha, arachidonic acid, also stimulated a similar response. The positive inotropic effect of PGF2 alpha was mediated through a protein kinase C signaling pathway that involved activation of the sarcolemmal Na+/H+ exchanger. We also found that the slow force response to stretch was attenuated in the presence of PGF2 alpha and by addition of indomethacin, a blocker of prostaglandin synthesis. In conclusion, PGF2 alpha was positively inotropic when acutely applied to trabeculae and contributed to the increased Ca2+ transients during the slow force response to stretch. Together, these data suggest that PGF2 alpha is important in maintaining homeostasis during volume loading in healthy hearts.
JTV-519 is a 1,4-benzothiazepine derivative with multichannel effects that inhibits Ca2+ release from the sarcoplasmic reticulum and stabilizes the closed state of the ryanodine receptor, preventing myocardial damage and the induction of arrhythmias during Ca2+ overload. Mechanical stretch increases cellular Na+ inflow, activates the reverse mode of the Na+/Ca2+ exchanger, and modifies Ca2+ handling and myocardial electrophysiology, favoring arrhythmogenesis. This study aims to determine whether JTV-519 modifies the stretch-induced manifestations of mechanoelectric feedback. The ventricular fibrillation (VF) modifications induced by acute stretch were studied in Langendorff-perfused rabbit hearts using epicardial multiple electrodes under control conditions (n=9) or during JTV-519 perfusion: 0.1 μM (n=9) and 1 μM (n=9). Spectral and mapping techniques were used to establish the baseline, stretch and post-stretch VF characteristics. JTV-519 slowed baseline VF and decreased activation complexity. These effects were dose-dependent (baseline VF dominant frequency: control=13.9±2.2 Hz; JTV 0.1 μM=11.1±1.1 Hz, p<0.01; JTV 1 μM=6.6±1.1 Hz, p<0.0001). The stretch-induced acceleration of VF (control=38.8%) was significantly reduced by JTV-519 0.1 μM (19.8%) and abolished by JTV 1 μM (-1.5%). During stretch, the VF activation complexity index was reduced in both JTV-519 series (control=1.60±0.15; JTV 0.1 μM=1.13±0.3, p<0.0001; JTV 1 μM=0.57±0.21, p<0.0001), and was independently related to VF dominant frequency (R=0.82; p<0.0001). The fifth percentile of the VF activation intervals, conduction velocity and wavelength entered the multiple linear regression model using dominant frequency as the dependent variable (R=-0.84; p<0.0001). In conclusion, JTV-519 slowed and simplified the baseline VF activation patterns and abolished the stretch-induced manifestations of mechanoelectric feedback. This article is protected by copyright. All rights reserved.
Prostaglandins are ubiquitous signalling molecules in the body that produce autocrine/paracrine effects on target cells in response to mechanical or chemical signals. In the heart, long term exposure to prostaglandin (PG) F2ɑ has been linked to the development of hypertrophy, however there is no consensus on the acute effect of PGF2ɑ. Our aim was to determine the response to exogenous PGF2ɑ in isolated trabeculae from rat hearts. 1 µM PGF2ɑ increased both the Ca transients and isometric stress in trabeculae, reaching steady-state after 10 - 15 min, without altering the time course of Ca transient decay. The precursor of PGF2ɑ, arachidonic acid, also stimulated a similar response. The positive inotropic effect of PGF2ɑ was mediated through a protein kinase C (PKC) signalling pathway that involved activation of the sarcolemmal Na/H exchanger. We also found that the slow force response to stretch was attenuated in the presence of PGF2ɑ, and by addition of indomethacin, a blocker of prostaglandin synthesis. In conclusion, PGF2ɑ was positively inotropic when acutely applied to trabeculae, and contributed to the increased Ca transients during the slow force response to stretch. Together, these data suggest PGF2ɑ is important in maintaining homeostasis during volume loading in healthy hearts.
The heart possesses the intrinsic ability to adjust to short- and long-term haemodynamic demands. These adaptive responses are dependent on the sensation of mechanical stimuli and transduction into cellular events. Recent evidence suggests that caveolae, flask-shaped invaginations of the cell membrane, may be an important part of the mechanotransductive pathway in the cardiac cell. Caveolae are ‘signalosomes’, microdomains enriched in components of signal transduction cascades, ion channels and exchangers, which are known to control some elements of cell signalling. The marker protein for caveolae, caveolin, acts as a scaffold for macromolecular signalling complexes, and can also regulate the activity of proteins with which it interacts. In this review, the morphological, biochemical and functional evidence to support a role for caveolae in mechanosensation and mechanotransduction will be presented. Although there is a paucity of direct evidence in the cardiac myocyte, the available data support the idea that caveolae are an integral part of downstream stretch-activated signalling, and that they are essential for the proper integration and co-ordination of mechanosensitive signalling pathways
The length-dependent activation of contraction is attenuated in the failing myocardium of adult male rats. This pathological change is not seen in adult female rats, possibly because of a protective effect of sex hormones. In the present study we evaluated length-dependent changes in isometric twitch, Ca(2+) transient (CaT) and action potential (AP) in the right ventricular myocardium of impuberal healthy male and female rats (control) and in rats treated with a single injection of 50 mg/kg monocrotaline (MCT). Compared with sex-matched control rats, MCT-treated male and female rats exhibited increased right ventricular weight (134% and 142% of control, respectively), decreased left ventricular weight (72% and 79%), twitch attenuation (48.8±2.7% and 57.5±1.2%) and prolongation (125±3% and 127±2%), CaT attenuation (37.8±0.4% and 39.1±1.1%) and prolongation (114±1% and 116±1%) and AP prolongation at 90% repolarization (195±2% and 203±1%). The MCT-treated male rats exhibited a 50% lower integral magnitude and an approximately 25% larger time-to-peak 'bump' compared with control male rats. These parameters in MCT-treated female rats tended to show similar changes to those seen in the control female rats, with no significant difference between the two groups. In all groups, integral magnitude and time-to-peak 'bump' increased with length. In conclusion, the length-dependent activation of contraction was equally blunted in the failing right ventricular myocardium of impuberal male and female rats. This was related to changes in CaT and AP, which were similar between male and female rats. Therefore, puberty is necessary for manifestation of the protective effects of sex hormones on this remodelling. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
Mechanical response to myocardial stretch has been explained by various mechanisms, which include Na(+)/H(+) exchanger activation by autocrine-paracrine system activity. Drug-induced changes were analyzed to investigate the role of these mechanisms in the electrophysiological responses to acute myocardial stretch.
Multiple epicardial electrodes and mapping techniques were used to analyze changes in ventricular fibrillation induced by acute myocardial stretch in isolated perfused rabbit hearts. Four series were studied: control (n = 9); during perfusion with the angiotensin receptor blocker losartan (1 μM, n = 8); during perfusion with the endothelin A receptor blocker BQ-123 (0.1 μM, n = 9), and during perfusion with the Na(+)/H(+) exchanger inhibitor EIPA (5-[N-ethyl-N-isopropyl]-amiloride) (1 μM, n = 9).
EIPA attenuated the increase in the dominant frequency of stretch-induced fibrillation (control=40.4%; losartan=36% [not significant]; BQ-123=46% [not significant]; and EIPA=22% [P<.001]). During stretch, the activation maps were less complex (P<.0001) and the spectral concentration of the arrhythmia was greater (greater regularity) in the EIPA series: control=18 (3%); EIPA = 26 (9%) (P < .02); losartan=18 (5%) (not significant); and BQ-123=18 (4%) (not significant).
The Na(+)/H(+) exchanger inhibitor EIPA attenuated the electrophysiological effects responsible for the acceleration and increased complexity of ventricular fibrillation induced by acute myocardial stretch. The angiotensin II receptor antagonist losartan and the endothelin A receptor blocker BQ-123 did not modify these effects.
Copyright © 2014 Sociedad Española de Cardiología. Published by Elsevier España, S.L.U. All rights reserved.
Resumen
Introducción y objetivos
Se han implicado diversos mecanismos en la respuesta mecánica al estiramiento miocárdico, que incluyen la activación del intercambiador Na⁺/H⁺ por acciones autocrinas y paracrinas. Se estudia la participación de estos mecanismos en las respuestas electrofisiológicas al estiramiento agudo miocárdico mediante el análisis de los cambios inducidos con fármacos.
Métodos
Se analizan las modificaciones de la fibrilación ventricular inducidas por el estiramiento agudo miocárdico en corazones de conejo aislados y perfundidos utilizando electrodos múltiples epicárdicos y técnicas cartográficas. Se estudian 4 series: control (n = 9); durante la perfusión del antagonista de los receptores de la angiotensina II, losartán (1 μM, n = 8); durante la perfusión del bloqueador del receptor de la endotelina A, BQ-123 (0,1 μM, n = 9), y durante la perfusión del inhibidor del intercambiador Na⁺/H⁺, EIPA (5-[N-ethyl-N-isopropyl]-amiloride) (1 μM, n = 9).
Resultados
EIPA atenuó el aumento de la frecuencia dominante de la fibrilación producido por el estiramiento (control = 40,4%; losartán = 36% [no significativo]; BQ-123 = 46% [no significativo], y EIPA = 22% [p < 0,001]). Durante el estiramiento, la complejidad de los mapas de activación fue menor en la serie con EIPA (p < 0,0001) y también en esta serie fue mayor la concentración espectral de la arritmia (mayor regularidad): control = 18 ± 3%; EIPA = 26 ± 9% (p < 0,02); losartán = 18 ± 5% (no significativo), y BQ-123 = 18 ± 4% (no significativo).
Conclusiones
El inhibidor del intercambiador Na⁺/H⁺ EIPA atenúa los efectos electrofisiológicos responsables de la aceleración y del aumento de la complejidad de la fibrilación ventricular producidos por el estiramiento agudo miocárdico. Por el contrario, el antagonista de los receptores de la angiotensina II, losartán, y el del receptor A de la endotelina, BQ-123, no modifican estos efectos.
This editorial refers to ‘Neuronal Na+ channel blockade suppresses arrhythmogenic diastolic Ca2+ release’ by P.B. Radwanski et al ., doi:10.1093/cvr/cvu262.
Arrhythmias are a well-accepted cause for morbidity and mortality. This is true both for atrial as well as ventricular arrhythmias. In general, atrial fibrillation can lead to the progression of cardiac diseases such as heart failure whereas ventricular arrhythmias can lead to sudden cardiac death.
Although several antiarrhythmic drugs are currently available and invasive ablation therapies have been established throughout the recent years both for atrial as well as ventricular arrhythmias, the prognosis has not been improved except for ICD therapy.
One reason for this caveat may be that the basic mechanisms for arrhythmias are still poorly understood. Moreover, some of the novel mechanisms for arrhythmias have not been translated into clinical routine yet.
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a rare congenital arrhythmogenic disease leading to sudden cardiac death without structural heart disease. Several mutations were described including the RyR2 gene1 encoding the cardiac isoform of the sarcoplasmic reticulum (SR) Ca-release channel (ryanodine receptor) as well as the SR Ca-binding protein calsequestrin (CASQ2).2,3
Radwanski et al. 4 describe a new proarrhythmogenic mechanism in the well-known CASQ2 (R33Q) CPVT mouse model3 that seems clinically …
Myocyte Na+ homeostasis is crucially involved in a number of vital cell functions, such as excitability, excitation–contraction coupling, energy metabolism, pH regulation, as well as cardiac development and growth. However, consideration of Na+ regulation is often relegated to a secondary position in the discussion of cardiac (patho-)physiology, where the focus is typically on contractile proteins, Ca2+ regulation and pH regulation which appear more directly related to contractile function. However, myocyte Na+ homeostasis is as complex as Ca2+ or pH homeostasis and [Na+]i very directly influences intracellular [Ca2+] and pH via powerful cardiac Na/Ca exchange, Na/H exchange and Na-bicarbonate cotransport systems. Na+ flux my even be central in mediating effects of mechanical loading of the heart on excitation–contraction coupling. Moreover, [Na+]i homeostasis is regulated by a delicate balance of Na+ channels and transporters in the surface and mitochondrial membrane that maintain a large [Na+] gradient across the sarcolemmal membrane.
Given this fundamental but often overlooked contribution of Na …
* Corresponding author. Tel.: +49-551-3989-25; fax: +49-551-3919-127.
Reactive oxygen species (ROS) are highly reactive oxygen-derived chemical compounds that are by-products of aerobic cellular metabolism as well as crucial second messengers in numerous signaling pathways. In excitation-contraction-coupling (ECC), which links electrical signaling and coordinated cardiac contraction, ROS have a severe impact on several key ion handling proteins such as ion channels and transporters, but also on regulating proteins such as protein kinases (e.g. CaMKII, PKA or PKC), thereby pivotally influencing the delicate balance of this finely tuned system. While essential as second messengers, ROS may be deleterious when excessively produced due to a disturbed balance in Na+ and Ca2 + handling, resulting in Na+ and Ca2 + overload, SR Ca2 + loss and contractile dysfunction. This may, in the end, result in systolic and diastolic dysfunction and arrhythmias. This review aims to provide an overview of the single targets of ROS in ECC and to outline the role of ROS in major cardiac pathologies, such as heart failure and arrhythmogenesis.
Stretch activates various signal transduction pathways including mitogen-activated protein kinases (MAPK). Stretch-induced phosphorylation of MAPK-contribution to contractility in human myocardium is unknown. We tested the effects of stretch on p44/42-, p38-MAPK and p90rsk phosphorylation and the functional relevance for force development in failing (F) and non-failing (NF) human myocardium. Trabeculae were stretched to a diastolic tension of 12mN/mm2 for 2.5 to 30 minutes and frozen for Western Blot analysis. Stretch induced a time-dependent increase in phosphorylation of p44/42-, p38-MAPK and p90rsk. For functional analysis, trabeculae from F myocardium were stretched and the immediate (Frank-Starling mechanism; FSM) and delayed (slow force response; SFR) increase in twitch force was assessed before and after blocking the activation of p44/42-MAPK (30 micromol/L U0126) and p38-MAPK (10 micromol/L SB203580). Inhibition of p44/42-MAPK almost completely blocked the SFR (106.7 3.7% vs. 125.4 2.9%), while p38-MAPK-blockade significantly increased the SFR (124.6 1.9% vs. 121.2 2.2%). Stretch induced a time-dependent increase in p44/42-, p38-MAPK and p90rsk phosphorylation in F and NF myocardium. While p44/42-MAPK phosphorylation contributed to the SFR, p38-MAPK activation antagonized the stretch-induced SFR.
Azelnidipine and amlodipine are dihydropyridine-type Ca(2+) channel blockers for the treatment of hypertension. Although these drugs have high vasoselectivity and small negative inotropic effects in vivo, little is known regarding their direct effects on cellular contractility without humoral regulation or the additive effects of these drugs with other antihypertensive drugs on myocardial contractility. To investigate the effects of Ca(2+) channel blockers on single cell mechanics, mouse cardiomyocytes were enzymatically isolated, and a pair of carbon fibers was attached to opposite cell-ends to stretch the cells. Cells were paced at 4Hz superfused in normal Tyrode solution at 37°C. Cell length and active/passive force calculated from carbon fiber bending were recorded in 6 different preload conditions. Slopes of end-systolic force-length relation curves (maximum elastance) were measured as an index of contractility before and after drugs were administered. Azelnidipine at 10nM and 100nM did not change maximum elastance, while amlodipine at 100nM did decrease maximum elastance. The combination of RNH-6270 (active form of angiotensin II receptor blocker, olmesartan, 10nM) and either amlodipine (10nM) or azelnidipine (10nM) did not affect maximum elastance. Although both amlodipine and azelnidipine can be used safely at therapeutically relevant concentrations even in combination with olmesartan, the present results suggest that azelnidipine has a less negative inotropic action compared to amlodipine.
Excitation-contraction coupling is the process by which electrical activation is translated into contraction of a cardiac myocyte and thus the heart. In heart failure, expression, phosphorylation, and function of several intracellular proteins that are involved in excitation-contraction coupling are altered. The present review article summarizes central principles and highlights novel aspects of alterations in heart failure, focusing especially on recent findings regarding altered sarcoplasmic reticulum Ca(2+)-leak and late Na(+)-current without being able to cover all changes in full detail. These two pathomechanisms seem to play interesting roles with respect to systolic and diastolic dysfunction and may also be important for cardiac arrhythmias. Furthermore, the article outlines the translation of these novel findings into potential therapeutic approaches.
The Na+ gradient produced across the cardiac sarcolemma by the ATP-dependent Na+-pump is a constant source of energy for Na+-dependent transporters. The plasma membrane Na+/H+ exchanger (NHE) is one such secondary active transporter, regulating intracellular pH, Na+ concentration, and cell volume. NHE1, the major isoform found in the heart, is activated in response to a variety of stimuli such as hormones and mechanical stress. This important characteristic of NHE1 is intimately linked to heart diseases, including maladaptive cardiac hypertrophy and subsequent heart failure, as well as acute ischemic-reperfusion injury. NHE1 activation results in elevation of pH and intracellular Na+ concentration, which potentially enhance downstream signaling cascades in the myocardium. Therefore, in addition to determining the mechanism underlying regulation of NHE1 activity, it is important to understand how the ionic signal produced by NHE1 is transmitted to the downstream targets. Extensive studies have identified many accessory factors that interact with NHE1. Here, we have summarized the recent progress on understanding the molecular mechanism underlying NHE1 regulation and have shown a possible signaling pathway leading to cardiac remodeling, which is initiated from NHE1. This article is part of a Special Issue entitled 'Na+ Regulation in Cardiac Myocytes'.
Myocardial stretching is an arrhythmogenic factor. Optical techniques and mechanical uncouplers are used to study the mechanoelectric feedback. The aim of this study is to determine whether the mechanical uncouplers 2,3-butanedione monoxime and Blebbistatin hinder or modify the electrophysiological effects of acute mechanical stretch.
The ventricular fibrillation (VF) modifications induced by acute mechanical stretch were studied in 27 Langendorff-perfused rabbit hearts using epicardial multiple electrodes and mapping techniques under control conditions (n = 9) and during the perfusion of 2,3-butanedione monoxime (15 mM) (n = 9) or Blebbistatin (10 μm) (n = 9).
In the control series, myocardial stretch increased the complexity of the activation maps and the dominant frequency (DF) of VF from 13.1 ± 2.0 Hz to 19.1 ± 3.1 Hz (P < 0.001, 46% increment). At baseline, the activation maps showed less complexity in both the 2,3-butanedione monoxime and Blebbistatin series, and the DF was lower in the 2,3-butanedione monoxime series (11.4 ± 1.2 Hz; P < 0.05). The accelerating effect of mechanical stretch was abolished under 2,3-butanedione monoxime (maximum DF = 11.7 ± 2.4 Hz, 5% increment, ns vs baseline, P < 0.0001 vs. control series) and reduced under Blebbistatin (maximum DF = 12.9 ± 0.7 Hz, 8% increment, P < 0.01 vs. baseline, P < 0.0001 vs. control series). The variations in complexity of the activation maps under stretch were not significant in the 2,3-butanedione monoxime series and were significantly attenuated under Blebbistatin.
The accelerating effect and increased complexity of myocardial activation during VF induced by acute mechanical stretch are abolished under the action of 2,3-butanedione monoxime and reduced under the action of Blebbistatin.
We address the question whether activation of the sodium-proton exchanger (NHE) does contribute to the stretch-induced accumulation of intracellular sodium and calcium in mouse ventricular myocytes. NHE-blocker cariporide (10 microM) were applied to the bath for 10 min. Axial stretch was applied for 2 min by increasing the distance between an adherent glass stylus and the patch pipette by 20%. Myocytes (stimulated at 3 Hz) were shock-frozen in diastole and the membrane currents monitored till cryofixation. Controls were treated identically, but not stretched. Total sodium and calcium concentrations ([Na], [Ca]=sum of free and bound Na and Ca) were measured by electron probe microanalysis (EPMA) in peripheral and central cytosol, mitochondria, nucleus and nuclear envelope. Cariporide did not reduce the stretch-activated negative current. The stretch-induced rise in [Na] was not different in the presence and in the absence of cariporide. Cariporide significantly reduced diastolic [Ca] in the cytosol of stretched myocytes. Since cariporide does not prevent the stretch-induced [Na] accumulation, we suggest that not NHE but the stretch-activated streptomycin-sensitive current I(SAC) causes the well documented stretch-induced [Na] accumulation. The discovery that cariporide prevents the stretch-induced rise in cytosolic [Ca] demonstrates an important additional effect of the drug on calcium handling.
The stretch of cardiac muscle elicits a chain of autocrine/paracrine events in which the Na+/H+ exchanger activation is the central step. This activation is induced by a sequential angiotensin II-endothelin release and results in an increase in intracellular Na+ [Na+]i without significant changes in intracellular pH. The increase in [Na+]i negatively shifts the reversal potential of the Na+/Ca2+ exchanger, thus inducing cell Ca2+ influx that augments myocardial contractility. This increase in force, which represents the mechanical counterpart of the autocrine/paracrine mechanism triggered by stretch and that has been called the slow force response to stretch (SFR), provides an explanation for the Anrep effect. In this article we discuss the potential link between the mechanisms involved in the Anrep effect and the development of cardiac hypertrophy.
Since the pioneering work of Henry Pickering Bowditch in the late 1800s to early 1900s, cardiac muscle contraction has remained an intensely studied topic for several reasons. The heart is located centrally in our body, and its pumping motion demands the attention of the observer. The contraction of the heart encompasses a complex interplay of mechanical, chemical, and electrical properties, and its function can thus be studied from any of these viewpoints. In addition, diseases of the heart are currently killing more people in the Westernized world than any other disease. When combined with the increasing emphasis of research to be clinically relevant, this contributes to the heart remaining a topic of continued basic and clinical investigation. Yet, there are significant aspects of cardiac muscle contraction that are still not well understood. A big complication of the study of cardiac muscle contraction is that there exists no equilibrium among many of the important governing parameters, which include pre- and afterload, intracellular ion concentrations, membrane potential, and velocity and direction of movement. Thus the classic approach of perturbing an equilibrium or a steady state to learn about the role of the perturbing factor in the system cannot be unambiguously interpreted, since each of the parameters that govern contraction are constantly changing, as well as constantly changing their interaction with each other. In this review, presented as the 54th Bowditch Lecture at Experimental Biology meeting in Anaheim in April 2010, I will revisit several governing factors of cardiac muscle relaxation by applying newly developed tools and protocols to isolated cardiac muscle tissue in which the dynamic interactions between the governing factors of contraction and relaxation can be studied.
Ventricular arrhythmias and contractile dysfunction are the main causes of death in human heart failure (HF). In a rabbit HF model reproducing these same aspects of human HF, we demonstrate that a 2-fold functional upregulation of Na+-Ca2+ exchange (NaCaX) unloads sarcoplasmic reticulum (SR) Ca2+ stores, reducing Ca2+ transients and contractile function, Whereas beta -adrenergic receptors (beta -ARs) are progressively downregulated in I-IF, residual beta -AR responsiveness at this critical HF stage allows SR Ca2+ load to increase, causing spontaneous SR Ca2+ release and transient inward current carried by NaCaX. A given Ca2+ release produces greater arrhythmogenic inward current in HF las a result of NaCaX upregulation), and approximate to 50% less Ca2+ release is required to trigger an action potential in HF. The inward rectifier potassium current (I-K1) is reduced by 49% in HF, and this allows greater depolarization for a given NaCaX current. partially blocking I-K1 in control cells with barium mimics the greater depolarization for a given current injection seen in HF. Thus, we present data to support a novel. paradigm in which changes in NaCaX and I-K1 and residual beta -AR responsiveness, conspire to greatly increase the propensity for triggered arrhythmias in HF. In addition, NaCaX upregulation appears to be a critical link between contractile dysfunction and arrhythmogenesis.
Reduction of the transsarcolemmal [Na] gradient in rabbit cardiac muscle leads to an increase in the force of contraction. This has frequently been attributed to alteration of Ca movements via the sarcolemmal Na/Ca exchange system. However, the specific mechanisms that mediate the increased force at individual contractions have not been clearly established. In the present study, the [Na] gradient was decreased by reduction of extracellular [Na] or inhibition of the Na pump by either the cardioactive steroid acetylstrophanthidin or by reduction of extracellular [K]. Contractile performance and changes in extracellular Ca (sensed by double-barreled Ca-selective microelectrodes) were studied in order to elucidate the underlying basis for the increase in force. In the presence of agents that inhibit sarcoplasmic reticulum (SR) function (10 mM caffeine, 100-500 nM ryanodine), reduction of the [Na] gradient produced increases in contractile force similar to that observed in the absence of caffeine or ryanodine. It is concluded that an intact, functioning SR is not required for the inotropic effect of [Na] gradient reduction (at least in rabbit ventricle). However, this does not exclude a possible contribution of enhanced SR Ca release in the inotropic response to [Na] gradient reduction in the absence of caffeine or ryanodine. Acetylstrophanthidin (3-5 microM) usually leads to an increase in the magnitude of extracellular Ca depletions associated with individual contractions. However, acetylstrophanthidin can also increase extracellular Ca accumulation during the contraction, especially at potentiated contractions. This extracellular Ca accumulation can be suppressed by ryanodine and it is suggested that this apparent enhancement of Ca efflux is secondary to an enhanced release of Ca from the SR. Under conditions where Ca efflux during contractions is minimized (after a rest interval in the presence of ryanodine), acetylstrophanthidin increased both the rate and the extent of extracellular Ca depletions. Thus, acetylstrophanthidin can increase both Ca influx and Ca efflux during the cardiac muscle contraction. These results can be explained by a simple model where the direction of net Ca flux via Na/Ca exchange during the action potential is determined by the changes in reversal potential of the Na/Ca exchange. Reduction of the [Na] gradient may well lead to net cellular Ca uptake (via Na/Ca exchange) and may also elevate the resting intracellular [Ca].(ABSTRACT TRUNCATED AT 400 WORDS)
No.7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea methanesulfonate), a selective inhibitor of the Na+/Ca2+ exchanger (NCX1), has been newly synthesized. It dose-dependently inhibited Na+i-dependent 45Ca2+ uptake and Na+i-dependent [Ca2+]i increase in cardiomyocytes, smooth muscle cells, and NCX1-transfected fibroblasts (IC50 = 1.2-2.4 μM). Inhibition was observed without prior incubation with the agent and was completely reversed by washing cells
with buffer for 1 min. Interestingly, No.7943 was much less potent in inhibiting Na+o-dependent 45Ca2+ efflux and Na+o-induced [Ca2+]i decline (IC50 = >30 μM), indicating that it selectively blocks the reverse mode of Na+/Ca2+ exchange in intact cells. In cardiac sarcolemmal preparations consisting mostly of inside-out vesicles, the agent inhibited
Na+i-dependent 45Ca2+ uptake and Na+o-dependent 45Ca2+ efflux with similar, but slightly lower, potencies (IC50 = 5.4-13 μM). Inhibition was noncompetitive with respect to Ca2+ and Na+ in both cells and sarcolemmal vesicles. These results suggest that No.7943 primarily acts on external exchanger site(s) other
than the transport sites in intact cells, although it is able to inhibit the exchanger from both sides of the plasma membrane.
No.7943 at up to 10 μM does not affect many other ion transporters nor several cardiac action potential parameters. This agent
at these concentrations also did not influence either diastolic [Ca2+]i or spontaneous beating in cardiomyocytes. Furthermore, No.7943 markedly inhibited Ca2+ overloading into cardiomyocytes under the Ca2+ paradox conditions. Thus, No.7943 is not only useful as a tool with which to study the transport mechanism and physiological
role of the Na+/Ca2+ exchanger but also has therapeutic potential as a selective blocker of excessive Ca2+ influx mediated via the Na+/Ca2+ exchanger under pathological conditions.
1. Major cellular structures involved in E-C coupling. 2. Myofilaments: The end effector of E-C Coupling. 3. Sources and sinks of activator calcium. 4. Cardiac action potentials and ion channels. 5. Ca influx via sarcolemmal Ca channels. 6. Na/Ca exchange and the sarcolemmal Ca-pump. 7. Sarcoplasmic reticulum Ca uptake, content and release. 8. Excitation-contraction coupling. 9. Control of cardiac contraction by SR and sarcolemmal Ca fluxes. 10. Cardiac inotropy and Ca mismanagement. References. Index.
Myocardial stretch produces an increase in developed force (DF) that occurs in two phases: the first (rapidly occurring) is generally attributed to an increase in myofilament calcium responsiveness and the second (gradually developing) to an increase in [Ca(2+)](i). Rat ventricular trabeculae were stretched from approximately 88% to approximately 98% of L(max), and the second force phase was analyzed. Intracellular pH, [Na(+)](i), and Ca(2+) transients were measured by epifluorescence with BCECF-AM, SBFI-AM, and fura-2, respectively. After stretch, DF increased by 1.94+/-0.2 g/mm(2) (P<0.01, n = 4), with the second phase accounting for 28+/-2% of the total increase (P<0.001, n = 4). During this phase, SBFI(340/380) ratio increased from 0.73+/-0.01 to 0.76+/-0.01 (P<0.05, n = 5) with an estimated [Na(+)](i) rise of approximately 6 mmol/L. [Ca(2+)](i) transient, expressed as fura-2(340/380) ratio, increased by 9.2+/-3.6% (P<0.05, n = 5). The increase in [Na(+)](i) was blocked by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA). The second phase in force and the increases in [Na(+)](i) and [Ca(2+)](i) transient were blunted by AT(1) or ET(A) blockade. Our data indicate that the second force phase and the increase in [Ca(2+)](i) transient after stretch result from activation of the Na(+)/H(+) exchanger (NHE) increasing [Na(+)](i) and leading to a secondary increase in [Ca(2+)](i) transient. This reflects an autocrine-paracrine mechanism whereby stretch triggers the release of angiotensin II, which in turn releases endothelin and activates the NHE through ET(A) receptors.
Background— Application of pyruvate was shown to improve contractile function in isolated animal myocardium and hemodynamics in patients with congestive heart failure. We assessed the influence of pyruvate on systolic and diastolic myocardial function and its subcellular mode of action in isolated myocardium from end-stage failing human hearts.
Methods and Results— In muscle strip preparations, concentration-dependent effects of pyruvate on developed and diastolic force (n=6), aequorin light emission reflecting intracellular Ca²⁺ transients (n=6), and rapid cooling contractures reflecting sarcoplasmic reticulum (SR) Ca²⁺ content (n=11) were measured. Pyruvate resulted in a concentration-dependent increase in developed force and a decrease in diastolic force, with a maximum effect of 155% and 21%, respectively, at 20 mmol/L pyruvate (P<0.05). This was associated with a dose-dependent prolongation of time to peak tension and relaxation time. Pyruvate increased rapid cooling contractures by 51% and aequorin light signals by 85% (at 15 and 20 mmol/L; P<0.05). This indicates increased SR Ca²⁺ content and increased intracellular Ca²⁺ transients. The inotropic effect of pyruvate was still present after elimination of SR Ca²⁺ storage function with 10 μmol/L cyclopiazonic acid and 1 μmol/L ryanodine (n=8). Pyruvate significantly increased intracellular pH from 7.31±0.03 to 7.40±0.04 by BCECF fluorescence (n=6).
Conclusions— The present findings indicate that pyruvate improves contractile performance of failing human myocardium by increasing intracellular Ca²⁺ transients as well as myofilament Ca²⁺ sensitivity. The former seem to result from increased SR Ca²⁺ accumulation and release, the latter from increased intracellular pH.
Endothelin (ET) isopeptides, ET-1, ET-2 and ET-3, elicit a positive inotropic effect (PIE) in association with a negative lusitropic effect, essentially with identical efficacies and potencies in the isolated rabbit papillary muscle, but with different concentration-dependent properties. Pharmacological analysis indicates that the PIE of ET-1 is mediated by an ETa2 subtype that is less sensitive to BQ123 and FR139317, whereas the PIE of ET-3 is mediated by an ETa1 subtype that is highly sensitive to these ETa antagonists. ETs increased the amplitude of intracellularCa2+ transient (CaT) in indo-1 loaded rabbit ventricular myocytes, but the increase was much smaller than that produced by elevation of [Ca2+]o or isoproterenol for a given extent of PIE, an indication of increased myofibrillar Ca2+ sensitivity. ETs stimulate phosphoinositide (PI) hydrolysis, which leads to production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Evidence for the role of IP3-induced Ca2+ release in cardiac E-C coupling is tenuous. Generation of IP3 induced by ET-1 was transient and returned to the baseline level when the PIE reached an elevated steady level. Protein kinase C (PKC) that is activated by DAG and also via other pathways triggered by ETs stimulates Na+-H+ exchanger to lead to an increased [Na+]i and alkalinization. The former may contribute to an increase in the amplitude of CaT through Na+-Ca2+ exchanger, and the latter, to an increase in myofibrillar Ca2+ sensitivity. A number of PKC inhibitors, such as staurosporine, H-7, calphostin C and chelerythrine, consistently and selectively inhibited the PIE of ET-3 without affecting the PIE of isoproterenol and Bay k 8644. The maximum inhibition was 20–30% of the total response. A Na+-H+ exchange inhibitor, [5-(N-ethyl-N-isopropyl) amiloride (EIPA)]or a Ca2+ antagonist, verapamil, could not completely inhibit the PIE of ET-3, but the combination of both inhibitors totally abolished the PIE of ET-3. These findings indicate that activation of PKC and subsequent activation of Na+-H+ exchanger and/or L-type Ca2+ channels may play a crucial role in the cardiac action of ET isopeptides in the rabbit ventricular myocardium.
The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+- dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.
The uncharged, colorless molecule fluorescein diacetate diffuses into Ehrlich ascites tumor cells at neutral pH, where intracellular esterases release the chromophore fluorescein. The negatively charged dye is retained by the cell, permitting the intracellular pH to be estimated from the shape of the pH-dependent absorption spectrum. The diacetate derivative of 6-carboxyfluorescein may be used similarly and has the additional advantage of a slower rate of leakage out of the cell but requires incubation at pH 6.2 to facilitate initial entry into the cell. After removal of external dye by centrifugation, 80-92% of the remaining dye is unresponsive to external pH changes. Calibration of the intracellular fluorescein spectra is obtained by equilibration of the internal and external pH with nigericin in K+ buffers. Results of intracellular pH measurements by this method are in good agreement with those obtained by measuring the distribution ratio of the weak acid 5,5-dimethyl[2-14C]oxazolidine-2,4-dione, under a variety of metabolic conditions. Besides the accurate estimation of intracellular pH, the method permits the kinetics of intracellular pH changes as small as 0.01 to be followed. Intracellular fluorescein reports pH changes occurring in both the cytoplasmic and the mitochondrial compartments, whereas 6-carboxyfluorescein reports only the cytoplasmic compartment. At equivalent concentrations, nigericin is more effective than valinomycin plus the protonophore 1799 in dissipating plasmalemma pH gradients. Either is effective at lower concentrations in dissipating mitochondrial pH gradients. Addition of glucose to Ehrlich ascites cells results in a transient acidification of the cytoplasm in close correspondence to the intracellular lactate levels. The transient acidification can be explained by the initial rapid rate of glycolysis exceeding the rate of lactate export.
Tension and action potentials have been measured simultaneously from isolated cat papillary muscles. Two groups of experiments are described. In the first group, the external conditions under which the muscle contracted were changed. Specifically, stimulation rate, extra-cellular [Ca++], extracellular [Na++] were altered, and adrenaline was added to the bathing fluid. A tendency for given levels of tension to be accompanied by action potentials of constant duration is demonstrated under some of these conditions. In the second group of experiments, tension and action potentials were recorded following some change in external conditions; specifically, after a long rest, after a change in muscle length, and after the muscle had been set up in the experimental apparatus (the 'running-in' period). In the period that followed each of these interventions, peak tension increased substantially over at least several minutes but all external conditions (for example, temperature, muscle length, stimulation rate, and composition of the bathing fluid) remained constant. In each of these three situations tension increased but in one case the action potential duration increased, in another it decreased, and in the third it was unchanged. It is concluded that change in action potential durations do not necessarily make an important contribution to the changes in tension of papillary muscles.
Transient diastolic dilatation of the isolated canine left ventricle predictably elicits arrhythmias. To test the hypothesis that such arrhythmias may be mediated by sarcolemmal stretch-activated channels, we attempted to inhibit stretch-induced arrhythmias with gadolinium (Gd3+), a potent stretch-activated channel blocker. In experiments with six isolated canine hearts, left ventricular volume was increased for 50 msec during early diastole and then returned to initial volume by a computerized servopump. The stretch volume was adjusted to yield a probability of eliciting a stretch-induced arrhythmia of 95 +/- 2% before treatment with Gd3+. When Gd3+ (1-10 microM) was administered, dose-dependent suppression of stretch-induced arrhythmias was observed. The probability of a stretch-induced arrhythmia was reduced to 13 +/- 10% (p less than 0.05) with 10 microM Gd3+. Washout of Gd3+ completely reversed this effect. Since Gd3+ is known to be a calcium channel antagonist, we compared the effect of Gd3+ on stretch-induced arrhythmias with that of verapamil and nifedipine. These calcium channel blockers produced no demonstrable inhibition of stretch-induced arrhythmias when administered at concentrations (1 microM) that substantially depressed left ventricular pressure development. Thus, our results indirectly implicate stretch-activated channels in the genesis of stretch-induced arrhythmias and provide preliminary evidence for a potential new mode of antiarrhythmic drug action--blockade of stretch-activated channels.
In isolated isovolumic ventricles and in in situ ventricles under nonsteady-state conditions, alterations in load have been shown to affect electrophysiological properties via contraction-excitation feedback. However, the effect of alterations in loading conditions on electrophysiological properties in normal ventricles under physiological loading conditions remains unknown. Furthermore, the arrhythmogenic significance of these load-induced electrophysiological changes is uncertain. We increased end-diastolic volume (27 +/- 4 ml vs. 51 +/- 6 ml) and assessed conduction, refractoriness, ventricular fibrillation thresholds (VFTs), and inducibility of ventricular arrhythmias in 14 isolated blood-perfused ejecting canine ventricles under steady-state conditions. We also examined the effect of increased end-diastolic volume on refractoriness and monophasic action potential (MAP) duration and contour under isovolumic versus ejecting conditions. Under ejecting conditions, increased end-diastolic volume resulted in very small (less than 1.5%) changes in the absolute refractory period (10 mA) and in local activation time but no change in local electrogram duration, overall dispersion of refractoriness, MAP duration or contour, VFT, or inducibility of ventricular arrhythmias. Increased volume loading under isovolumic conditions resulted in a very slight (less than 1%) shortening of MAP duration and refractoriness but had no effect on the MAP contour. These findings provide strong evidence that alterations in volume load are of little electrophysiological or arrhythmogenic importance in normal canine ventricles under physiologically loaded conditions (contraction-excitation feedback, load and arrhythmias, volume load).
We have used the sarcoplasmic reticulum (SR) inhibitor ryanodine to assess the contribution of the SR to the increase in twitch tension seen on cooling the mammalian myocardium. To select a suitable concentration of ryanodine, i.e., one that will exert a maximal effect at all temperatures studied, concentration-response curves for ryanodine action were constructed at 37 degrees, 29 degrees, and 23 degrees C in ventricular muscle from rabbit and rat. Using a concentration of ryanodine (1 microM) that exerted a maximal effect at all temperatures studied, the ability of ryanodine to inhibit SR function at 37 degrees, 29 degrees, and 23 degrees C was then confirmed by using rapid cooling contractures (RCCs) to provide an indirect assessment of the SR calcium content. To estimate the rest decay of the SR calcium content in the absence and presence of ryanodine (1 microM), RCCs were initiated after a range of rest intervals (0.3-300 seconds) in rabbit muscles maintained at 37 degrees, 29 degrees, or 23 degrees C. In the absence of ryanodine, low temperatures elevated RCCs at all rest intervals studied. In the presence of ryanodine, RCCs were only seen at rest intervals shorter than 2.0 seconds, even at 23 degrees C, the lowest temperature studied. Thus, even at 23 degrees C, ryanodine appears to be effective at inhibiting SR calcium release in muscles stimulated at 0.5 Hz (i.e., after 2 seconds rest). Therefore, using this concentration of ryanodine (1 microM) and a stimulation rate of 0.5 Hz, we have investigated the contribution of the SR to the positive inotropic response to hypothermia. Under these conditions, the positive inotropic response to cooling in rabbit ventricle was almost unaffected by the inhibition of the SR with ryanodine. In rat ventricle, a tissue in which SR calcium release may dominate excitation-contraction (EC) coupling, the inotropic response to hypothermia was still observed, although developed tension was strongly depressed at all temperatures. These results suggest that a change in SR function is not the principal mediator of the large (400-500%) increase in force associated with cooling mammalian ventricular muscle from 37 degrees to 25 degrees C. The ryanodine-sensitive fraction of tension development was greatest at 37 degrees C, suggesting that the relative contribution of the SR to tension development in rabbit ventricle is reduced at temperatures below 37 degrees C.(ABSTRACT TRUNCATED AT 400 WORDS)
When cardiac muscle shortens during a contraction, the duration of mechanical activity is abbreviated (shortening deactivation), but the duration of the action potential is prolonged. Neither of these phenomena is fully understood, but both may be related to changes in the myoplasmic free calcium concentration. In these experiments, isolated papillary muscles from cats and ferrets were allowed to contract under various mechanical conditions while myoplasmic calcium was monitored with aequorin, or in parallel experiments the membrane potential was recorded with microelectrodes or a sucrose gap. When shortening occurred, myoplasmic calcium was increased and the membrane potential was more positive than in isometric contractions. The changes in calcium apparently precede the depolarization. We propose that muscle shortening reduces calcium binding to the contractile proteins and leads to a rise in myoplasmic calcium, and that this rise in myoplasmic calcium activates an inward current leading to the observed changes in the action potential. These processes may be important contributory factors in some arrhythmias.
1. The calcium-sensitive photoprotein aequorin was micro-injected into cells of rat and cat ventricular muscles. The resulting light emission is a function of intracellular free calcium concentration ([Ca2+]i). The transient increases in [Ca2+]i that accompany contraction were monitored. 2. After an increase in muscle length, the developed tension increased immediately and then showed a slow increase over a period of minutes. The peak [Ca2+]i in each contraction was initially unchanged after an increase in muscle length but then showed a slow increase with a time course similar to that of the slow tension change. 3. As a consequence of these slow changes, the shape of the tension-length relation depends on the procedure used to determine it and this change in shape can be attributed to changes in activation. 4. Immediately after an increase in muscle length the calcium transient was abbreviated. 5. When a quick release was performed during a contraction, a short-lived increase in the [Ca2+]i was observed following the release. 6. The two previous observations can both be explained if the binding constant of troponin for calcium is a function of developed tension.
The paper reports on changes of the configuration of the action potential recorded from atrial trabecular preparations of the rabbit. Trabeculae, which were isolated from the right atrial cavum (initial length between 2.6 and 5.4 mm) were stretched up to 160% of the initial length. After stretching the steady state action potentials were analysed. While increasing the trabecular length, the duration of 90% repolarization was prolonged and the duration of 25% repolarization was shortened drastically. The conductance for a time-independent potassium outward current gK, measured from the trajectories of the repolarization phase of the action potential, was reduced by increasing the trabecular length. The reconstruction of the measured action potential suggests an increased anomalous rectification for the time-independent potassium current, a decreased amount of the inward transported calcium during the action potential, and a more rapid inactivation of the Ca++-inward current by stretching the atrial myocardium. The results concerning the gK and the anomalous rectification for the potassium current may explain the alleviated generation of the extrasystoles and arrhythmias in the stretched atrial myocardium. The reduction of the Ca++-transport during the action potential may be a control mechanism protecting the heart tissue from the destabilizing effect of an increased anomalous rectification for a time-independent potassium current.
Hypertrophy is a fundamental adaptive process employed by postmitotic cardiac and skeletal muscle in response to mechanical load. How muscle cells convert mechanical stimuli into growth signals has been a long-standing question. Using an in vitro model of load (stretch)-induced cardiac hypertrophy, we demonstrate that mechanical stretch causes release of angiotensin II (Ang II) from cardiac myocytes and that Ang II acts as an initial mediator of the stretch-induced hypertrophic response. The results not only provide direct evidence for the autocrine mechanism in load-induced growth of cardiac muscle cells, but also define the pathophysiological role of the local (cardiac) renin-angiotensin system.
The effects of No. 7943 on the Na ⁺ /Ca ²⁺ exchange current and on other membrane currents were investigated in single cardiac ventricular cells of guinea‐pig with the whole‐cell voltage‐clamp technique.
No. 7943 at 0.1–10 μ m suppressed the outward Na ⁺ /Ca ²⁺ exchange current in a concentration‐dependent manner. The suppression was reversible and the IC 50 value was approximately 0.32 μ m .
No. 7943 at 5–50 μ m suppressed also the inward Na ⁺ /Ca ²⁺ exchange current in a concentration‐dependent manner but with a higher IC 50 value of approximately 17 μ m .
In a concentration‐response curve, No. 7943 raised the K m Ca ²⁺ value, but did not affect the I max value, indicating that No. 7943 is a competitive antagonist with external Ca ²⁺ for the outward Na ⁺ /Ca ²⁺ exchange current.
The voltage‐gated Na ⁺ current, Ca ²⁺ current and the inward rectifier K ⁺ current were also inhibited by No. 7943 with IC 50 s of approximately 14, 8 and 7 μ m , respectively.
In contrast to No. 7943, 3′,4′‐dichlorobenzamil (DCB) at 3–30 μ m suppressed the inward Na ⁺ /Ca ²⁺ exchange current with IC 50 of 17 μ m , but did not affect the outward exchange current at these concentrations.
We conclude that No. 7943 inhibits the outward Na ⁺ /Ca ²⁺ exchange current more potently than any other currents as a competitive inhibitor with external Ca ²⁺ . This effect is in contrast to DCB which preferentially inhibits the inward rather than the outward Na ⁺ /Ca ²⁺ exchange current.
The present study examines the intracellular pH (pHi) dependence of angiotensin (ANG) II-induced positive inotropic effect in cat papillary muscles contracting isometrically (0.2 Hz, 30 degrees C). Muscles were loaded with the fluorescent dye 2'-7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester for simultaneous measurement of pHi and contractility. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) buffer (n = 4), there was a temporal dissociation between the positive inotropic and the alkalinizing effects of ANG II (0.5 microM). The positive inotropic effect of ANG II peaked at 9.7 +/- 0.8 min (240 +/- 57% above control) without significant changes in pHi. The increase in pHi became significant (0.05 +/- 0.01 pH units) only after 16 min of exposure to the drug, when the positive inotropic effect of ANG II was already fading. In HCO3- buffer (n = 7), the ANG II-induced positive inotropic effect occurred without significant pHi changes. In the presence of 5 microM ethyl isopropyl amiloride (EIPA, to specifically inhibit the Na+/H+ exchanger), the alkalinizing effect of ANG II was changed to a significant decrease in pHi, despite which ANG II still increased contractility by 87 +/- 16% (n = 6). The results indicate that in HEPES buffer only a fraction of the ANG II-induced positive inotropic effect can be attributed to a pHi change, whereas in a physiological CO2-HCO3- medium the positive inotropic effect of ANG II is independent of pHi changes. Furthermore, an ANG II-induced increase in myocardial contractility was observed even when ANG II administration elicited a decrease in pHi, as occurred after Na+/H+ exchanger blockade. The results show that in feline myocardium, the increase in contractility evoked by ANG II in a physiological CO2-HCO3- medium is not due to an increase in Ca2+ myofilament sensitivity secondary to an increase in myocardial pHi.
Changes in contractile force of the myocardium may depend on changes in the intracellular Ca2+ concentration, changes in the responsiveness of the myofibrils for Ca2+, or a combination of both. We investigated in isolated muscle strip preparations from human nonfailing and endstage failing hearts the influence of physical (changes in preload, stimulation rate, or rhythm), and pharmacological interventions (alpha- or beta-adrenoceptor-stimulation, endothelin) on developed force of contraction and the corresponding intracellular Ca2+ transients.
Isometric contraction, electrical stimulation, 37 degrees C. Simultaneous registration of force of contraction and intracellular Ca2+ transients (aequorin method).
Increases in preload, alpha- and endothelin-receptor stimulation resulted in increases in force of contraction without increasing aequorin light emission. Increasing stimulation rate or increasing rest intervals resulted in parallel increases (nonfailing myocardium) or decreases (failing myocardium) of force of contraction and aequorin light emission. beta-Adrenoceptor-stimulation exerted inotropic and lusitropic effects in human failing myocardium associated with a large, overproportional increase in aequorin light emission.
The human heart regulates intrinsic contractility via several subcellular mechanisms. Increases in preload (Frank-Starling-mechanism) and alpha- or endothelin-receptor-stimulation enhance myocardial contractility by increasing the Ca2+ responsiveness of the myofilaments; rate- and rhythm-dependent modulation of the contractile state directly depend on changes in the intracellular Ca(2+)-transients; beta-adrenoceptor stimulation results in an overproportional large increase in intracellular Ca2+ transients, probably due to additional cAMP-dependent Ca(2+)-desensitizing effects on the level of the myofibrils.
1. Changes in cytosolic [Ca2+] ([Ca2+]i) were measured in isolated rat trabeculae that had been micro-injected with fura-2 salt, in order to investigate the mechanism by which twitch force changes following an alteration of muscle length. 2. A step increase in length of the muscle produced a rapid potentiation of twitch force but not of the Ca2+ transient. The rapid rise of force was unaffected by inhibiting the sarcoplasmic reticulum (SR) with ryanodine and cyclopiazonic acid. 3. The force-[Ca2+]i relationship of the myofibrils in situ, determined from twitches and tetanic contractions in SR-inhibited muscles, showed that the rapid rise of force was due primarily to an increase in myofibrillar Ca2+ sensitivity, with a contribution from an increase in the maximum force production of the myofibrils. 4. After stretch of the muscle there was a further, slow increase of twitch force which was due entirely to a slow increase of the Ca2+ transient, since there was no change in the myofibrillar force-[Ca2+]i relationship. SR inhibition slowed down, but did not alter the magnitude of, the slow force response. 5. During the slow rise of force there was no slow increase of diastolic [Ca2+]i, whether or not the SR was inhibited. The same was true in unstimulated muscles. 6. We conclude that the rapid increase in twitch force after muscle stretch is due to the length-dependent properties of the myofibrils. The slow force increase is not explained by length dependence of the myofibrils or the SR, or by a rise in diastolic [Ca2+]i. Evidence from tetani suggests the slow force responses result from increased Ca2+ loading of the cell during the action potential.
The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l-sotalol and flecainide.
The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid-filled balloon placed in the left ventricle. Pacing was performed alternately using epi- and endocardial monophasic action potential (MAP)-pacing catheters at cycle lengths from 1,000 to 300 msec. d,l-Sotalol (10 microM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5% +/- 3.1%), MAP duration (+14.9% +/- 3.2%), and QT interval (+15.5% +/- 4.1%) (mean +/- SEM at 1,000 msec). These effects had a reverse rate-dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of AVD, d,l-sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2 microM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate-dependent increase of pacing thresholds (+47.6% +/- 8.2%), prolongation of QRS (+48.8% +/- 5.6%), and conduction time (+78.6% +/- 8.6%) (mean +/- SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate-dependent prolongation of QRS (+16.7% +/- 5.5%) and conduction time (+17.1% +/- 4.3%).
Our data demonstrate that, during AVD, the Class III effect of d,l-sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide-related proarrhythmia in certain clinical situations.
Myocardial stretch is a well-known stimulus that leads to hypertrophy. Little is known, however, about the intracellular pathways involved in the transmission of myocardial stretch to the cytoplasm and nucleus. Studies in neonatal cardiomyocytes demonstrated stretch-induced release of angiotensin II (Ang II). Because intracellular alkalinization is a signal to cell growth and Ang II stimulates the Na+/H+ exchanger (NHE), we studied the relationship between myocardial stretch and intracellular pH (pHi). Experiments were performed in cat papillary muscles fixed by the ventricular end to a force transducer. Muscles were paced at 0.2 Hz and superfused with HEPES-buffered solution. pHi was measured by epifluorescence with the acetoxymethyl ester form of the pH-sensitive dye 2',7'-bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF-AM). Each muscle was progressively stretched to reach maximal developed force (Lmax) and maintained in a length that was approximately 92% Lmax (Li). During the "stretch protocol," muscles were quickly stretched to Lmax for 10 minutes and then released to Li; pHi significantly increased during stretch and came back to the previous value when the muscle was released to Li. The increase in pHi was eliminated by (1) specific inhibition of the NHE (EIPA, 5 micromol/L), (2) AT1-receptor blockade (losartan, 10 micromol/L), (3) inhibition of protein kinase C (PKC) (chelerythrine, 5 micromol/L), (4) blockade of endothelin (ET) receptors with a nonselective (PD 142,893, 50 nmol/L) or a selective ETA antagonist (BQ-123, 300 nmol/L). The increase in pHi by exogenous Ang II (500 nmol/L) was also reduced by both ET-receptor antagonists. Our results indicate that after myocardial stretch, pHi increases because of stimulation of NHE activity. This involves an autocrine-paracrine mechanism in which protein kinase C, Ang II, and ET play crucial roles.
Disturbed sarcoplasmic reticulum (SR) Ca2+ content may underlie the altered force-frequency and postrest contractile behavior in failing human myocardium. We used rapid cooling contractures (RCCs) to assess SR Ca2+ content in ventricular muscle strips isolated from nonfailing and end-stage failing human hearts. With an increase in rest intervals (1 to 240 s; 37 degrees C), nonfailing human myocardium (n=7) exhibited a parallel increase in postrest twitch force (at 240 s by 121+/-44%; P<0.05) and RCC amplitude (by 69+/-53%; P<0.05). In contrast, in failing myocardium (n=30), postrest twitch force decreased at long rest intervals and RCC amplitude declined monotonically with rest (by 25+/-9% and 53+/-9%, respectively; P<0.05). With an increase in stimulation frequencies (0.25 to 3 Hz), twitch force increased continuously in nonfailing human myocardium (n=7) by 71+/-17% (at 3 Hz; P<0.05) and RCC amplitude increased in parallel by 247+/-55% (P<0.05). In contrast, in failing myocardium (n=26), twitch force declined by 29+/-7% (P<0. 05) and RCC amplitude increased only slightly by 36+/-14% (P<0.05). Paired RCCs were evoked to investigate the relative contribution of SR Ca2+ uptake and Na+/Ca2+ exchange to cytosolic Ca2+ removal during relaxation. SR Ca2+ uptake (relative to the Na+/Ca2+ exchange) increased significantly in nonfailing but not in failing human myocardium as stimulation rates increased. We conclude that the negative force-frequency relation in failing human myocardium is due to an inability of SR Ca2+ content to increase sufficiently at high frequencies and thus cannot overcome the frequency-dependent refractoriness of SR Ca2+ release. The rest-dependent decay in twitch force in failing myocardium is due to rest-dependent decline in SR Ca2+ content. These alterations could be secondary to depressed SR Ca2+-ATPase combined with enhanced cytosolic Ca2+ extrusion via Na+/Ca2+ exchange.
The Na(+)-H(+) exchange (NHE) is a major mechanism by which the heart adapts to intracellular acidosis during ischemia and recovers from the acidosis after reperfusion. There are at least 6 NHE isoforms thus far identified with the NHE1 subtype representing the major one found in the mammalian myocardium. This 110-kDa glycoprotein extrudes protons concomitantly with Na(+) influx in a 1:1 stoichiometric relationship rendering the process electroneutral, and its activity is regulated by numerous factors, including phosphorylation-dependent processes. There is convincing evidence that NHE mediates tissue injury during ischemia and reperfusion, which probably reflects the fact that under conditions of tissue stress, including ischemia, Na(+)-K(+) ATPase is inhibited, thereby limiting Na(+) extrusion, resulting in an elevation in [Na(+)](i). The latter effect, in turn, will increase [Ca(2+)](i) via Na(+)-Ca(2+) exchange. In addition, NHE1 mRNA expression is elevated in response to injury, which may further contribute to the deleterious consequence of pathological insult. Extensive studies using NHE inhibitors have consistently shown protective effects against ischemic and reperfusion injury in a large variety of experimental models and has led to clinical evaluation of NHE inhibition in patients with coronary artery disease. Emerging evidence also implicates NHE1 in other cardiac disease states, and the exchanger may be particularly critical to postinfarction remodeling responses resulting in development of hypertrophy and heart failure.
We investigated the inhibitory effect of KB-R7943 on ‘bi-directional’ Na+/Ca2+ exchange current (iNCX) with the reversal potential of iNCX (ENCX) in the middle of the ramp voltage pulse employed.
Bi-directional iNCX was recorded with ‘full’ ramp pulses given every 10 s from the holding potential of −60 mV over the voltage range between 30 and −150 mV under the ionic conditions of 140 mM [Na]o, 20 mM [Na]i, 1 mM [Ca]o and 433 nM [Ca]i with calculated ENCX at −50 mV.
KB-R7943 (0.1–100 μM) concentration-dependently inhibited the current, which reversed near the calculated ENCX, indicating that the blocked current was iNCX.
The inhibition levels were not significantly different between outward and inward iNCX measured at 0 and −120 mV, respectively. IC50 of KB-R7943 was approximately 1 μM for both directions of iNCX.
Under the bi-directional ionic conditions, only an outward or inward iNCX was induced by positive or negative ‘half’ ramp pulses, respectively, from the holding potential of −60 mV. KB-R7943 inhibited both direction of iNCX and the concentration-inhibition relations were superimposable to the ones obtained by ‘full’ ramp pulses.
These results indicate that KB-R7943 inhibits iNCX direction-independently under bi-directional conditions. This conclusion is different from that of our previous results obtained from iNCX under uni-directional ionic conditions, where KB-R7943 inhibited iNCX direction-dependently. The difference could be attributed to slow dissociation of the drug from the exchanger.
British Journal of Pharmacology (1999) 128, 969–974; doi:10.1038/sj.bjp.0702869
The Na(+)/Ca(2+) exchange (NCX) extrudes Ca(2+) from cardiac myocytes, but it can also mediate Ca(2+) influx, load the sarcoplasmic reticulum with Ca(2+), and trigger Ca(2+) release from the sarcoplasmic reticulum. In ischemia/reperfusion or digitalis toxicity, increased levels of intracellular [Na(+)] ([Na(+)](i)) may raise levels of intracellular [Ca(2+)] ([Ca(2+)](i)) via NCX, leading to cell injury and arrhythmia.
We used KB-R7943 (KBR) to selectively block Ca(2+) influx via NCX to study the role of NCX-mediated Ca(2+) influx in intact rat ventricular myocytes. Removing extracellular Na(+) caused [Ca(2+)](i) to rise, due to Ca(2+) influx via NCX, and this was blocked by 90% with 5 micromol/L KBR. However, KBR did not alter [Ca(2+)](i) decline due to NCX. Thus, we used 5 micromol/L KBR to selectively block Ca(2+) entry but not efflux via NCX. Under control conditions, 5 micromol/L KBR did not alter steady-state twitches, Ca(2+) transients, Ca(2+) load in the sarcoplasmic reticulum, or rest potentiation, but it did prolong the late low plateau of the rat action potential. When Na(+)/K(+) ATPase was inhibited by strophanthidin, KBR reduced diastolic [Ca(2+)](i) and abolished the spontaneous Ca(2+) oscillations, but it did not prevent inotropy.
In rat ventricular myocytes, Ca(2+) influx via NCX is not important for normal excitation-contraction coupling. Furthermore, the inhibition of Ca(2+) efflux alone (as [Na(+)](i) rises) may be sufficient to cause glycoside inotropy. In contrast, Ca(2+) overload and spontaneous activity at high [Na(+)](i) was blocked by KBR, suggesting that net Ca(2+) influx (not merely reduced efflux) via NCX is involved in potentially arrhythmogenic Ca(2+) overload.
To determine sarcolemmal Na+/H+ exchanger (NHE) activity and expression in human ventricular myocardium.
Although the sarcolemmal NHE has been implicated in various physiological and pathophysiological phenomena in animal studies, its activity and expression in human myocardium have not been studied.
Ventricular myocardium was obtained from unused donor hearts with acute myocardial dysfunction (n = 5) and recipient hearts with chronic end stage heart failure (n = 11) through a transplantation program. Intracellular pH (pHi) was monitored in enzymatically isolated single ventricular myocytes by microepifluorescence. As the index of sarcolemmal NHE activity, the rate of H+ efflux at a pHi of 6.90 J(H6.9)) was determined after the induction of intracellular acidosis in bicarbonate-free medium. Na+/H+ exchanger isoform 1 (NHE1) expression in ventricular myocardium was determined by immunoblot analysis.
Human ventricular myocytes exhibited readily detectable sarcolemmal NHE activity after the induction of intracellular acidosis, and this activity was suppressed by the NHE1-selective inhibitor HOE-642 (cariporide) at 1 micromol/L. Sarcolemmal NHE activity of myocytes was significantly greater in recipient hearts (JH6.9 = 1.95+/-0.18 mmol/L/min) than it was in unused donor hearts (J(H6.9 = 1.06+/-0.15 mmol/L/min). In contrast, NHE1 protein was expressed in similar abundance in ventricular myocardium from both recipient and unused donor hearts.
Sarcolemmal NHE activity of human ventricular myocytes arises from the NHE1 isoform and is inhibited by HOE-642. Sarcolemmal NHE activity is significantly greater in recipient hearts with chronic end-stage heart failure than it is in unused donor hearts, and this difference is likely to arise from altered posttranslational regulation.
We tested the hypothesis that overstretching the myocardium could induce and/or exacerbate contractile dysfunction via stretch-activated (SA) ion channels. Maximum developed tension (T(max)), normalized to a control value, was compared in guinea pig papillary muscles held at one of three resting lengths (physiological stretch, overstretch, and unloaded) for 85 min. Overstretched muscles exhibited decreased contractile force (T(max) = 0.77 +/- 0.03) compared with physiological and unloaded muscles (T(max) = 0.93 +/- 0.05 and 1.03 +/- 0.07, respectively). Gd(3+), an SA channel antagonist, eliminated the adverse effect of overstretching (T(max) = 0.98 +/- 0.06), but nifedipine, a dihydropyridine (DHP) antagonist of L-type calcium channels, did not (T(max) = 0.82 +/- 0.04). Exposure to modified hypoxia-reoxygenation (MHR) during physiological stretch resulted in decreased contractility (T(max) = 0.63 +/- 0.07), an effect that was exacerbated by overstretching (T(max) = 0.44 +/- 0.04). Gd(3+) mitigated the effects of overstretch during MHR (T(max) = 0.64 +/- 0.05), but DHP did not (T(max) = 0.48 +/- 0.04). These data suggest that overstretching of the myocardium contributes to contractile abnormalities via SA channels that are distinct from L-type calcium channels.
We investigated the contribution of angiotensin II and endothelin I to the slow positive inotropic response observed following stretch of isolated ferret papillary muscle from 88% to 98% of the length at which maximum force is generated. Angiotensin antagonists losartan and saralasin did not affect the magnitude of the slow response in ferret papillary muscle. The ETA-selective antagonist BQ123 slightly reduced the magnitude of the slow response (P > 0.05). In the presence of PD145065 (an ETA and ETB antagonist), the magnitude of the slow response was reduced significantly by 50%. Removal of the endothelium with 1% Triton X-100 reversed the slow response to stretch. We conclude that, in the ferret, endothelin I acting through ETA and ETB receptors, contributes to the slow response although it is not the sole mediator. Angiotensin II is not a prerequisite for the slow response to stretch. We have shown for the first time that the endocardial endothelium plays a pivotal role in this phenomenon in cardiac papillary muscle.
This study was designed to gain additional insight into the mechanism of the slow force response (SFR) to stretch of cardiac muscle. SFR and changes in intracellular Na(+) concentration ([Na(+)](i)) were assessed in cat papillary muscles stretched from 92% to approximately 98% of L(max). The SFR was 120+/-0.6% (n=5) of the rapid initial phase and coincided with an increase in [Na(+)](i). The SFR was markedly depressed by Na(+)-H(+) exchanger inhibition, AT(1) receptor blockade, nonselective endothelin-receptor blockade and selective ET(A)-receptor blockade, extracellular Na(+) removal, and inhibition of the reverse mode of the Na(+)-Ca(2+) exchange by KB-R7943. KB-R7943 prevented the SFR but not the increase in [Na(+)](i). Inhibition of endothelin-converting enzyme activity by phosphoramidon suppressed both the SFR and the increase in [Na(+)](i). The SFR and the increase in [Na(+)](i) after stretch were both present in muscles with their endothelium (vascular and endocardial) made functionally inactive by Triton X-100. In these muscles, phosphoramidon also suppressed the SFR and the increase in [Na(+)](i). The data provide evidence that the last step of the autocrine-paracrine mechanism leading to the SFR to stretch is Ca(2+) entry through the reverse mode of Na(+)-Ca(2+) exchange.
The steep relationship between active force and length in cardiac muscle is based on a length dependence of myofilament Ca(2+) sensitivity. However, it is not muscle length but the lateral spacing between actin and myosin filaments that sets the level of Ca(2+) sensitivity, mainly through modulation of myosin-mediated activation of the thin filament.
Augmented preload increases myocardial excitability by shortening action potential duration (APD). The mechanism governing this phenomenon is unknown. Because myocardial stretch increases intracellular cAMP, we hypothesized that load-dependent changes in myocardial excitability are mediated by beta-adrenergic stimulation of a cAMP-sensitive K(+) current.
The effects of propranolol on load-induced changes in electrical excitability were studied in 7 isolated ejecting canine hearts. LV monophasic APD at 50% and 90% repolarization (MAPD(50) and MAPD(90)) and refractoriness were determined at low (9+/-3 mL) and high (39+/-4 mL) load before and after beta-adrenergic blockade. During control, the MAPD(50) decreased from 193+/-26 to 184+/-26 ms with increased load, as did the MAPD(90) (238+/-28 to 233+/-28 ms), P</=0.04. Similar changes were observed in ventricular refractoriness. Treatment with propranolol completely abolished these load-induced effects. Myocardial catecholamine depletion with reserpine in 2 hearts also abolished changes in MAPD and excitability in response to increased preload.
Increases in ventricular load mediate a decrease in ventricular APD and refractoriness through activation of the beta-adrenergic receptor. An increase in a cAMP-mediated K(+) current, possibly the slowly activating delayed rectifier I(Ks), may account in part for this form of mechanoelectrical coupling.
Of the ions involved in the intricate workings of the heart, calcium is considered perhaps the most important. It is crucial to the very process that enables the chambers of the heart to contract and relax, a process called excitation-contraction coupling. It is important to understand in quantitative detail exactly how calcium is moved around the various organelles of the myocyte in order to bring about excitation-contraction coupling if we are to understand the basic physiology of heart function. Furthermore, spatial microdomains within the cell are important in localizing the molecular players that orchestrate cardiac function.
Chronic stimulation of the beta(1)-adrenergic receptor leads to hypertrophy and heart failure in beta(1)-adrenergic receptor transgenic mice and contributes to disease progression in heart failure patients. The cellular mechanisms underlying these detrimental effects are largely unknown. In this study, we have identified the cardiac Na(+)-H(+) exchanger (NHE1) as a novel mediator of adrenergically induced heart failure. beta(1)-Adrenergic receptor transgenic mice showed upregulation of both NHE1 mRNA (+140+/-6%) and protein (+42+/-19%). In order to test whether increased NHE1 is causally related to beta(1)-adrenergic-induced hypertrophy, fibrosis, and heart failure, beta(1)-adrenergic receptor transgenic (TG) and wild-type (WT) littermates were treated with a diet containing 6000 ppm of the NHE1 inhibitor cariporide or control chow for 8 months. There was significant hypertrophy of cardiac myocytes in beta(1)-adrenergic receptor transgenic mice (2.3-fold increase in myocyte cross-sectional area), which was virtually absent in cariporide-fed animals. Interstitial fibrosis was prominent throughout the left ventricular wall in nontreated beta(1)-adrenergic receptor transgenic mice (4.8-fold increase in collagen volume fraction); cariporide treatment completely prevented this development of fibrosis. Left ventricular catheterization showed that cariporide also prevented the loss of contractile function in beta(1)-adrenergic receptor transgenic mice: whereas untreated transgenic mice showed a significant decrease in left ventricular contractility (5250+/-570 mm Hg/s TG versus 7360+/-540 mm Hg/s WT, dp/dt(max)), this decrease was completely prevented by cariporide (8150+/-520 mm Hg/s TG cariporide). Inhibition of NHE1 prevented the development of heart failure in beta(1)-receptor transgenic mice. We conclude that the cardiac Na(+)-H(+) exchanger 1 is essential for the detrimental cardiac effects of chronic beta(1)-receptor stimulation in the heart.
The role of the Na+-Ca2+ exchanger as a major determinant of cell Ca2+ is well defined in cardiac tissue, and there has been much effort to develop specific inhibitors of the exchanger. We use a novel system to test the specificity of two putative specific inhibitors, KB-R7943 and SEA0400. The drugs are applied to electrically stimulated heart tubes from control mouse embryos or embryos with the Na+-Ca2+ exchanger knocked out. We monitored effects of the drugs on Ca2+ transients. Both drugs depress the Ca2+ transients at low concentrations even in the absence of any Na+-Ca2+ exchanger. KB-R7943 and SEA0400 are not completely specific and should be used with caution as Na+-Ca2+ exchange inhibitors.
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