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Mechanisms Underlying the Increase in Force and Ca2+ Transient That Follow Stretch of Cardiac Muscle : A Possible Explanation of the Anrep Effect
Abstract
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.
... However, several of the studies by Sarnoff et al. (1960) showed a small and short-lived preload stretch that reversed within 10-12 s. This 'transient stretch' was the argument for many subsequent studies to equate the Anrep effect with the SFR (Alvarez et al. 1999;Dowrick et al. 2019), although Sarnoff et al. (1960) did not consider this stretch per se essential for triggering the Anrep effect. ...
... The mechanistic subcellular background of SFR includes increased intracellular Ca 2+ transient either activated by the non-specific stretch-activated ion channels (SACs) and/or by the stretch-activated G-protein coupled angiotensin and endothelin receptor, working both via the Na + /H + and Na + /Ca 2+ exchanger (Calaghan & White, 2004;Dowrick et al. 2019). This is considered as a valid mechanism for the Anrep effect (Alvarez et al. 1999). If the Anrep effect, however, can be triggered without stretch, its subcellular mechanism then would differ from the latter ones, turning the view in a different direction. ...
... the SRF, however, is completed after 2-15 min depending on species and experimental preparations (Alvarez et al. 1999;Dowrick et al. 2019). Additionally, SFR is mediated by SACs and can be inhibited in mice by streptomycin (Ward et al. 2008). ...
Key points
The Anrep effect represents the alteration of left ventricular (LV) contractility to acutely enhanced afterload in a few seconds, thereby preserving stroke volume (SV) at constant preload. As a result of the missing preload stretch in our model, the Anrep effect differs from the slow force response and has a different mechanism.
The Anrep effect demonstrated two different phases. First, the sudden increased afterload was momentary equilibrated by the enhanced LV contractility as a result of higher power strokes of strongly‐bound myosin cross‐bridges. Second, the slightly delayed recovery of SV is perhaps dependent on Ca²⁺/calmodulin‐dependent protein kinase II activation caused by oxidation and myofilament phosphorylation (cardiac myosin‐binding protein‐C, myosin light chain 2), maximizing the recruitment of available strongly‐bound myosin cross‐bridges.
Short‐lived oxidative stress might present a new facet of subcellular signalling with respect to cardiovascular regulation.
Relevance for human physiology was demonstrated by echocardiography disclosing the Anrep effect in humans during handgrip exercise.
Abstract
The present study investigated whether oxidative stress and Ca²⁺/calmodulin‐dependent protein kinase II (CaMKII) activity are involved in triggering the Anrep effect. LV pressure–volume (PV) analyses of isolated, preload controlled working hearts were performed at two afterload levels (60 and 100 mmHg) in C57BL/6N wild‐type (WT) and CaMKII‐double knockout mice (DKOCaMKII). In snap‐frozen WT hearts, force–pCa relationship, H2O2 generation, CaMKII oxidation and phosphorylation of myofilament and Ca²⁺ handling proteins were assessed. Acutely raised afterload showed significantly increased wall stress, H2O2 generation and LV contractility in the PV diagram with an initial decrease and recovery of stroke volume, whereas end‐diastolic pressure and volume, as well as heart rate, remained constant. Afterload induced increase in LV contractility was blunted in DKOCaMKII‐hearts. Force development of single WT cardiomyocytes was greater with elevated afterload at submaximal Ca²⁺ concentration and associated with increases in CaMKII oxidation and phosphorylation of cardiac‐myosin binding protein‐C, myosin light chain and Ca²⁺ handling proteins. CaMKII activity is involved in the regulation of the Anrep effect and associates with stimulation of oxidative stress, presumably starting a cascade of CaMKII oxidation with downstream phosphorylation of myofilament and Ca²⁺ handling proteins. These mechanisms improve LV inotropy and preserve stroke volume within few seconds.
... It is known that this secondary behaviour is independent of the Frank-Starling mechanism 6 ; rather, it is thought to be the adaptive phase of the Anrep effect. 7 The SFR occurs in response to a gradual augmentation of calcium transient magnitude 8,9 during which there is no apparent increase in diastolic calcium, [9][10][11] even when the calcium handling of the sarcoplasmic reticulum (SR) is compromised. 12 Calcium transients are the basis of force generation of cardiac muscle. ...
... Blockade of the G-protein coupled receptors (GPCR) ATR and endothelin receptor (ETR) has been shown to blunt or eliminate the SFR. 10,73,82,83 While ETR blockade prevents an inotropic response to exogenous Ang II application, ATR blockade does not do the same in the case of exogenous endothelin (ET) application, indicating a directionality of interaction. 83 There are two classes of ATR: ATR 1 and ATR 2 . ...
... 162,163 Some SFR experiments using bicarbonate-free solutions observed alkalization in response to stretch, indicating an enhanced NHE 1 activity. 10,83,165 In contrast, Shen et al 94 recorded cytosolic acidification, proposing that increased force production is mirrored by an increased production of protons, and that the upregulation of NHE 1 activity occurs in order to maintain intracellular pH. These discrepant findings regarding pH regulation also conflict temporally as observations of alkalinization occurred once the maximal SFR force was reached, 10,83,165 whereas acidification commenced almost immediately poststretch. ...
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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... This review will focus on the third of these cellular mechanisms since there is still debate about this aspect of cardiac regulation. The Ca 2+ -dependent response to stretch is commonly referred to as the "slow force response" (SFR) and is thought to form the basis of the Anrep effect which describes the heart's increase in contractility that develops following an increase in afterload (Alvarez et al. 1999;Cingolani et al. 2013;Nichols et al. 1988). ...
... They are thought to act acutely by binding to their receptors and activating a signaling cascade that includes activation of the cardiac Na + /H + exchanger (NHE1). Cingolani and colleagues reported an intracellular alkalosis in isolated rabbit muscle following stretch, consistent with activation of NHE1, which was blocked by the NHE1 inhibitor EIPA, as well as by angiotensin and endothelin receptor blockers (Alvarez et al. 1999;Cingolani et al. 1998). Increased NHE1 activity increases [Na + ] i , leading to increased [Ca 2+ ] i via the cardiac Na + /Ca 2+ exchanger (NCX). ...
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.
... 15,16 They not only regulate the intracellular pH (pH i ) but also modulate the intracellular sodium concentration ([Na + ] i ) and indirectly, through sodium/calcium antiporter activity, regulate the intracellular calcium concentration ([Ca 2+ ] i ). [17][18][19][20] It has been fully demonstrated that the overactivation of NHE and NBC is involved in Na + and Ca 2+ overload, 17,20 leading to the development and progression of several CVD types. [21][22][23][24][25][26][27][28][29] At present, it is known that at least 2 functional isoforms of NBC exist in ventricular myocytes: the electroneutral (stoichiometry 1Na + /1HCO À 3 ), called NBCn1, and the electrogenic (stoichiometry 1Na + /2HCO À 3 ), called NBCe1. ...
... 15,16 They not only regulate the intracellular pH (pH i ) but also modulate the intracellular sodium concentration ([Na + ] i ) and indirectly, through sodium/calcium antiporter activity, regulate the intracellular calcium concentration ([Ca 2+ ] i ). [17][18][19][20] It has been fully demonstrated that the overactivation of NHE and NBC is involved in Na + and Ca 2+ overload, 17,20 leading to the development and progression of several CVD types. [21][22][23][24][25][26][27][28][29] At present, it is known that at least 2 functional isoforms of NBC exist in ventricular myocytes: the electroneutral (stoichiometry 1Na + /1HCO À 3 ), called NBCn1, and the electrogenic (stoichiometry 1Na + /2HCO À 3 ), called NBCe1. ...
Background
It is well known that after menopause women are exposed to a greater cardiovascular risk, but the intracellular modifications are not properly described. The sodium/proton exchanger (NHE) and the sodium/bicarbonate cotransporter (NBC) regulate the intracellular pH and, indirectly, the intracellular sodium concentration ([Na⁺]). There are 2 isoforms of NBC in the heart: the electrogenic (1Na⁺/2HCO3−; NBCe1) and the electroneutral (1Na⁺/1HCO3−; NBCn1). Because NHE and NBCn1 hyperactivity as well as the NBCe1 decreased activity have been associated with several cardiovascular pathologies, the aim of this study was to investigate the potential alterations of the alkalinizing transporters during the postmenopausal period.
Methods and Results
Three‐month ovariectomized rats (OVX) were used. The NHE activity and protein expression are significantly increased in OVX. The NBCe1 activity is diminished, and the NBCn1 activity becomes predominant in OVX rats. p‐Akt levels showed a significant diminution in OVX. Finally, NHE activity in platelets from OVX rats is also higher in comparison to sham rats, resulting in a potential biomarker of cardiovascular diseases.
Conclusions
Our results demonstrated for the first time that in the cardiac ventricular myocytes of OVX rats NHE and NBC isoforms are altered, probably because of the decreased level of p‐Akt, compromising the ionic intracellular homeostasis.
... 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). ...
... 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.
... 76 Mechanistically, endothelin-1-mediated enhancement of Ca 2+ entry results from (1) activation of Na + /Ca 2+ exchanger forward and reverse modes via protein kinase C downstream of the endothelin A receptor [25][26][27][28] and (2) indirect activation of the Na + /Ca 2+ exchanger reverse mode by protein kinase Cdependent increase of Na + -H + exchanger activity, effectively increasing intracellular Na + and thus driving the Na + /Ca 2+ exchanger to increase intracellular calcium. 29,30 Other mechanisms, such as endothelin-1 activation of L-type calcium channel current, remain controversial [31][32][33][34] but are also likely mediated via protein kinase C downstream of the endothelin receptor. 35 The increased extrusion of H + via the Na + -H + exchanger was originally hypothesized to affect myofilament calcium sensitivity via cytosolic alkalinization. ...
... 76,77 Paracrine Effects of Cardiac Fibroblasts on Myocyte ECC Conventionally, healthy cardiac tissue is believed to have a cellular composition that is up to two thirds nonmyocytes, the Attenuate β-adrenergic inotropic effects cGMP-dependent degradation of cAMP via PDEII 19,20 ; PKG-mediated decrease of LTCC activity. 19,21 Positive lusitropy cGMP-dependent PKG phosphorylation of troponin I. [22][23][24] Endothelin-1 ↑ Ca 2+ entry PKC-mediated activation of NCX forward/reverse modes [25][26][27][28][29][30] ; PKC-mediated activation of LTCC. [31][32][33][34][35] Alter myofilament Ca 2+ sensitivity PKC/D-dependent phosphorylation of troponin I and myosin-binding protein C. [36][37][38][39][40] ↑ calcium-induced calcium release IP 3 -induced calcium release from the SR via IP 3 R, which subsequently sensitizes RyR2. ...
Cardiac excitation-contraction coupling (ECC) is the orchestrated process of initial myocyte electrical excitation, which leads to calcium entry, intracellular trafficking, and subsequent sarcomere shortening and myofibrillar contraction. Neurohumoral β-adrenergic signaling is a well-established mediator of ECC; other signaling mechanisms, such as paracrine signaling, have also demonstrated significant impact on ECC but are less well understood. For example, resident heart endothelial cells are well-known physiological paracrine modulators of cardiac myocyte ECC mainly via NO and endothelin-1. Moreover, recent studies have demonstrated other resident noncardiomyocyte heart cells (eg, physiological fibroblasts and pathological myofibroblasts), and even experimental cardiotherapeutic cells (eg, mesenchymal stem cells) are also capable of altering cardiomyocyte ECC through paracrine mechanisms. In this review, we first focus on the paracrine-mediated effects of resident and therapeutic noncardiomyocytes on cardiomyocyte hypertrophy, electrophysiology, and calcium handling, each of which can modulate ECC, and then discuss the current knowledge about key paracrine factors and their underlying mechanisms of action. Next, we provide a case example demonstrating the promise of tissue-engineering approaches to study paracrine effects on tissue-level contractility. More specifically, we present new functional and molecular data on the effects of human adult cardiac fibroblast conditioned media on human engineered cardiac tissue contractility and ion channel gene expression that generally agrees with previous murine studies but also suggests possible species-specific differences. By contrast, paracrine secretions by human dermal fibroblasts had no discernible effect on human engineered cardiac tissue contractile function and gene expression. Finally, we discuss systems biology approaches to help identify key stem cell paracrine mediators of ECC and their associated mechanistic pathways. Such integration of tissue-engineering and systems biology methods shows promise to reveal novel insights into paracrine mediators of ECC and their underlying mechanisms of action, ultimately leading to improved cell-based therapies for patients with heart disease.
... The fact that the rise in Na + is cancelled both by EIPA and losartan is consistent with the notion that stretch induces the release of prestored Ang II which activates NHE through AT 1 receptors. Modified with permission from Alvarez et al. 8 To further characterize the mechanism, the possible involvement of ET-1 in NHE activation by stretch was explored. Compelling evidence demonstrates that many effects previously adscribed to Ang II are actually due to the release/formation of endogenous ET. Figure 5 shows that the increase in Na + i elicited by stretch is suppressed by either blockade of ET A receptors with BQ 123 or inhibition of the endothelin-converting enzyme with phosphoramidon. ...
... By this time, the CaT has increased by ~20%. Experiments from other 11 and our 8,12 laboratories have shown that this increase in CaT is responsable for the second, slow increase in DF observed after stretch. Relatively small increases in Na + i can have a large impact on the Ca 2+ fluxes mediated by NCX. ...
The sarcolemmal Na+/H+ exchanger (NHE) extrudes intracellular H+ in exchange for extracellular Na+ and has a role in the regulation of not only intracellular pH (pHi) but also intracellular Na+ (Na+i homeostasis. Both H+ and Na+ are important determinants of cardiac contractility and therefore, the level of NHE activity may become relevant to many (patho)physiological conditions. We have examined the participation of NHE activity in the response to myocardial stretch. Pioneering studies performed by several independent research groups on neonatal rat cardiomyocytes cultured on deformable silicone dishes showed that mechanical stress induces a whole set of cell responses including the expression of immediate-early and fetal genes, secretion of growth factors such as angiotensin II (Ang II) and endothelin-1 (ET-1), activation of PKC and MAPK-dependent intracellular signaling pathways, and increased protein synthesis (1–4). Since protein synthesis and MAPK activation were partially abrogated by inhibitors of NHE activity (4), the possibility of stretch-induced NHE stimulation was suspected. Our experiments were designed to assess, in papillary muscles of adult animals, the mechanism(s) of NHE activation by myocardial stretch and its contribution to the subsequent increase in force. The experimental set-up is schematically illustrated in Figure 1.
... Therefore, while the Frank-Starling effect is regulated at the myofibrillar level, the Anrep effect is likely caused by a gradual change in intracellular Ca 2+ homeostasis in response to an increase in afterload. Though numerous mechanisms have been reported regarding the rise of CaT (e.g., [14,36]), it is generally accepted today that a cardinal mechanism is the increased sarcolemmal Ca 2+ influx through the Na + /Ca 2+ exchanger operating in a reverse mode (e.g., [6,14]). ...
Of the ions involved in myocardial function, Ca ²⁺ is the most important. Ca ²⁺ is crucial to the process that allows myocardium to repeatedly contract and relax in a well-organized fashion; it is the process called excitation–contraction coupling. In order, therefore, for accurate comprehension of the physiology of the heart, it is fundamentally important to understand the detailed mechanism by which the intracellular Ca ²⁺ concentration is regulated to elicit excitation–contraction coupling. Aequorin was discovered by Shimomura, Johnson and Saiga in 1962. By taking advantage of the fact that aequorin emits blue light when it binds to Ca ²⁺ within the physiologically relevant concentration range, in the 1970s and 1980s, physiologists microinjected it into myocardial preparations. By doing so, they proved that Ca ²⁺ transients occur upon membrane depolarization, and tension development (i.e., actomyosin interaction) subsequently follows, dramatically advancing the research on cardiac excitation–contraction coupling.
... Stretching cardiomyocytes increases the calcium sensitivity of their sarcomeres resulting in enhanced contractility. In situations where ventricular preloading is maintained, there is a further progressive increase in the force of contractility termed the slow force response (SFR) which is driven by elevated, TRPC3 and TRPC6 dependent (9), Ca 2+ transients (18). Chronic ventricular loading, for example after myocardial ischemia, results in cardiac remodeling and pathological cardiomyocyte hypertrophy leading, ultimately, to heart failure. ...
Myocardial damage caused, for example, by cardiac ischemia leads to ventricular volume overload resulting in increased stretch of the remaining myocardium. In adult mammals, these changes trigger an adaptive cardiomyocyte hypertrophic response which, if the damage is extensive, will ultimately lead to pathological hypertrophy and heart failure. Conversely, in response to extensive myocardial damage, cardiomyocytes in the adult zebrafish heart and neonatal mice proliferate and completely regenerate the damaged myocardium. We therefore hypothesized that in adult zebrafish, changes in mechanical loading due to myocardial damage may act as a trigger to induce cardiac regeneration. Based on this notion we sought to identify mechanosensors which could be involved in detecting changes in mechanical loading and triggering regeneration. Here we show using a combination of knockout animals, RNAseq and in vitro assays that the mechanosensitive ion channel Trpc6a is required by cardiomyocytes for successful cardiac regeneration in adult zebrafish. Furthermore, using a cyclic cell stretch assay, we have determined that Trpc6a induces the expression of components of the AP1 transcription complex in response to mechanical stretch. Our data highlights how changes in mechanical forces due to myocardial damage can be detected by mechanosensors which in turn can trigger cardiac regeneration.
... The usage and limitations of various techniques for applying mechanical loading at the single-cell level have been analyzed in a review paper (Chen- Izu and Izu, 2017). One technique is to use a pair of micro-cantilevers (i.e., carbon fibers, glass fibers, etc.) attached to the cell surface to stretch the rod-shaped cardiomyocyte along the long axis (Le Guennec et al., 1990;Alvarez et al., 1999;Calaghan and White, 2004;Iribe et al., 2009;Prosser et al., 2011). Such a one-dimensional stretching method applies longitudinal tension on the cell but no transverse compression or surface traction on lateral surfaces. ...
The heart pumps blood into circulation against vascular resistance and actively regulates the contractile force to compensate for mechanical load changes. Our experimental data show that cardiomyocytes have a mechano-chemo-transduction (MCT) mechanism that increases intracellular Ca2+ transient to enhance contractility in response to increased mechanical load. This study advances the cardiac excitation-Ca2+ signaling-contraction (E-C) coupling model on conceptual and technical fronts. First, we developed analytical and computational models to perform 3-dimensional mechanical analysis of cardiomyocytes contracting in a viscoelastic medium under mechanical load. Next, we proposed an MCT feedback loop in the E-C coupling dynamic system to shift the feedforward paradigm of cardiac E-C coupling to an autoregulation model. Our combined modeling and experimental studies reveal that MCT enables autoregulation of E-C coupling and contractility in single cardiomyocytes, which underlies the heart’s intrinsic autoregulation in compensatory response to load changes in order to maintain the stroke volume and cardiac output.
... Some authors suggest pathways that involve autocrine and/or paracrine factors released by mechanical stress such as endothelin-1 or angiotensin. 35 Others have suggested pathways that are partially or completely independent from humoral or neurohumoral factors but involve cellular mechanoreceptors like stretch-activated channels, 36 Na + /H + -exchanger, 37 Z-disc proteins like MLP, 38 or costamere proteins like melusin. 39 For Akt/GSK3β, a link to the myofilament proteins melusin 39 and zyxin 40 has been described, indicating that preload is sensed at the level of myofilaments. ...
Aims:
Pressure overload (PO) and volume overload (VO) lead to concentric or eccentric hypertrophy. Previously, we could show that activation of signalling cascades differ in in vivo mouse models. Activation of these signal cascades could either be induced by intrinsic load sensing or neuro-endocrine substances like catecholamines or the renin-angiotensin-aldosterone system.
Methods and results:
We therefore analysed the activation of classical cardiac signal pathways [mitogen-activated protein kinases (MAPKs) (ERK, p38, and JNK) and Akt-GSK3β] in in vitro of mechanical overload (ejecting heart model, rabbit and human isolated muscle strips). Selective elevation of preload in vitro increased AKT and GSK3β phosphorylation after 15 min in isolated rabbit muscles strips (AKT 49%, GSK3β 26%, P < 0.05) and in mouse ejecting hearts (AKT 51%, GSK49%, P < 0.05), whereas phosphorylation of MAPKs was not influenced by increased preload. Selective elevation of afterload revealed an increase in ERK phosphorylation in the ejecting heart (43%, P < 0.05), but not in AKT, GSK3β, and the other MAPKs. Elevation of preload and afterload in the ejecting heart induced a significant phosphorylation of ERK (95%, P < 0.001) and showed a moderate increased AKT (P = 0.14) and GSK3β (P = 0.21) phosphorylation, which did not reach significance. Preload and afterload elevation in muscles strips from human failing hearts showed neither AKT nor ERK phosphorylation changes.
Conclusions:
Our data show that preload activates the AKT-GSK3β and afterload the ERK pathway in vitro, indicating an intrinsic mechanism independent of endocrine signalling.
... An important mediator of the Anrep effect is β-adrenergic stimulation as Anrep himself found [4]. Other extrinsic factors such as pH [10], glucagon [11,12,28], and angiotensin [29,30] are also likely to be involved because when β-adrenergic receptors are blocked [28,31], saturated [14], or when they are reduced [8] the Anrep effect still occurs. ...
The heart has two intrinsic mechanisms to enhance contractile strength that compensate for increased mechanical load to help maintain cardiac output. When vascular resistance increases the ventricular chamber initially expands causing an immediate length-dependent increase of contraction force via the Frank-Starling mechanism. Additionally, the stress-dependent Anrep effect slowly increases contraction force that results in the recovery of the chamber volume towards its initial state. The Anrep effect poses a paradox: how can the cardiomyocyte maintain higher contractility even after the cell length has recovered its initial length? Here we propose a surface mechanosensor model that enables the cardiomyocyte to sense different mechanical stresses at the same mechanical strain. The cell-surface mechanosensor is coupled to a mechano-chemo-transduction feedback mechanism involving three elements: surface mechanosensor strain, intracellular Ca2+ transient, and cell strain. We show that in this simple yet general system, contractility autoregulation naturally emerges, enabling the cardiomyocyte to maintain contraction amplitude despite changes in a range of afterloads. These nontrivial model predictions have been experimentally confirmed. Hence, this model provides a new conceptual framework for understanding the contractility autoregulation in cardiomyocytes, which contributes to the heart’s intrinsic adaptivity to mechanical load changes in health and diseases.
... The stretch-dependent changes in the cardiac contraction force have biphasic properties: first, a rapid and larger increase in force, and second, a slow increase in force [138,139]. Stretching of the ventricle and atrium is accompanied by increases in Ca 2+ transient amplitude [140][141][142]. Stretch-induced augmentation of Ca 2+ transients may result from enhanced unitary Ca 2+ releases in ventricular myocytes. ...
Homeostasis in the level of reactive oxygen species (ROS) in cardiac myocytes plays a critical role in regulating their physiological functions. Disturbance of balance between generation and removal of ROS is a major cause of cardiac myocyte remodeling, dysfunction, and failure. Cardiac myocytes possess several ROS-producing pathways, such as mitochondrial electron transport chain, NADPH oxidases, and nitric oxide synthases, and have endogenous antioxidation mechanisms. Cardiac Ca2+-signaling toolkit proteins, as well as mitochondrial functions, are largely modulated by ROS under physiological and pathological conditions, thereby producing alterations in contraction, membrane conductivity, cell metabolism and cell growth and death. Mechanical stresses under hypertension, post-myocardial infarction, heart failure, and valve diseases are the main causes for stress-induced cardiac remodeling and functional failure, which are associated with ROS-induced pathogenesis. Experimental evidence demonstrates that many cardioprotective natural antioxidants, enriched in foods or herbs, exert beneficial effects on cardiac functions (Ca2+ signal, contractility and rhythm), myocytes remodeling, inflammation and death in pathological hearts. The review may provide knowledge and insight into the modulation of cardiac pathogenesis by ROS and natural antioxidants.
... The slow force response (SFR) to myocardial stretch is a second increase in developed force that occurs just after the Frank-Starling mechanism takes place. It is well-known that an augmented calcium transient amplitude underlies its development (1,2) but the exact genesis of this increase is still a matter of debate. We have proposed that the SFR is the mechanical expression of a stretchtriggered autocrine mechanism where oxidative stress targeting the Na + /H + exchanger (NHE1) plays a crucial role (3). ...
The stretch of cardiac muscle increases developed force in two phases. The first phase occurs immediately after stretch and is the expression of the Frank–Starling mechanism, while the second one or slow force response (SFR) occurs gradually and is due to an increase in the calcium transient amplitude. An important step in the chain of events leading to the SFR generation is the increased production of reactive oxygen species (ROS) leading to redox sensitive ERK1/2, p90RSK, and NHE1 phosphorylation/activation. Conversely, suppression of ROS production blunts the SFR. The purpose of this study was to explore whether overexpression of the ubiquitously expressed antioxidant molecule thioredoxin-1 (TRX1) affects the SFR development and NHE1 phosphorylation. We did not detect any change in basal phopho-ERK1/2, phopho-p90RSK, and NHE1 expression in mice with TRX1 overexpression compared to wild type (WT). Isolated papillary muscles from WT or TRX1-overexpressing mice were stretched from 92 to 98% of its maximal length. A prominent SFR was observed in WT mice that was completely canceled in TRX1 animals. Interestingly, myocardial stretch induced a significant increase in NHE1 phosphorylation in WT mice that was not detected in TRX1-overexpressing mice. These novel results suggest that magnification of cardiac antioxidant defense power by overexpression of TRX1 precludes NHE1 phosphorylation/activation after stretch, consequently blunting the SFR development.
... The scheme of Figure 2 summarizes this sequence. Interestingly, although activation of the NHE1 is critical, pHi does not play a role because of simultaneous activation of the acidifying Cl − /HCO − 3 anion exchanger by endothelin (33)(34)(35), which compensates the rise in pHi but not in Na + , leading to Ca 2+ increase. ...
Current evidence reveals that cardiac mineralocorticoid receptor (MR) activation following myocardial stretch plays an important physiological role in adapting developed force to sudden changes in hemodynamic conditions. Its underlying mechanism involves a previously unknown nongenomic effect of the MR that triggers redox-mediated Na ⁺ /H ⁺ exchanger (NHE1) activation, intracellular Na ⁺ accumulation, and a consequent increase in Ca ²⁺ transient amplitude through reverse Na ⁺ /Ca ²⁺ exchange. However, clinical evidence assigns a detrimental role to MR activation in the pathogenesis of severe cardiac diseases such as congestive heart failure. This mini review is meant to present and briefly discuss some recent discoveries about locally triggered cardiac MR signals with the objective of shedding some light on its physiological but potentially pathological consequences in the heart.
... CaMKII contributes to afterload-induced Ca 2+ sparks (Guo et al., 2006a) and the inhibition of both nNOS and CaMKII in cardiomyocytes eliminates the Ca 2+ sparks, suggesting mechano-transduction activates nNOS and CaMKII (Jian et al., 2014). Mechano-chemotransduction through NOS and CaMKII signaling pathways suggests possible therapeutic targets for treating mechanical stress-induced Ca 2+ dysregulation, arrhythmias (Jian et al., 2014), and cardiomyopathy (Moolman et al., 1997;Alvarez et al., 1999). The effects of NO in mediating Ca 2+ handling in the cardiomyocyte are shown in Figure 5. ...
The ability§ of the heart to adapt to changes in the mechanical environment is critical for normal cardiac physiology. The role of nitric oxide is increasingly recognized as a mediator of mechanical signaling. Produced in the heart by nitric oxide synthases, nitric oxide affects almost all mechano-transduction pathways within the cardiomyocyte, with roles mediating mechano-sensing, mechano-electric feedback (via modulation of ion channel activity), and calcium handling. As more precise experimental techniques for applying mechanical stresses to cells are developed, the role of these forces in cardiomyocyte function can be further understood. Furthermore, specific inhibitors of different nitric oxide synthase isoforms are now available to elucidate the role of these enzymes in mediating mechano-electrical signaling. Understanding of the links between nitric oxide production and mechano-electrical signaling is incomplete, particularly whether mechanically sensitive ion channels are regulated by nitric oxide, and how this affects the cardiac action potential. This is of particular relevance to conditions such as atrial fibrillation and heart failure, in which nitric oxide production is reduced. Dysfunction of the nitric oxide/mechano-electrical signaling pathways are likely to be a feature of cardiac pathology (e.g., atrial fibrillation, cardiomyopathy, and heart failure) and a better understanding of the importance of nitric oxide signaling and its links to mechanical regulation of heart function may advance our understanding of these conditions.
... Subsequently, in isolated ventricular muscle strips exposed to sudden myocardial stretch, a gradual secondary increase in isometric/isotonic force was observed to follow the initial rise in contractility induced by the Frank-Starling mechanism (Parmley and Chuck, 1973). It has since been proposed that this "Slow Force Response (SFR)" (Figure 1, SFR) is the in vitro equivalent of the Anrep effect (Alvarez et al., 1999). Unlike the Frank-Starling mechanism, SFR is induced by a gradual increase in Ca 2+ transient amplitude (Allen and Kurihara, 1982;Kentish and Wrzosek, 1998) through the activation of multiple intracellular components and ion transporters (Cingolani et al., 2013). ...
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.
... Since a phosphorylation-mediated increase in NHE-1 activity is a key factor in the chain of events leading to the SFR [19][20][21][22][23], we evaluated whether p38-MAPK inhibition had an effect upon this parameter. To this purpose, papillary muscles were frozen 5 minutes after myocardial stretch and processed for NHE-1 phosphorylation quantitation. ...
Abstract
Background/Aims: Myocardial stretch increases cardiac force in two consecutive phases:
The first one due to Frank-Starling mechanism, followed by the gradually developed slow
force response (SFR). The latter is the mechanical counterpart of an autocrine/paracrine
mechanism involving the release of angiotensin II (Ang II) and endothelin (ET) leading to
Na+/H+ exchanger 1 (NHE-1) phosphorylation and activation. Since previous evidence
indicates that p38-MAP kinase (p38-MAPK) negatively regulates the Ang II-induced NHE-
1 activation in vascular smooth muscle and the positive inotropic effect of ET in the heart,
we hypothesized that this kinase might modulate the magnitude of the SFR to stretch.
Methods: Experiments were performed in isolated rat papillary muscles subjected to sudden
stretch from 92 to 98% of its maximal length, in the absence or presence of the p38-MAPK
inhibitor SB202190, or its inactive analogous SB202474. Western blot technique was used
to determine phosphorylation level of p38-MAPK, ERK1/2, p90RSK and NHE-1 (previously
immunoprecipitated with NHE-1 polyclonal antibody). Dual specificity phosphatase 6 (DUSP6)
expression was evaluated by RT-PCR and western blot. Additionally, the Na+-dependent
intracellular pH recovery from an ammonium prepulse-induced acid load was used to asses
NHE-1 activity. Results: The SFR was larger under p38-MAPK inhibition (SB202190), effect that
was not observed in the presence of an inactive analogous (SB202474). Myocardial stretch
activated p38-MAPK, while pre-treatment with SB202190 precluded this effect. Inhibition
of p38-MAPK increased stretched-induced NHE-1 phosphorylation and activity, key event
in the SFR development. Consistently, p38-MAPK inhibition promoted a greater increase in
ERK1/2-p90RSK phosphorylation/activation after myocardial stretch, effect that may certainly
be responsible for the observed increase in NHE-1 phosphorylation under this condition.
Myocardial stretch induced up-regulation of the DUSP6, which specifically dephosphorylates
ERK1/2, effect that was blunted by SB202190. Conclusion: Taken together, our data support
the notion that p38-MAPK activation after myocardial stretch restricts the SFR by limiting
ERK1/2-p90RSK phosphorylation, and consequently NHE-1 phosphorylation/activity, through
a mechanism that involves DUSP6 up-regulation.
... Mechanical stretch of β 1 integrin at the cardiomyocyte surface initiates a number of different signalling pathways by causing release of angiotensin II (Ang II) from myocytes (Browe & Baumgarten, 2004). The Ang II then activates angiotensin II type 1 receptor (AT 1 receptor), which begins a cascade that ultimately activates chloride-permeant stretch-activated channels (Browe & Baumgarten, 2004) or the Na + -H + exchanger (NHE) and NCX, leading to a secondary increase in the Ca 2+ transient (Alvarez et al. 1999;Calaghan & White, 2004;Cingolani et al. 2013). ...
Cardiac excitation‐contraction (E‐C) coupling is influenced by (at least) three dynamic systems that couple and feedback to one another (see Abstract Figure). Here we review the mechanical effects on cardiomyocytes that include mechano‐electro‐transduction (commonly referred to as mechano‐electric coupling, MEC) and mechano‐chemo‐transduction (MCT) mechanisms at cell and molecular levels which couple to Ca²⁺‐electro and E‐C coupling reviewed elsewhere. These feedback loops from muscle contraction and mechano‐transduction to the Ca²⁺ homeodynamics and to the electrical excitation are essential for understanding the E‐C coupling dynamic system and arrhythmogenesis in mechanically loaded hearts. This white paper comprises two parts, each reflecting key aspects from the 2018 UC Davis symposium: MEC (how mechanical load influences electrical dynamics) and MCT (how mechanical load alters cell signalling and Ca²⁺ dynamics). Of course, such separation is artificial since Ca²⁺ dynamics profoundly affect ion channels and electrogenic transporters and vice versa. In time, these dynamic systems and their interactions must become fully integrated, and that should be a goal for a comprehensive understanding of how mechanical load influences cell signalling, Ca²⁺ homeodynamics and electrical dynamics. In this white paper we emphasize current understanding, consensus, controversies and the pressing issues for future investigations. Space constraints make it impossible to cover all relevant articles in the field, so we will focus on the topics discussed at the symposium. image
... The stretch-dependent changes in the cardiac contractility have two components: first, a rapid and larger increase in force, and second, a slow increase in force [69][70][71]. Increases in muscle length in the ventricle or atrium are accompanied by increases in Ca 2+ transient amplitude [64,67,[72][73][74]. ...
... Toutefois, le mécanisme de Frank -Starling étant une réponse à l'étirement, la diminution du VTD au-delà de 50% de la VO2max suggère que le mécanisme de Frank -Starling n'est plus impliqué dans l'augmentation de la contractilité audelà de ces intensités. (Alvarez et al. 1999). ...
Initiée par les travaux des lauréats du prix Nobel Krogh et Hill l’étude de l’adaptation du système cardiorespiratoire lors de la transition du repos à l’exercice représente un champ majeur en physiologie de l’exercice. En début d’effort, la consommation d’oxygène (VO2) doit rapidement s’adapter afin de minimiser le déficit en dioxygène (O2), les perturbations intracellulaires et par conséquent optimiser la performance. Pendant cette phase dite « cardiodynamique » le ventricule gauche (VG) doit rapidement augmenter l’apport en O2 aux muscles actifs. Le volume d’éjection systolique (VES) et les nombreux paramètres qui le conditionnent doivent aussi rapidement s’adapter malgré une réduction très importante de la durée de diastole. Comment le VG parvient-il à relever ce défi dans les premières minutes de l’exercice et comment ces différents paramètres interagissent ? L’objectif de ce travail de thèse est de répondre à ces questions en combinant l’utilisation des dernières avancées de l’échocardiographie en mode « Speckle Tracking » (STE), permettant d’apprécier finement la fonction myocardique régionale grâce à l’évaluation de la mécanique du VG, avec une approche originale caractérisée par des enregistrements à intervalles réguliers proches au cours de répétitions d’épreuves à charges constantes (épreuves dites « rectangulaires »). Une première partie va consister à décrire finement les cinétiques cardiaques chez le sujet jeune actif, alors qu’une deuxième partie s’attachera à évaluer l’impact de l’entraînement aérobie chez des cyclistes de très bon niveau rapportant de nombreuses années de pratique. Les résultats mettent en évidence que la fonction diastolique et ses mécanismes sous-jacents jouent un rôle clé dans l’adaptation du VES à l’exercice. En permettant l’augmentation de la vitesse de remplissage, la pression de remplissage et la relaxation favorisent l’adaptation du VES dans la première minute. L’adaptation de la vitesse de détorsion jusqu’à 120s prolonge l’adaptation de la vitesse de remplissage et permet le maintien du VES au-delà de la première minute. Par ailleurs, nos résultats indiquent que l’entraînement aérobie induit une adaptation du VES plus importante grâce à une amélioration de la vitesse et du débit de remplissage du VG. Ces améliorations sont le résultat d’une adaptation plus rapide et plus importante de la relaxation à la base et d’une adaptation plus importante de la vitesse de détorsion. Ce travail de thèse basé sur l’étude de la mécanique cardiaque à l’exercice d’intensité modérée a permis de montrer le rôle clé de la diastole, et plus particulièrement de la détorsion du VG, dans l’adaptation rapide du VES ainsi que son amélioration chez le sportif aérobie. En perspective, il serait intéressant d’étudier les cinétiques cardiaques à des intensités d’exercice supérieures ou dans des populations qui présentent des intolérances à l'effort.
... It has been conceived that slow force response in normal myocardium is regulated by complex pathway of molecular event involving Ang II/endothelin secretion (Alvarez et al. 1999;Cingolani et al. 2013). To test whether the pathway is in operation under our experimental conditions, we compared twitch characteristics during slow responses to stretch in normal rat myocardium before and after inhibition of AT 1 receptors by perfusion of muscles with modified Krebs-Henseleit solution + 100 μmol/l losartan for 30 min (n = 12). ...
The slow force response (SFR) of a cardiac muscle to a sudden stretch is thought to be important in the regulatory adaptation of myocardial contraction. Autocrine-paracrine regulation pathways which involve angiotensin II are participating in this mechanism. On the other hand, renin-angiotensin-aldosterone system (RAS) is altered in hypertrophic or failing myocardium. We compared the effects of sudden stretch to SFR as well as to twitch and Ca2+ transient characteristics in rat myocardium with monocrotaline-induced heart failure with those in normal rat myocardium without and with inhibition of angiotensin II type-1 (AT1) receptors. Our findings indicate that the myocardium of rats with monocrotaline-induced right ventricular failure is deficient with activation of local RAS and therefore expresses blunted SFR, very similar to the depression of SFR observed in normal myocardium under inhibition of AT1 receptors. The "failing" myocardium does not further respond to the "putative" inhibition of AT1 receptors by losartan. In conclusion, SFR is related to autocrine-paracrine regulation of myocardial contraction in normal rat myocardium and that the involvement of RAS into stretch-induced modulation of contractility may be significantly altered in failing heart.
... 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.
... Thus, activation of SACs may cause an increase [Na + ] i and [Ca 2+ ] I , although the increase in [Ca 2+ ] i may be indirect, as SACs have greater permeability for Na + than Ca 2+ . Hence, they could indirectly increase [Ca 2+ ] i via the reversemode NCX (Alvarez et al. 1999;Baartscheer et al. 2003;Isenberg et al. 2003;Kamkin et al. 2003). ...
Calcium (Ca²⁺) is a universal intracellular second messenger. In the heart, it plays a key role by activating contraction through the excitation-contraction coupling (EC coupling) mechanism. Although this is its key role in the heart, Ca²⁺ has other important functions, not only being involved in cell growth (in the heart named excitation-transcription coupling, ET coupling) but also in mitochondrial function (excitation-metabolism coupling, EM coupling) and cell death. Moreover, as Ca²⁺ is electrically charged, its movement across membranes generates an electrical current, which is important in cardiomyocyte electrophysiology and, if disturbed, may be involved in arrhythmias. The cardiac myocyte may discriminate between Ca²⁺ signals by creating “spaces” where Ca²⁺ diffusion is limited, creating gradients of [Ca²⁺]i at the micrometer scale, which are named microdomains. They are maintained by the cellular architecture and location of Ca²⁺-handling proteins and buffers.
... 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.
... The proposed mechanism underlying the slow Ca 2þ response is an increase in intracellular sodium (Na þ ) resulting from complex interactions between multiple mechanosensitive pathways. The initial step probably involves auto-/paracrine activation of angiotensin receptors and downstream release of endothelin ( (Alvarez et al., 1999;Calaghan and White, 2001;P erez et al., 2001), but see also conflicting results by (Kocksk€ amper et al., 2008)). 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). ...
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.
... 10,11 Further experiments could show that activation of the Na + /H + exchanger results in enhanced transsarcolemmal Na + entry followed by a [Na + ] i -dependent Ca 2+ entry via the Na + /Ca 2+ exchanger working in its reverse mode. 12,13 During the cardiac contraction cycle, two different types of load-pre-load during diastole and after-load during systole -can be differentiated. Pre-load builds up during diastolic filling and stretches cardiomyocytes, whereas after-load is generated by each cardiomyocyte to produce adequate cardiac stroke work against vascular resistance. ...
Aims
The Frank–Starling mechanism (rapid response (RR)) and the secondary slow response (SR) are known to contribute to increases contractile performance. The contractility of the heart muscle is influenced by pre‐load and after‐load. Because of the effect of pre‐load vs. after‐load on these mechanisms in not completely understood, we studied the effect in isolated muscle strips.
Methods and results
Progressive stretch lead to an increase in shortening/force development under isotonic (only pre‐load) and isometric conditions (pre‐ and after‐load). Muscle length with maximal function was reached earlier under isotonic (Lmax‐isotonic) compared with isometric conditions (Lmax‐isometric) in nonfailing rabbit, in human atrial and in failing ventricular muscles. Also, SR after stretch from slack to Lmax‐isotonic was comparable under isotonic and isometric conditions (human: isotonic 10 ± 4%, isometric 10 ± 4%). Moreover, a switch from isotonic to isometric conditions at Lmax‐isometric showed no SR proving independence of after‐load. To further analyse the degree of SR on the total contractile performance at higher pre‐load muscles were stretched from slack to 98% Lmax‐isometric under isotonic conditions. Thereby, the SR was 60 ± 9% in rabbit and 51 ± 14% in human muscle strips.
Conclusions
This work shows that the acute contractile response largely depends on the degree and type of mechanical load. Increased filling of the heart elevates pre‐load and prolongs the isotonic part of contraction. The reduction in shortening at higher levels of pre‐load is thereby partially compensated by the pre‐load‐induced SR. After‐load shifts the contractile curve to a better ‘myofilament function’ by probably influencing thin fibers and calcium sensitivity, but has no effect on the SR.
... This phenomenon is manifested as a relationship between the degree of muscle stretch (preload) and the strength of contraction in isolated myocardial preparations. The molecular events that underlie the Frank−Starling phenomenon are the following: a) change in the num ber of available actin-myosin binding sites upon a change in the degree of overlap of actin-myosin fila ments [1], b) changes in the kinetics of calcium ion binding to the regulatory protein troponin C (TnC), including those related to cooperative effects [2,3], Abbreviations: TnC, troponin C; SERCA2a, ATP dependent calcium pump of the sarcoplasmic reticulum, cardiac isoform; "bump", a phase of short lived deceleration of the decay of the calcium transient; CaT, calcium transient; TTP, time required to attain the peak of contraction or CaT. and c) changes in the function of the exchange mech anism that is involved in the regulation of the ion bal ance [4,5]. Moreover, the length dependent changes of the contractility of cardiac muscle are affected by changes in the cycling kinetics of myosin cross bridges [6,7], structural changes in the myosin molecule [8], and additional sensitization of contractile proteins to calcium mediated by the giant protein titin [9]. ...
The characteristics of tension increase and decline, as well as those of the calcium transients, have been measured in the trabeculae of the right cardiac ventricle of the guinea pig and rat in the isometric contraction mode with different preloads. Measurements were performed at different temperatures of physiological saline and the effects of inhibition of calcium removal from the cytosol mediated by Na+–Ca2+ exchange and the ATP-dependent Ca2+ pump of the sarcoplasmic reticulum (SERCA2a) were analyzed. Emergence of the “bump” phase (a phase of brief deceleration of the decay of the calcium transient) was observed in the guinea pig myocardium as the temperature was increased from 25 to 30°C; earlier observations of this phenomenon were reported only for rats. As the temperature was elevated to 35°C, the “bump” phase in the guinea pig myocardium transformed into a “plateau” phase of the calcium transient. The effect of temperature on the course of the decay of the calcium transient in the rat myocardium was negligible. In contrast, a gradual stretching of the right ventricular trabecula of the rat was accompanied by the emergence of the “bump” phase and a gradual increase of its parameters (amplitude, integral intensity, and duration), whereas preload did not exert a similar effect on the guinea pig myocardium. Selective inhibition of the reverse mode of Na+–Ca2+ exchange did not affect the characteristics of the decay of the calcium transient in guinea pig myocardium. Selective inhibition of SERCA2a in the guinea pig and rat myocardium had a significant modifying effect on the decay phase of the calcium transient and resulted in emergence of the “bump” phase or an increase in the intensity of this phase in the myocardium of these animal species. The characteristics of this phase can be used to quantify the length-dependent activation of myocardial contraction.
... The myocardial vascular compartment is also composed of mechanosensitive cells that may contribute to the interpretation of mechanical signals, and alter cardiac structure and function. For instance, rapidly increasing coronary perfusion pressure has long been known to increase myocardial oxygen consumption (Gregg's phenomenon) and enhance cardiac contractility (the garden-hose effect) [10][11][12], whereas a sudden increase in afterload increases myocardial contractility (the Anrep effect) [13,14]. Similarly, increased coronary blood flow increases shear stress on the coronary endothelium, which in turn regulates the release of vasoactive intermediaries (such as nitric oxide, endothelial hyperpolarizing factor, and endothelin-1) that can secondarily affect contractile performance [15,16]. ...
Mechanotransduction is the process by which load-bearing cells sense physical forces, transduce the forces into biochemical signals, and generate adaptive or maladaptive responses that lead to alterations in cell structure and function. Mechanotransduction in the heart not only affects the beat-to-beat regulation of cardiac performance, but also profoundly affects the growth, differentiation, and survival of the cellular components that comprise the human myocardium. Understanding the molecular basis for mechanotransduction is, therefore, important to our overall understanding of growth regulation and function during cardiac hypertrophy and ischemia. Cardiomyocytes rely on several intracellular components to sense mechanical load, and convert mechanical stimuli into biochemical events that affect cellular structure and function. These sensors include protein components within the myofilaments and Z-discs, integrins and other membrane-associated proteins that link the extracellular matrix to the cytoskeleton, and stretch-activated ion channels. A complex signaling web then transmits signals from mechanosensors to the nucleus and other organelles. Ultimately, it is hoped that new pharmacological and molecular genetic approaches targeted to specific components of the mechanotransduction machinery will be developed to translate this wealth of basic knowledge into therapeutic strategies designed to reduce cardiac hypertrophy, further protect the ischemic myocardium, and prevent its transition to heart failure.
... Length-dependent changes in myofibril-relaxation properties occur within only a few milliseconds [ 31 ]. Conversely, it has been shown that the slow-phase response of Frank-Starling relationship takes minutes and is dependent on multiple mechanisms such as increase in calcium transient [32,33]. Additionally, during this slow phase response, contractile proteins are also phosphorylated [24,34]. ...
Cross-bridge cycling kinetics play an essential role in the heart's ability to contract and relax. The rate of tension redevelopment (ktr) slows down as a muscle length is increased in intact human myocardium. We set out to determine the effect of rapid length step changes and protein kinase A (PKA) and protein kinase C-βII (PKC-βII) inhibitors on the ktr in ultra-thin non-failing and failing human right ventricular trabeculae. After stabilizing the muscle either at L90 (90% of optimal length) or at Lopt (optimal length), we rapidly changed the length to either Lopt or L90 and measured ktr. We report that length-dependent changes in ktr occur very rapidly (in the order of seconds or faster) in both non-failing and failing muscles and that the length at which a muscle had been stabilized prior to the length change does not significantly affect ktr. In addition, at L90 and at Lopt, PKA and PKC-βII inhibitors did not significantly change ktr. Our results reveal that length-dependent regulation of cross-bridge cycling kinetics predominantly occurs rapidly and involves the intrinsic properties of the myofilament rather than post-translational modifications that are known to occur in the cardiac muscle as a result of a change in muscle/sarcomere length.
... This second progressive increase in force development, was demonstrated in vitro by Parmley and Chuck in 1973, being since then synonymously called slow force response (SFR; Parmley and Chuck, 1973). Contrarily to the FSM, the main mechanism responsible for the SFR appears to be a progressive increase in Ca 2+ transient (Alvarez et al., 1999). ...
Myocardial stretch, as result of acute hemodynamic overload, is one of the most frequent challenges to the heart and the ability of the heart to intrinsically adapt to it is essential to prevent circulatory congestion. In this review, we highlight the historical background, the currently known mechanisms, as well as the gaps in the understanding of this physiological response. The systolic adaptation to stretch is well-known for over 100 years, being dependent on an immediate increase in contractility—known as the Frank-Starling mechanism—and a further progressive increase—the slow force response. On the other hand, its diastolic counterpart remains largely unstudied. Mechanosensors are structures capable of perceiving mechanical signals and activating pathways that allow their transduction into biochemical responses. Although the connection between these structures and stretch activated pathways remains elusive, we emphasize those most likely responsible for the initiation of the acute response. Calcium-dependent pathways, including angiotensin- and endothelin-related pathways; and cGMP-dependent pathways, comprising the effects of nitric oxide and cardiac natriuretic hormones, embody downstream signaling. The ischemic setting, a paradigmatic situation of acute hemodynamic overload, is also touched upon. Despite the relevant knowledge accumulated, there is much that we still do not know. The quest for further understanding the myocardial response to acute stretch may provide new insights, not only in its physiological importance, but also in the prevention and treatment of cardiovascular diseases.
The Anrep effect is an adaptive response that increases left ventricular contractility following an acute rise in afterload. Although the mechanistic origin remains undefined, recent findings suggest a two-phase activation of resting myosin for contraction, involving strain-sensitive and posttranslational phases. We propose that this mobilization represents a transition among the relaxed states of myosin—specifically, from the super-relaxed (SRX) to the disordered-relaxed (DRX)—with DRX myosin ready to participate in force generation. This hypothesis offers a unified explanation that connects myosin’s SRX-DRX equilibrium and the Anrep effect as parts of a singular phenomenon. We underscore the significance of this equilibrium in modulating contractility, primarily studied in the context of hypertrophic cardiomyopathy, the most common inherited cardiomyopathy associated with diastolic dysfunction, hypercontractility, and left ventricular hypertrophy. As we posit that the cellular basis of the Anrep effect relies on a two-phased transition of myosin from the SRX to the contraction-ready DRX configuration, any dysregulation in this equilibrium may result in the pathological manifestation of the Anrep phenomenon. For instance, in hypertrophic cardiomyopathy, hypercontractility is linked to a considerable shift of myosin to the DRX state, implying a persistent activation of the Anrep effect. These valuable insights call for additional research to uncover a clinical Anrep fingerprint in pathological states. Here, we demonstrate through noninvasive echocardiographic pressure-volume measurements that this fingerprint is evident in 12 patients with hypertrophic obstructive cardiomyopathy before septal myocardial ablation. This unique signature is characterized by enhanced contractility, indicated by a leftward shift and steepening of the end-systolic pressure-volume relationship, and a prolonged systolic ejection time adjusted for heart rate, which reverses post-procedure. The clinical application of this concept has potential implications beyond hypertrophic cardiomyopathy, extending to other genetic cardiomyopathies and even noncongenital heart diseases with complex etiologies across a broad spectrum of left ventricular ejection fractions.
Although pathologically a disease of the pulmonary vasculature, pulmonary arterial hypertension (PAH) and chronic thromboembolic pulmonary hypertension (CTEPH) manifest clinically as a syndrome of right heart failure. In the PAH syndrome, the increased afterload produced by the characteristic plexogenic pulmonary arteriopathy of PAH is just one element that contributes to a progressive decline in right-heart function. Likewise, in CTEPH, mechanical obstruction of proximal and distal pulmonary vessels by persistent thromboemboli sets off a sequence of pathological changes that ultimately cause right-heart pressure overload. Maladaptive coupling of the RV to the pulmonary circulation, left ventricular-right ventricular interactions, impaired myocardial contractility, attenuated myocardial relaxation, and systemic vascular congestion all contribute to a decline in RV function and ultimately to adverse outcomes in these conditions. Here, an overview of RV involvement in PAH and CTEPH is provided, with a focus on methods utilized to assess and characterize RV function, as well as the mechanisms underlying maladaptive changes that have been elucidated to date.
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.
Gleb von Anrep, a Russian-born Egyptian physiologist, observed in 1912 the gradual partial recovery of left ventricular dilation following acute aortic constriction. Anrep effect deals with epicardial to endocardial redistribution of blood flow. Recovery from ischemia is the primary mechanism of the Anrep effect. Developed Force is the force because of an increase in the troponin C sensitivity to the influx of calcium (L-type channel). When a cardiac muscle stretches, there is an expeditious rise in the developed force known as the Frank-Starling mechanism. After an initial rapid response to stretch, there is a slow (gradual) increase in the developed force, called slow flow response, over 10-15 minutes, because of the rise in calcium-transient amplitude, called the Anrep effect. A vast number of mechanisms are responsible for the rise in the calcium transient during the Anrep effect. A cardinal mechanism is the increased sarcolemmal calcium influx through the sodium-calcium exchanger operating in a reverse mode. The prolongation of the action potential duration by cardiac myocyte stretch could favor reverse-mode sodium-calcium exchange and escalate calcium influx. A greater action potential duration will approve the reverse-mode sodium-calcium exchanger activity.
Right ventricular failure (RV-F) is a complex, heterogeneous clinical syndrome, characterized by dyspnea—fatigue complaints and normally systemic congestion, which “can result from any structural or functional cardiovascular disorder that impairs the ability of the RV to fill or to eject blood” [1–3].
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.
The heart has the ability to adjust to changing mechanical loads. The Frank-Starling law and the Anrep effect describe exquisite intrinsic mechanisms the heart has for autoregulating the force of contraction to maintain cardiac output under preload and afterload. Although these mechanisms have been known for more than a century, their cellular and molecular underpinnings are still debated. How does the cardiac myocyte sense a change in preload or afterload? How does the myocyte adjust its response to compensate for such changes? In cardiac myocytes Ca(2+) is a crucial regulator of contractile force and in this review we compare and contrast recent results from different labs that address two important questions. The "dimensionality" of the mechanical milieu under which experiments are carried out provide important clues to the location of the mechanosensors and the kinds of mechanical forces they can sense and respond to. As a first approximation, sensors inside the myocyte appear to modulate reactive oxygen species (ROS) while sensors on the cell surface appear to also modulate nitric oxide (NO) signalling; both signalling pathways affect Ca(2+) handling. Undoubtedly, further studies will add layers to this simplified picture. Clarifying the intimate links from cellular mechanics to ROS and NO signalling and to Ca(2+) handling will deepen our understanding of the Frank-Starling law and the Anrep effect, and also provide a unified view on how arrhythmias may arise in seemingly disparate diseases that have in common altered myocyte mechanics. This article is protected by copyright. All rights reserved.
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.
In Chaps. 6–8, a pure electrophysiology model—the ten Tusscher et al. human ventricular cell model (TNNP) [1]—representing the electrical activity of the human ventricle was used to investigate the functional consequences of the SQTS on cardiac electrical excitation wave conduction
It is probably not an exaggeration to suggest that the modern era of cardiovascular research began with the classical studies of Otto Frank and Ernest Starling on the relationship between end diastolic volume and systolic function in the isolated heart (Frank, 1895; Patterson and Starling, 1914). From the standpoint of integrative physiology, their work provided a mechanism for linking cardiac output to peripheral vascular perturbations (blood volume, venous compliance, skeletal muscle activity, etc.) which can alter central venous pressure and, hence, the degree of stretch of the myocardial muscle fibers. Their findings, still valid, are central to our understanding of the events that take place in such situations as exercise, hemorrhage, and congestive heart failure. For the muscle physiologist, their work provided the challenge of explaining cardiac function in terms of the basic cellular and molecular mechanisms which control force generation and shortening in striated muscle fibers. This effort has continued to the present day. Building on recent advances in the areas of muscle ultrastructure, contractile protein function, and Ca2+ regulation, it is now possible to formulate a generally plausible mechanism which accounts, at least in large part, for the linkage between ventricular filling pressure and systolic performance. The intracellular signaling pathways which mediate this process remain to be clarified.
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.
Introduction Hemodynamic instability is a common cause for intensive care unit admission. This instability is often described as an inadequate arterial blood pressure, or as unspecific signs of inadequate perfusion of organs and tissues such as metabolic acidosis, hyperlactemia, decreased urine output, and prolonged capillary repletion time. Sustained in time, hemodynamic instability will result in inadequate oxygen delivery and activation of cellular apoptosis and organ failure, so that hemodynamic stabilization represents a life-saving intervention. The causes of cardiovascular dysfunction can be grouped into three categories:
This unique book provides clinicians and administrators with a comprehensive understanding of perioperative hemodynamic monitoring and goal directed therapy, emphasizing practical guidance for implementation at the bedside. Successful hemodynamic monitoring and goal directed therapy require a wide range of skills. This book will enable readers to: • Detail the rationale for using perioperative hemodynamic monitoring systems and for applying goal directed therapy protocols at the bedside • Understand the physiological concepts underlying perioperative goal directed therapy for hemodynamic management • Evaluate hemodynamic monitoring systems in clinical practice • Learn about new techniques for achieving goal directed therapy • Apply goal directed therapy protocols in the perioperative environment (including emergency departments, operating rooms and intensive care units) • Demonstrate clinical utility of GDT and hemodynamic optimization using case presentations. Illustrated with diagrams and case examples, this is an important resource for anesthesiologists, emergency physicians, intensivists and pneumonologists as well as nurses and administrative officers.
Intracellular pH (pHi) homeostasis in cardiac myocytes is achieved principally by the integrated action of 4 different sarcolemmal ion transporters (1). When the myocyte cytoplasm becomes acidic, the Na+/H+ exchanger (NHE) and the Na+/HCO3- cotransporter (NBC) extrude acid from the cell, while under conditions of intracellular alkalosis, the Cl-/HCO3- and Cl-/OH- exchangers effectively import acid. In order to investigate the function and regulation of NHE, experimental protocols are often performed in the absence of bicarbonate, which renders NBC inactive and thereby makes NHE the sole acid extrusion pathway. NHE activity is regulated primarily by pHi, and increases markedly in response to intracellular acidosis (1) through the interaction of H+ with an allosteric modifier site on the transport domain (2,3). The basal activity of the sarcolemmal NHE is low under physiological conditions, while increasing intracellular acidosis leads to a pHi-dependent increase in NHE activity, with a Hill coefficient of around 3 (4). This indicates that more than 1 proton binds to the NHE protein during the transport cycle, and has led to the suggestion that the NHE protein contains a non-transporting proton-binding site which allosterically modifies NHE activity. Thus, as pHi falls, the proton modifier site becomes increasingly occupied, leading to a greater increase in NHE activity than would be expected by simply in creasing the availability of transportable protons.
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
Recent studies have shown that at the same endsystolic volume, ejecting beats can achieve a higher end-systolic pressure than isovolumic beats. The purpose of this study was to assess the metabolic cost, in terms of oxygen consumption (MVO2), and efficiency, in terms of the relation between MVO2 and pressure-volume area (PVA), of this increase in strength during ejection. The slope of the end-systolic pressure-volume relation (ESPVR) (Ees) was greater during ejecting than isovolumic contractions when ejection fraction (EF) was greater than approximately 30%, indicating an increase in contractile strength. The difference in Ees between the two modes of contraction was as much as 30% at EFs of 60%. In contrast, the slope of the MVO2-PVA relation was less during ejecting than isovolumic contractions, indicating a decrease in MVO2 at any given PVA. The difference in slope was as much as 20% at EFs of 60%. Thus afterload conditions, allowing substantial fiber shortening, shift the ESPVR toward greater contractile strength and increase the metabolic efficiency when viewed in terms of the relation between MVO2 and total mechanical energy generation (PVA) by the ventricle. This may reflect an energetically favorable effect of shortening on muscle force-generating capability.
We have recently shown that mechanical stress induces cardiomyocyte hypertrophy partly through the enhanced secretion of angiotensin II (ATII). Endothelin-1 (ET-1) has been reported to be a potent growth factor for a variety of cells, including cardiomyocytes. In this study, we examined the role of ET-1 in mechanical stress-induced cardiac hypertrophy by using cultured cardiomyocytes of neonatal rats. ET-1 (10(-8) approximately 10(-7) M) maximally induced the activation of both Raf-1 kinase and mitogen-activated protein (MAP) kinases at 4 and 8 min, respectively, followed by an increase in protein synthesis at 24 h. All of these hypertrophic responses were completely blocked by pretreatment with BQ123, an antagonist selective for the ET-1 type A receptor subtype, but not by BQ788, an ET-1 type B receptor-specific antagonist. BQ123 also suppressed stretch-induced activation of MAP kinases and an increase in phenylalanine uptake by approximately 60 and 50%, respectively, but BQ788 did not. ET-1 was constitutively secreted from cultured cardiomyocytes, and a significant increase in ET-1 concentration was observed in the culture medium of cardiomyocytes after stretching for 10 min. After 24 h, an approximately 3-fold increase in ET-1 concentration was observed in the conditioned medium of stretched cardiomyocytes compared with that of unstretched cardiomyocytes. ET-1 mRNA levels were also increased at 30 min after stretching. Moreover, ET-1 and ATII synergistically activated Raf-1 kinase and MAP kinases in cultured cardiomyocytes. In conclusion, mechanical stretching stimulates secretion and production of ET-1 in cultured cardiomyocytes, and vasoconstrictive peptides such as ATII and ET-1 may play an important role in mechanical stress-induced cardiac hypertrophy.
We have measured force, sarcomere length, and Ca2+ during twitches in rat cardiac trabeculae. To avoid the difficulties associated with fura-2/acetoxymethyl ester (AM), fura-2 salt was iontophoretically microinjected into the preparation using a single impalement site; this is possible because fura-2 diffuses through the gap junctions between cells. By use of this method, the estimated peak of the [Ca2+] transient during a twitch was not statistically different at different sarcomere lengths: 875 +/- 92 nM at a sarcomere length of 2.15 mum vs. 905 +/- 67 nM at a sarcomere length of 1.65 mum (means +/- SD, n = 10). When trabeculae were loaded using fura-2/AM, the estimated peak of the [Ca2+] transient at a sarcomere length of 2.15 mum was 540 +/- 180 nM (means +/- SD, n = 5). The time course of the Ca2+ transients at different sarcomere lengths is qualitatively similar, but small systematic differences were observed during the relaxation period. On the other hand, the duration of twitch force increases dramatically as the muscle length is increased. As a result, when the trabeculae were held at short muscle lengths,the temporal relationship between force and the Ca2+ transient resembled the relationship between cell shortening and the Ca2+ transient measured in isolated myocytes. At longer lengths the temporal relationship between force and the Ca2+ transient more closely resembles that obtained in papillary muscles using aequorin.
The effect of angiotensin II (Ang II) on the activity of the cardiac Na+-independent Cl--HCO3- exchanger (anionic exchanger [AE]) was explored in cat papillary muscles. pHi was measured by epifluorescence with BCECF-AM. Ang II (500 nmol/L) induced a 5-(N-ethyl-N-isopropyl)amiloride-sensitive increase in pHi in the absence of external HCO3- (HEPES buffer), consistent with its stimulatory action on Na+-H+ exchange (NHE). This alkalinizing effect was not detected in the presence of a CO2-HCO3- buffer (pHi 7.07+/-0.02 and 7.08+/-0.02 before and after Ang II, respectively; n=17). Moreover, in Na+-free HCO3--buffered medium, in which neither NHE nor Na+-HCO3- cotransport are acting, Ang II decreased pHi, and this effect was canceled by previous treatment with SITS. These findings suggested that the Ang II-induced activation of NHE was masked, in the presence of the physiological buffer, by a HCO3--dependent acidifying mechanism, probably the AE. This hypothesis was confirmed on papillary muscles bathed with HCO3- buffer that were first exposed to 1 micromol/L S20787, a specific inhibitor of AE activity in cardiac tissue, and then to 500 nmol/L Ang II (n=4). Under this condition, Ang II increased pHi from 7.05+/-0.05 to 7.22+/-0.05 (P<.05). The effect of Ang II on AE activity was further explored by measuring the velocity of myocardial pHi recovery after the imposition of an intracellular alkali load in a HCO3--containing solution either with or without Ang II. The rate of myocardial pHi recovery was doubled in the presence of Ang II, suggesting a stimulatory effect on AE. The enhancement of the activity of this exchanger by Ang II was also detected when the AE activity was reversed by the removal of extracellular Cl- in a Na+-free solution. Under this condition, the rate of intracellular alkalinization increased from 0.053+/-0.016 to 0.108+/-0.026 pH unit/min (n=6, P<.05) in the presence of Ang II. This effect was canceled either by the presence of the AT1 receptor antagonist, losartan, or by the previous inhibition of protein kinase C with chelerythrine or calphostin C. The above results allow us to conclude that Ang II, in addition to its stimulatory effect on alkaline loading mechanisms, activates the AE in ventricular myocardium and that the latter effect is mediated by a protein kinase C-dependent regulatory pathway linked to the AT1 receptors.
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.
Angiotensin II (ANG II) is a stimulus for positive chronotropic and inotropic effects, protein synthesis, and hypertrophic growth in cardiac tissue. These short- and long-term effects of ANG II are mediated through specific plasma membrane receptors. Indirect evidence suggests that ANG II synthesized in the myocardium may be important in regulating cardiac function. The cell types in the myocardium that produce components of the renin-angiotensin system have not been determined. In this study, we evaluated whether cultured cardiomyocytes and fibroblasts obtained from ventricles of neonatal rat hearts were capable of synthesizing ANG I and II. Both cardiomyocytes and fibroblasts were found to have immunofluorescent staining for ANG I, ANG II, and angiotensin-converting enzyme (ACE). The amounts of ANG I and II in cell extracts and conditioned media obtained from cardiomyocytes and fibroblasts were quantified by radioimmunoassay. The amounts of ANG I and II detected in cardiomyocyte cultures (1.48 x 10(6) cells/dish) were 32.2 +/- 16.2 (n = 4) and 6.2 +/- 2.9 (n = 4) ng/10(6) cells, respectively. The amounts of ANG I and II detected in the media conditioned by a 48-h exposure to cardiomyocytes were 5.2 +/- 1.2 (n = 3) and 2.1 +/- 1.2 (n = 3) ng/10(6) cells, respectively. The amounts of ANG I and II detected in fibroblast cultures (5.38 x 10(6) cells/dish) were 34.8 +/- 4.9 (n = 4) and 8.0 +/- 3.5 (n = 4) ng/10(6) cells, respectively. The amounts of ANG I and II obtained from media conditioned by a 48-h exposure to fibroblasts were 4.7 +/- 0.6 (n = 4) and 3.3 +/- 2.1 (n = 4) ng/10(6) cells, respectively. The identity of the radioimmunoassayable materials as ANG I and II peptides was confirmed in cardiomyocytes using an in vitro bioassay based on displacement of 125I-ANG II from receptor binding sites in cardiac membranes prepared from neonatal pig heart. Identification of ANG I and II and ACE in vitro in cultures of cardiac myocytes and fibroblasts supports the hypothesis that there is an intracardiac renin-angiotensin system that produces these peptides.
We have investigated the effect of a CO2-induced (respiratory) acidosis on contraction and on intracellular Ca2+, Na+, and pH (measured using the fluorescent dyes fura-2, sodium-binding benzofuran isophthalate, and 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein, respectively) in ventricular myocytes isolated from rat hearts. Initial exposure to acidosis led to a rapid decrease in intracellular pH that was accompanied by an abrupt decline in contractility. There were no consistent changes of intracellular Na+ or Ca2+ during this period. The rapid decline of contractility was followed by a slower partial recovery, which was accompanied by increases in intracellular Na+, systolic and diastolic Ca2+, and an increase in the Ca2+ content of the sarcoplasmic reticulum (estimated using caffeine). Intracellular pH did not change during this slow recovery. The slow rise of intracellular Na+ and the recovery of the twitch were blocked by the Na(+)-H+ exchange inhibitor amiloride. The sarcoplasmic reticulum inhibitor ryanodine blocked the recovery of the twitch but had no effect on the rise of intracellular Na+ induced during acidosis. It is concluded that a major cause of the initial decline of the twitch during acidosis is a decrease in the response of the contractile proteins to Ca2+ due to the decrease of intracellular pH. The subsequent slow recovery of the twitch is due to the decrease of intracellular pH activating the Na(+)-H+ exchange mechanism. This elevates intracellular Na+ and presumably, via the Na(+)-Ca2+ exchange mechanism, intracellular Ca2+. This in turn may lead to increased Ca2+ loading of, and hence release from, the sarcoplasmic reticulum, and it is this that underlies the partial recovery of contraction during acidosis in this preparation.
Intracellular [Ca2+] ([Ca2+]i), intracellular Na+ activity (aiNa), and contraction have been monitored in single myocytes isolated from the ventricles of rat hearts. Some of these cells showed an increase in the size of the twitch as stimulation frequency was increased (positive force-frequency relationship), while others showed a decrease in the strength of contraction as the frequency of stimulation was increased (negative force-frequency relationship). In cells that showed a positive force-frequency relationship, increasing stimulation frequency resulted in increases in aiNa, diastolic [Ca2+]i, systolic [Ca2+]i, and the amount of Ca2+ that could be released from the sarcoplasmic reticulum by caffeine. The rate of decline of the [Ca2+]i transient and the twitch also increased as stimulation frequency was increased. In cells that showed a negative force-frequency relationship, increasing stimulation frequency had less effect on aiNa and had either no effect or decreased systolic [Ca2+]i with no change in the amount of Ca2+ that could be released from the sarcoplasmic reticulum using caffeine. The rate of relaxation of the [Ca2+]i transient and the twitch again increased as stimulation frequency increased. The pattern and time course of mechanical restitution was the same in both cell types. Although these data are essentially descriptive, it is consistent with the hypothesis that the final contractile response observed during changes of stimulation frequency may be dependent on how the Ca2+ loading of the preparation varies with stimulation frequency.
Developed twitch tension and action potentials were recorded in rabbit ventricular muscle in physiological saline at 30 degrees C stimulated at 0.5 Hz. Addition of 5 microM nifedipine to block Ca entry via Ca channels almost abolished twitches (to 2.5 +/- 0.7%, S.E.M., n = 10 of control). This suggests that under normal conditions Ca entry via Na-Ca exchange is insufficient to activate contractions. However, when muscles are first exposed to 4 microM acetylstrophanthidin to elevate [Na]i the same exposure to nifedipine only partially suppresses twitches (to 59 +/- 12% of the original control). This suggests that when [Na]i is elevated, Ca entry via the Na-Ca exchange may be adequate to partially activate contraction. From this result it is not clear whether Ca entry via Na-Ca exchange is sufficient to activate contraction directly or whether sarcoplasmic reticulum (SR) Ca release is required. When these experiments were carried out in the presence of 5 to 10 mM caffeine or 100 nM ryanodine similar results were obtained. That is, nifedipine still abolished contractions in the presence of caffeine or ryanodine (to 3.8 +/- 0.3% and 1.3 +/- 0.4%, respectively), but only partially inhibited contractions in the presence of caffeine + acetylstrophanthidin (to 21 +/- 5%) or ryanodine + acetylstrophanthidin (10 +/- 2%). Thus, it appears that even in the absence of a functional SR and with Ca current blocked, Na-Ca exchange might bring sufficient Ca into the cell to activate appreciable contractions, but only when [Na]i is elevated. Action potential duration is decreased by nifedipine and acetylstrophanthidin and is further decreased when nifedipine is added on top of acetylstrophanthidin. If this Ca entry is by an electrogenic 3 Na: 1 Ca exchange, Ca entry will be favored at more positive membrane potentials. If the action potential were not so abbreviated with these drugs, Na-Ca exchange might bring in more Ca and activate additional tension.
In cat papillary muscles contracting physiologically, increasing the afterload caused a biphasic change in contractility. In response to an increase in afterload, contractility (as measured by peak shortening, peak developed force, or peak dF/dt) initially decreased (antihomeometric autoregulation) over the first few beats and then increased slowly with t 1/2 of about 3 min at 30 degrees C and about 1 min at 37 degrees C (homeometric autoregulation). The antihomeometric autoregulation is due to decreased active shortening when the afterload is increased, since it also occurs in response to increased afterload in isotonic contractions. The secondary slow increase in contractility is primarily due to the increase in mean diastolic length that occurs as a result of increased afterload. The time course and the magnitude of the biphasic change in contractility are very similar to those observed in response to afterload increase in intact hearts; we suggest that the secondary slow increase in contractility that we observed is a contributory mechanism to homeometric autoregulation (or the Anrep effect), as it is observed in the whole heart.
There is evidence that the steep ascending limb of the force-length curve in cardiac muscle (Frank-Starling relation) is based on a length-dependence of myofilament Ca2+ sensitivity. Previous work from this laboratory has indicated that in the sarcomere length range corresponding to the ascending limb of the cardiac force length curve (1.7 to 2.3 microns) the Ca2+-troponin C affinity is length-dependent. In this study Ca2+ binding to chemically skinned bovine cardiac muscle bundles was measured during ATP-induced force generation with fiber bundles having sarcomere lengths of 2.2 to 2.4 microns and 1.6 to 1.8 microns. A double isotope technique was used to make concurrent determinations of the force-pCa and bound Ca2+-pCa relationships. At the longer sarcomere lengths the fibers bound, at saturation, an amount of Ca2+ equivalent to approximately 3 mol Ca2+/mol troponin C. Force development appeared to be coupled to titration of the single, low-affinity Ca2+-specific site. In the pCa range 7.0 to 6.0 sarcomere length had no effect on Ca2+ binding. In the pCa range 6.0 to 5.0, in which force increased steeply, there was, in addition to a decreased relative force, a significant reduction in bound Ca2+ at the shorter sarcomere length. Thus sarcomere length appears to influence the Ca2+ binding properties of the regulatory site on troponin C. These data provide direct evidence that length-dependent modulation of Ca2+-troponin C affinity may make a major contribution to the force-length relationship in cardiac muscle.
1. Ferret papillary muscles were isolated and injected with aequorin to measure intracellular Ca2+ concentration [( Ca2+]i). Developed tension and [Ca2+]i were measured in response to length changes. 2. A maintained reduction in muscle length produced an immediate decrease in developed tension followed by slow decline over 10-20 min. This slow decline in tension was accompanied by a slow decline in the amplitude of the systolic [Ca2+]i rise (the Ca2+ transient). The immediate decrease in tension was accompanied by a prolongation of the Ca2+ transient and an abbreviation of the twitch. 3. Repeated reductions in muscle length timed to occur only during the period of contraction (systolic shortening) produced an immediate decrease of developed tension but the subsequent slow decline was substantially smaller. The slow decline in the amplitude of the Ca2+ transients was also smaller. The prolongation of the Ca2+ transient and abbreviation of the twitch were similar to those observed with a maintained reduction of length. 4. Repeated reductions in muscle length during the period between contractions (diastolic shortening) did not produce the immediate decrease of tension but the slow decline of tension was present. The slow decline in the amplitude of the Ca2+ transients was also present. However no change in the duration of the Ca2+ transient or the twitch was present under these conditions. 5. These results suggest that diastolic muscle length can influence the amplitude of the Ca2+ transients achieved during systole. This conclusion was confirmed by experiments in which the recovery of tension and Ca2+ transients was observed after periods of rest. Both developed tension and Ca2+ transients on recovery from a rest were reduced when the rest occurred at a short length in comparison with a long length. 6. We suggest that muscle length influences resting [Ca2+]i and this in turn affects the Ca2+ transients and developed tension.
The relation between muscle length or sarcomere length and developed tension for lengths up to the optimal for contraction (Lmax) is much steeper in cardiac muscle than in skeletal muscle. The steepness of the cardiac length--tension relation arises because the degree of activation of the cardiac myofibrils by calcium increases as muscle length is increased. Two processes contribute to this length-dependence of activation: (i) the calcium sensitivity of the myofibrils increases with muscle length and (ii) the amount of calcium supplied to the myofibrils during systole increases with muscle length. Of these two, the change in calcium sensitivity is the most clearly defined and is responsible for a large part of the rapid change in developed tension when muscle length is altered. It is likely that this change in calcium sensitivity is due to a change in the affinity of troponin for calcium but the underlying mechanism has not been identified. There is good evidence that changes in the calcium supply to the myofibrils can account for the slow changes in tension that follow an alteration in length; there may also be rapid changes in calcium supply but this is less clearly established at present.
Isometrically contracting cat papillary muscles were studied. Muscle length was changed during diastole and returned to control just before the next contraction such that developed force was always measured at the same length. When the diastolic length was increased from a control length, systolic force at the control length increased slowly over several minutes. When the muscle was then held at the increased length, there was an immediate increase in systolic force followed by a small secondary slow increase. Conversely, a decrease in diastolic length from a control length resulted in a slow decrease in systolic force at the control length. When the muscle was then held at the decreased length there was an immediate decrease in systolic force followed by a small secondary decrease. No change in the time course of contraction accompanied the slow force changes after a maintained change of length or a change of diastolic length alone. The magnitude of the slow change of force was proportional to the duration of time in each diastole for which the length was altered and independent of the onset time of a given duration of diastolic length change. The contractility changes were not linearly related to the amplitude of the diastolic length changes. The potentiating effect of a given stretch was greater than the depotentiating effect of a similar release. The development of inotropic changes as a result of diastolic length changes occurred whether or not the muscle was stimulated during the period of the length changes.
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.
To elucidate the mechanisms controlling pHi in myocardium of the spontaneously hypertensive rat (SHR), experiments were performed in papillary muscles (isometrically contracting at 0.2 Hz) from SHR and age-matched normotensive Wistar-Kyoto (WKY) rats loaded with the pH-sensitive fluorescent probe BCECF-AM. An enhanced activity of the Na(+)-H+ exchanger was detected in the hypertrophic myocardium of SHR. This conclusion was based on the following: (1) The myocardial pHi was more alkaline in SHR (7.23 +/- 0.03) than in WKY rats (7.10 +/- 0.03) (P < .05) in HEPES buffer. (2) SITS (0.1 mmol/L in HEPES buffer) did not alter pHi in the SHR (pHi 7.26 +/- 0.03 and 7.28 +/- 0.03 before and after SITS, respectively). (3) The fall in pHi observed after 20 minutes of Na(+)-H+ exchanger inhibition [5 mumol/L 5-(N-ethyl-N-isopropyl)amiloride (EIPA)] was greater in SHR (-0.16 +/- 0.01) than in WKY rats (-0.09 +/- 0.02, P < 0.05). (4) The velocity of pHi recovery from an intracellular acid load was faster in SHR than in WKY rats (0.068 +/- 0.02 versus 0.014 +/- 0.002 pH units/min at pHi 6.99, P < .05). (5) After EIPA inhibition, the rate of pHi recovery from the same acid load decreased to a similar value in both rat strains (0.0032 +/- 0.002 pH units/min in SHR and 0.0032 +/- 0.002 pH units/min in WKY rats). Under the more physiological HCO3(-)-CO2 buffer, no significant difference in steady state myocardial pHi was detected between rat strains (7.15 +/- 0.03 in SHR and 7.11 +/- 0.05 in WKY rats).(ABSTRACT TRUNCATED AT 250 WORDS)
Mechanically activated ion channels (MACs) in rat ventricular myocytes were activated by stretching membrane patches attached to microelectrodes. Charge injection from MACs, consisting mainly of inward K+, was sometimes of sufficient magnitude (0.2 pC) during brief (50 ms) periods, to trigger action currents. Action currents were shown to be extracellular records of action potentials. These results demonstrate the activation of myocytes by injection of approximately 4 pA from a few MACs in a membrane patch.
1. The effects of mechanical loading and changes of length on the contraction of single guinea-pig ventricular myocytes has been investigated. 2. Cell shortening was monitored during isotonic contractions (in which the cell shortened freely) and after attaching carbon fibres of known compliance to the ends of the cell, so that the cell contracted auxotonically (the cell both shortened and developed force). 3. Mechanically loading the cells decreased the amount of shortening during a contraction and abbreviated the contraction. There were, however, no consistent changes in the action potential or the [Ca2+]i transient (measured with the fluorescent dye fura-2). 4. Increasing stimulation rate increased the size of the contraction and the [Ca2+]i transient in both isotonic and auxotonic conditions. The increase in the size of the contraction induced by an increase in stimulation rate was greater in auxotonic conditions but the increase in the size of the [Ca2+]i transient was not. 5. When cells were stretched, there was a step increase in the size of the contraction and a prolongation of its time course. However, neither the size nor the time course of the accompanying [Ca2+]i transient was significantly altered by this intervention. 6. When a stretch was maintained, a further, slow increase in the size of the contraction occurred during the following 3-11 min, in about half the cells studied. The probability of this slow response occurring was increased if the initial degree of activation of the cell was decreased. 7. These data suggest that the mechanisms underlying the responses to mechanical loading and changes of length are the same in both multicellular and single cell preparations of cardiac muscle.
Volume-induced increases in left ventricular (LV) contractility were studied in 18 anesthetized dogs. Denervation eliminated cardiac reflexes, and hearts were paced at 100 +/- 9 (SD) beats/min. Acute volume loading increased LV end-diastolic pressure from approximately 4 to 14 mmHg within 1 min. Contractility increased over 10 min as measured by a decrease in end-systolic length (ESL) (sonomicrometer) at matched LV end-systolic pressure (ESP) or increase in LVESP measured at matched ESL. Volume-dependent increase in contractility was not attenuated by verapamil (0.3 +/- 0.2 microgram/kg i.v., n = 6) or myocardial stunning (15 min ischemia, 30 min reperfusion, n = 6) but was attenuated by ryanodine (1-16 micrograms/kg i.v., n = 6), which alters calcium release from the sarcoplasmic reticulum. From 1 to 10 min after volume loading, LV anterior ESL (measured at LVESP 132 +/- 12 mmHg) decreased by 6.7 +/- 0.5% before, but only by 4.0 +/- 1.7% after 1 microgram/kg ryanodine (P < 0.05). The LVESP (measured at anterior ESL 11.6 mm) increased 32 +/- 4 mmHg before, but only by 17 +/- 12 mmHg after 1 microgram/kg ryanodine. In conclusion, acute volume loading produces a time-dependent increase in LV contractility, which is mediated in part by an increase in calcium release from the sarcoplasmic reticulum.
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.
We have measured force, sarcomere length, and Ca2+ during twitches in rat cardiac trabeculae. To avoid the difficulties associated with fura-2/acetoxymethyl ester (AM), fura-2 salt was iontophoretically microinjected into the preparation using a single impalement site; this is possible because fura-2 diffuses through the gap junctions between cells. By use of this method, the estimated peak of the [Ca2+] transient during a twitch was not statistically different at different sarcomere lengths: 875 +/- 92 nM at a sarcomere length of 2.15 microns vs. 905 +/- 67 nM at a sarcomere length of 1.65 microns (means +/- SD, n = 10). When trabeculae were loaded using fura-2/AM, the estimated peak of the [Ca2+] transient at a sarcomere length of 2.15 microns was 540 +/- 180 nM (means +/- SD, n = 5). The time course of the Ca2+ transients at different sarcomere lengths is qualitatively similar, but small systematic differences were observed during the relaxation period. On the other hand, the duration of twitch force increases dramatically as the muscle length is increased. As a result, when the trabeculae were held at short muscle lengths, the temporal relationship between force and the Ca2+ transient resembled the relationship between cell shortening and the Ca2+ transient measured in isolated myocytes. At longer lengths the temporal relationship between force and the Ca2+ transient more closely resembles that obtained in papillary muscles using aequorin.
An accumulating body of experimental data supports the presence of a paracrine pathway for the modulation of myocardial function by cardiac endothelial cells. Cardioactive substances released by endothelial cells include nitric oxide, endothelin-1, prostanoids, adenylpurines, natriuretic peptides, and other agents that have so far only been characterised in bioassay studies. Endothelial cells also possess enzymatic activities, in particular ACE/kininase activity, which can alter local levels of angiotensin II and bradykinin. Many of the "endothelial" mediators can be produced by cardiac myocytes themselves, often under pathological conditions, suggesting a potential parallel autocrine pathway. Complex reciprocal relationships exist between individual mediators, which affect both their release and actions. Most studies to date have focused on the acute influence of these agents on contractile function; the longer-term modulation both of cardiac structure and function could be equally important. A notable feature of the action of several of the endothelial mediators is that they modify myocardial contractile behaviour predominantly through changes in myofilament properties rather than by altering cytosolic Ca2+ transients. This mode of action often results in a disproportionate effect on myocardial relaxation and diastolic tone. The opposing contractile effects and differing time-scales of action of agents such as nitric oxide and endothelin-1 are reminiscent of the interplay between these factors in the regulation of blood vessel tone. The endothelial paracrine pathway is likely to act in concert and to interact with other cardiovascular regulatory pathways, e.g., the Frank-Starling mechanism, neurohumoral influences, the effects of heart rate, coronary perfusion and load. A better understanding of its physiological and pathophysiological roles may lead to novel therapeutic strategies.
Isolated rat ventricular myocytes were stretched using carbon fibres to investigate the mechanisms underlying the increase in contraction following stretch. 2. [Ca2+]i and [Na+]i were monitored using the fluorescent indicators fura-2 and sodium-binding benzofuran isophthalate, respectively. The L-type Ca2+ current was recorded simultaneously with contraction using the perforated patch-clamp technique. 3. Mechanical stretch caused an immediate increase in contraction, followed by a slow increase. Contraction was prolonged immediately after the stretch, but did not change during the slow phase. 4. The Ca2+ transient did not change immediately after the stretch. The slow increase in contraction was accompanied by an increase in the amplitude of the Ca2+ transient. However, diastolic [Ca2+]i did not change significantly following stretch. 5. [Na+]i did not change significantly either immediately, or during the slow increase in contraction, after the stretch. 6. The L-type Ca2+ current was not significantly altered either by mechanical loading of the cell with carbon fibres or by stretching the cell. 7. These results suggest that: (1) the rapid increase in contraction following a stretch is due to an increase in myofilament Ca2+ sensitivity rather than to changes in the L-type Ca2+ current or [Na+]i; and (2) a slow increase in the Ca2+ transient underlies the slow increase in contraction in isolated myocytes, but is not caused by either an increase in diastolic [Ca2+]i or a change in [Na+]i (and hence Ca2+ influx via Na(+)-Ca2+ exchange) or a change in myofilament Ca2+ sensitivity.
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.
Basic fibroblast growth factor (bFGF; fibroblast growth factor-2) and angiotensin II (ANG II), among other peptide signaling autacoids (cytokines), are known to regulate the phenotypic adaptation of cardiac muscle to physiological stress. The cell type(s) in cardiac muscle responsible for ANG II synthesis and secretion and the role of endogenous cytokines in the regulation of bFGF induction remain unclear. With the use of confluent, serum-starved, low-passage cultures of cardiac microvascular endothelial cells (CMEC), ANG II could be detected in cellular lysates and in medium conditioned by these cells with the use of high-performance liquid chromatography followed by radioimmunoassay. The secretion of angiotensins by individual CMEC could be detected with a cell-blot assay technique. ANG II secretion was decreased by brefeldin A, an agent that interrupts constitutive and regulated secretory pathways for peptide autacoid/ hormone synthesis, suggesting de novo synthesis, activation, and secretion of angiotensins by CMEC. In primary isolates of adult rat ventricular myocytes (ARVM) and CMEC, ANG II, acting at ANG II type 1 receptors in both cell types, was found to increase bFGF mRNA levels measured by ribonuclease protection assay. Endothelin-1 (ET-1), which is known to be synthesized by CMEC, and bFGF itself, which has been detected in both ARVM and CMEC, increased bFGF transcript levels in both cell types. Interleukin-1beta (IL-1beta), which like ANG II and ET-1 is known to activate mitogen-activated protein kinases in both ARVM and CMEC, increased bFGF mRNA levels only in cardiac myocytes. Thus cytokines such as ANG II, ET-1, bFGF, and IL-1beta locally generated by cellular constituents of cardiac muscle, including CMEC, regulate bFGF mRNA levels in a cell type-specific manner.
In cardiac hypertrophy, both excessive enlargement of cardiac myocytes and progressive interstitial fibrosis are well known to occur simultaneously. In the present study, to investigate the interaction between ventricular myocytes (MCs) and cardiac nonmyocytes (NMCs), mostly fibroblasts, during cardiocytes hypertrophy, we examined the change in cell size and gene expression of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) in cultured MCs as markers for hypertrophy in the neonatal rat ventricular cardiac cell culture system.
The size of cultured MCs significantly increased in the MC-NMC coculture. Concomitantly, secretions of ANP and BNP into culture media were significantly increased in the MC-NMC coculture compared with in the MC culture (with the possible contamination of NMC <1% of MC). Moreover, in the MC culture, enlargement of MC and an increase in ANP and BNP secretions were induced by treatment with conditioned media of the NMC culture. A considerable amount of endothelin (ET)-1 production was detected in the NMC-conditioned media. BQ-123, an ET-A receptor antagonist, and bosentan, a nonselective ET receptor antagonist, significantly blocked the hypertrophic response of MCs induced by treatment with NMC-conditioned media. Angiotensin II (Ang II) (10(-10) to 10(-6) mol/L) and transforming growth factor-beta1 (TGF-beta1) (10(-13) to 10(-9) mol/L), both of which are known to be cardiac hypertrophic factors, did not induce hypertrophy in MC culture, but both Ang II and TGF-beta1 increased the size of MCs and augmented ANP and BNP productions in the MC-NMC coculture. This hypertrophic activity of Ang II and TGF-beta1 was associated with the potentiation of ET-1 production in the MC-NMC coculture, and the effect of Ang II or TGF-beta1 on the secretions of ANP and BNP in the coculture was significantly suppressed by pretreatment with BQ-123.
These results demonstrate that NMCs regulate MC hypertrophy at least partially via ET-1 secretion and that the interaction between MCs and NMCs plays a critical role during the process of Ang II- or TGF-beta1-induced cardiocyte hypertrophy.
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.
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.
The effect of increasing the activity of the ventricle on its contractility was investigated. Several beats after the ventricle increases the amount of tension it develops per unit of time, it exhibits an increased contractility as shown by the increase in work and the more rapid development of pressure from a given end-diastolic pressure or fiber length. This has been termed homeometric autoregulation in contradistinction to the Frank-Starling or heterometric type of autoregulation. It was found that changes in coronary flow are not essential to the exhibition of this phenomenon. Possible mechanisms and the physiologic significance of the findings are discussed.
Effects of muscle length on diastolic [Ca 2 ] i in isolated guinea-pig ventricular trabeculae
- Ds Steele
- Gl Smith
Steele DS, Smith GL. Effects of muscle length on diastolic [Ca 2 ] i in
isolated guinea-pig ventricular trabeculae. J Physiol (Lond). 1993;
467:328P. Abstract.
The adaptation of ventricular muscle to different circulatory conditions
- A Rosenblueth
- J Alanais
- E Lopez
- R Rubio
Rosenblueth A, Alanais J, Lopez E, Rubio R. The adaptation of ventricular muscle to different circulatory conditions. Arch Int Physiol Biochim.
1959;67:358 -373.
Effects of muscle length on diastolic [Ca 2ϩ ] i in isolated guinea-pig ventricular trabeculae
- D S Steele
- G L Smith
Steele DS, Smith GL. Effects of muscle length on diastolic [Ca 2ϩ ] i in
isolated guinea-pig ventricular trabeculae. J Physiol (Lond). 1993;
467:328P. Abstract.