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Effect of streptomycin on wall-stress-induced arrhythmias in working rat heart
Abstract
To assess whether streptomycin, an inhibitor of mechano-sensitive cation channels, has an effect on arrhythmias-induced by an increase of ventricular wall stress in the rat heart.
The isolated working rat heart preparation was used. Arrhythmias were induced by increasing the afterload (i.e., aortic pressure) against which the left ventricle (LV) pumped for 20 s. This led to an increase of LV pressure, stretch of the LV and an increase in LV wall stress. The number of ventricular premature beats induced by each afterload step was compared in the absence and presence of streptomycin, a compound known to block mechano-sensitive cation channels in the heart.
Perfusion with 200 microM streptomycin caused a significant reduction in wall-stress-induced arrhythmias. The effect of streptomycin on arrhythmias reached steady-state within 10 min of application. In the presence of streptomycin, arrhythmias elicited by a 40 mmHg afterload increase were reduced to 38% of control. Arrhythmias induced by an 80 mmHg afterload increase were reduced to 61% of control. Complex arrhythmias (ventricular tachycardia) induced by an afterload increase were also reduced in the presence of 200 microM streptomycin. There was no change in inotropic state with streptomycin, as assessed either by cardiac output or by maximum developed LV pressure. Streptomycin 50 microM (a typical therapeutic plasma concentration in patients) had no effect on wall-stress-induced arrhythmias.
The results were inconsistent with streptomycin acting by modulating inositol phosphate production, or altering the level of intracellular calcium or inotropic state. The anti-arrhythmic effect of streptomycin appears more consistent with inhibition of mechano-sensitive cation channels, suggesting that these ion channels might be involved in causing wall-stress-induced arrhythmias.
... While it is an efficient blocker of SAC NS in isolated or cultured cardiac cells (40), it has been shown to have a limited efficacy in native myocardium (119). This limitation results in the need for higher concentrations to affect stretch-induced responses (Ͼ200 M) (161,534) and in a disparity of positive and negative results, some of which may have been caused by lack of effect or off-target actions (492). It is also important to note that streptomycin (like other aminoglycosidic antibiotics) is a common component of standard cell-culture media, so caution is needed when interpreting studies on stretch effects in cultured cells, as background SAC NS availability may be reduced in these preparations. ...
... It is also important to note that streptomycin (like other aminoglycosidic antibiotics) is a common component of standard cell-culture media, so caution is needed when interpreting studies on stretch effects in cultured cells, as background SAC NS availability may be reduced in these preparations. Thankfully, streptomycin seems to wash off reasonably well in vitro (161,534). ...
The heart is vital for biological function in almost all chordates, including human. It beats continually throughout our life, supplying the body with oxygen and nutrients while removing waste products. If it stops - so does life. The heartbeat involves precise coordination of the activity of billions of individual cells, as well as their swift and well-coordinated adaption to changes in physiological demand. Much of the vital control of cardiac function occurs at the level of individual cardiac muscle cells, including acute beat-by-beat feedback from the local mechanical environment to electrical activity (as opposed to longer-term changes in gene expression, and functional or structural remodelling). This process is known as Mechano-Electric Coupling (MEC). In the current review, we: present evidence for, and implications of, MEC in health and disease in human; summarise our understanding of MEC effects gained from whole animal, organ, tissue, and cell studies; identify potential molecular mediators of MEC responses; and demonstrate the power of computational modelling in developing a more comprehensive understanding of 'what makes the heart tick'.
... Streptomycin is commonly used as an SAC blocker. However, while there is some inconsistency in the literature, it is typically reported to be effective in blocking SACs in isolated cells, but not in intact tissue 6,[18][19][20] . Gadolinium (Gd 3+ ) is another commonly used SAC blocker; it has been reported to modulate stretch-induced arrhythmias in intact myocardium, but again, the literature is inconsistent [21][22][23] . ...
... Bode et al. showed that GsMtx-4, a more specific SAC blocker than that used in the present study, effectively inhibited atrial fibrillation following mechanical stretch 24 . Similar antiarrhythmic effects have also been reported after streptomycin and Gd 3+ treatment 18,22,23 . On the other hand, antiarrhythmic effects of these SAC blockers were not observed in other studies [19][20][21]45 . ...
Mechanical stimulation can elicit electrical activation of the heart. This mechanosensitivity can start life-threatening arrhythmias (commotio cordis) or terminate them (precordial thump). Mechanosensitivity may also be involved in arrhythmogenesis in other settings. Stretch-activated ion channels (SACs) are thought to be important in mechanosensitivity and a number of agents that block them have been identified. Such agents could potentially be used as tools in experimental investigation of mechanosensitivity. However, studies using them in intact-heart preparations have yielded inconsistent results. In the present study, we used isolated, perfused hearts from 25-35 kg pigs and a computer-controlled device that repeatably delivered focal mechanical stimuli. The concentration-dependent ability of the SAC blocker gadolinium to suppress mechanical activation was assessed by the success rate of mechanical stimulation and by the delay between successful mechanical stimulation and electrical activation. In six hearts, perfusate was recirculated. In an additional six hearts, perfusate was not recirculated to prevent gadolinium from forming complexes with metabolic waste and possibly precipitating. Gadolinium did not suppress mechanically-induced activation. Although gadolinium has been shown to be an effective SAC blocker in isolated cells, using it to probe the role of mechanical stimulation in whole heart preparations should be done with great caution.
... Acute stretch can also trigger arrhythmias in both ventricles ( Franz et al., 1992) and atria ( Bode et al., 2001) possibly via the activation of mechanosensitive ion channels (MSCs) see articles in ( Kohl and Ravens, 2003;Kohl et al., 2005) and ( White, 2006). There is some evidence to suggest that stretch-activated arrhythmias are more prominent in diseased hearts, e.g. in atrial fibrillation ( Bode et al., 2001), heart failure ( Wang et al., 1994) and compensated hypertensive hypertrophy ( Salmon et al., 1997). This may be due to changes in MSC activity and/or a pro-arrhythmic state in disease, caused by changes such as altered dispersion of repolarisation ( Antzelevitch, 2005). ...
... MSCs are thought to modulate action potential shape and generate stretch-activated arrhythmias both by the generation of ion currents and in the case of MSC NS , the modulation of intracellular Ca 2þ , but there has been relatively few studies of MSCs in diseased tissue ( Kohl and Ravens, 2003;Kohl et al., 2005). The spontaneously hypertensive rat is more susceptible to stretchactivated arrhythmias ( Evans et al., 1995;Kim et al., 2012) via mechanisms associated with MSC NS ( Salmon et al., 1997) and these animals are reported to have a greater density of such currents ( Kamkin et al., 2000). We are interested in the electrical and mechanical remodelling that occurs in heart failure. ...
We demonstrate the synergistic benefits of using multiple technologies to investigate complex multi-scale biological responses. The combination of reductionist and integrative methodologies can reveal novel insights into mechanisms of action by tracking changes of in vivo phenomena to alterations in protein activity (or vice versa). We have applied this approach to electrical and mechanical remodelling in right ventricular failure caused by monocrotaline-induced pulmonary artery hypertension in rats.
We show arrhythmogenic T-wave alternans in the ECG of conscious heart failure animals. Optical mapping of isolated hearts revealed discordant action potential duration (APD) alternans. Potential causes of the arrhythmic substrate; structural remodelling and/or steep APD restitution and dispersion were observed, with specific remodelling of the Right Ventricular Outflow Tract. At the myocyte level, [Ca2+]i transient alternans were observed together with decreased activity, gene and protein expression of the sarcoplasmic reticulum Ca2+-ATPase (SERCA). Computer simulations of the electrical and structural remodelling suggest both contribute to a less stable substrate.
Echocardiography was used to estimate increased wall stress in failure, in vivo. Stretch of intact and skinned single myocytes revealed no effect on the Frank-Starling mechanism in failing myocytes. In isolated hearts acute stretch-induced arrhythmias occurred in all preparations. Significant shortening of the early APD was seen in control but not failing hearts. These observations may be linked to changes in the gene expression of candidate mechanosensitive ion channels (MSCs) TREK-1 and TRPC1/6. Computer simulations incorporating MSCs and changes in ion channels with failure, based on altered gene expression, largely reproduced experimental observations.
... While it is an efficient blocker of SAC NS in isolated or cultured cardiac cells (40), it has been shown to have a limited efficacy in native myocardium (119). This limitation results in the need for higher concentrations to affect stretch-induced responses (Ͼ200 M) (161,534) and in a disparity of positive and negative results, some of which may have been caused by lack of effect or off-target actions (492). It is also important to note that streptomycin (like other aminoglycosidic antibiotics) is a common component of standard cell-culture media, so caution is needed when interpreting studies on stretch effects in cultured cells, as background SAC NS availability may be reduced in these preparations. ...
... It is also important to note that streptomycin (like other aminoglycosidic antibiotics) is a common component of standard cell-culture media, so caution is needed when interpreting studies on stretch effects in cultured cells, as background SAC NS availability may be reduced in these preparations. Thankfully, streptomycin seems to wash off reasonably well in vitro (161,534). ...
Cardiac electrical and mechanical activity are closely interrelated not only via the chain of events, commonly referred to
as “excitation–contraction coupling” (ECC), that links electrical excitation to contraction, but equally via feedback from
the heart’s mechanical environment to the origin and spread of cardiac excitation. The latter has been termed mechano-electric
feedback (MEF) and complements ECC to form an intracardiac electro-mechanical regulatory loop. This chapter will review MEF
effects on heart rate (HR, number of beats per unit time, usually 1 min) and rhythm (regularity of cardiac contractions),
distinguishing between pro- and anti-arrhythmic effects, and elucidate the extent to which stretch-activated ion channels
(SAC) may explain observed responses. A concluding section addresses current shortfalls in insight and presents theories regarding
the physiological relevance of MEF in the heart.
... In our hands, streptomycin had no significant effect (P > 0.05) on the force of contraction or on [Ca 2+ ] i transient amplitude prior to stretch (data not shown). We used a higher concentration of streptomycin in muscle (80 µm), as there is evidence that intact muscle is less sensitive to streptomycin than myocytes (see Salmon et al. 1997). In papillary muscle, 80 µm (but not 40 µm) streptomycin significantly reduced the magnitude of the slow response to stretch (34 ± 5%; n = 6, P < 0.05) compared with that recorded under control conditions (44 ± 4%; Fig. 6A). ...
... We also saw no significant effect of 40 µm streptomycin on force or [Ca 2+ ] i in unstretched cells. Neither did 80 µm streptomycin modulate contractility in the absence of stretch in muscle; Salmon et al. (1997) have reported similar findings with 200 µm streptomycin in the intact heart. ...
We present the first direct comparison of the major candidates proposed to underlie the slow phase of the force increase seen following myocardial stretch: (i) the Na(+)-H(+) exchanger (NHE) (ii) nitric oxide (NO) and the ryanodine receptor (RyR) and (iii) the stretch-activated channel (SAC) in both single myocytes and multicellular muscle preparations from the rat heart. Ventricular myocytes were stretched by approximately 7% using carbon fibres. Papillary muscles were stretched from 88 to 98% of the length at which maximum tension is generated (L(max)). Inhibition of NHE with HOE 642 (5 microm) significantly reduced (P < 0.05) the magnitude of the slow force response in both muscle and myocytes. Neither inhibition of phosphatidylinositol-3-OH kinase (PtdIns-3-OH kinase) with LY294002 (10 microm) nor NO synthase with L-NAME (1 mm) reduced the slow force response in muscle or myocytes (P > 0.05), and the slow response was still present in the single myocyte when the sarcoplasmic reticulum was rigorously inhibited with 1 microm ryanodine and 1 microm thapsigargin. We saw a significant reduction (P < 0.05) in the slow force response in the presence of the SAC blocker streptomycin in both muscle (80 microm) and myocytes (40 microm). In fura 2-loaded myocytes, HOE 642 and streptomycin, but not L-NAME, ablated the stretch-induced increase in [Ca(2+)](i) transient amplitude. Our data suggest that in the rat, under our experimental conditions, there are two mechanisms that underlie the slow inotropic response to stretch: activation of NHE; and of activation of SACs. Both these mechanisms are intrinsic to the myocyte.
... We found that the anti-arrhythmic effects of VRP in this study AFRICA are similar to the preliminary data from different studies using both streptomycin and gadolinium. 25,26 According to the KEGG analysis of the DEGs, the enriched pathways included the pathways of adherens junctions and those involved in regulation of actin cytoskeleton and MAPK signalling. The actin cytoskeleton pathways play an important role in the morphology of cardiomyocytes and in the maintenance of membrane subdomains such as intercalated discs, caveolae and T-tubules. ...
Background:
Life-threatening ventricular arrhythmias can lead to sudden cardiac death in patients. This study aimed to investigate the changes in gene profiles involved when verapamil (VRP) affects increased wall stress (pressure overload)-induced ventricular arrhythmias, thus revealing the potential causative molecular mechanisms and therapeutic targets through gene-expression identification and functional analysis.
Methods:
Animal models with wall stress-induced ventricular arrhythmias were established. Low (0.5 mg/kg) and high (1 mg/kg) doses of VRP were administered intravenously 10 minutes before transverse aortic constriction, and average ventricular arrhythmia scores were calculated. Next, we evaluated the molecular role of VRP by characterising differential gene-expression profiles between VRP-pretreated (1 mg/kg) and control groups using RNA-sequencing technology. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were used to reveal molecular function. A protein-protein interaction (PPI) network was then developed.
Results:
VRP exerted its anti-arrhythmic effects in response to increases in left ventricular (LV) afterload. We detected differentially expressed genes (DEGs), of which 36 were upregulated and 1 397 downregulated, between the VRP-pretreated and model groups during acute increases in LV wall stress. GO analysis demonstrated that the DEGs were associated with cytoskeletal protein binding. KEGG analysis showed that enriched pathways were mainly distributed in adherens junctions, actin cytoskeleton regulation and the MAPK signalling pathway. Centralities analysis of the PPI identified Rac1, Grb2, Rbm8a and Mapk1 as hub genes.
Conclusions:
VRP prevented acute pressure overload-induced ventricular arrhythmias, possibly through the hub genes Rac1, Grb2, Rbm8a and Mapk1 as potential targets of VRP.
... Ranolazine is an antiarrhythmic drug inhibiting late sodium current with high selectivity over the transient phase [20,51,52,154,221]. Considering that myocardial wall stretch has been linked to clinical arrhythmias for a long time [222][223][224], these data may help to develop new therapeutic strategies in the antiarrhythmic pharmacology. Currently, myocardial wall stress is reduced in clinical practice by reducing central venous pressure with diuretics. ...
Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.
... Further studies to dissect the role of the I(f) channel in the process are warranted. Cardiac fibrosis damages the mechanical-electrical coupling system, evokes trigger mechanisms, and results in arrhythmia and sudden death [51][52][53]. Our results revealed that the excitatory conduction of the ventricular tissue was abnormally disordered, and the normal wave-shaped conduction mode was disrupted, leading to a significant increase in the induction rate of ventricular tachycardia (VT) in HFrEF mice. ...
Heart failure (HF) is a clinical syndrome caused by impairment of ventricular filling, ejection of blood, or both and is categorized as HF with reduced ejection fraction (HFrEF) or HF with preserved ejection fraction (HFpEF) based on left ventricular function. Cardiac fibrosis contributes to left ventricular dysfunction and leads to the development of HF. Ivabradine, an If current selective specific inhibitor, has been shown to improve the prognosis of patients with HF. However, the effects of ivabradine on cardiac function and fibrosis in HFpEF and HFrEF and the underlying mechanism remain unclear. In the present study, we utilized mouse models to mimic HFpEF and HFrEF and evaluated the therapeutic effects of ivabradine. By treating mice with different doses (10 mg/kg/d and 20 mg/kg/d) of ivabradine for 4 or 8 weeks, we found that a high dose of ivabradine improved cardiac diastolic function in HFpEF mice and ameliorated cardiac diastolic and systolic function and ventricular tachycardia incidence in HFrEF mice. Moreover, ivabradine significantly reduced the activation of cardiac fibroblasts and myocardial fibrosis in mice. Mechanistically, microRNA-133a, which was upregulated by ivabradine, targeted connective tissue growth factor and collagen 1 in cardiac fibroblasts and might contribute to the protective role of ivabradine. Together, our work utilized mouse models to study HFpEF and HFrEF, demonstrated the protective role of ivabradine in HFpEF and HFrEF, and elucidated the potential underlying mechanism, which provides an effective strategy for related diseases.
... According to Laplace's law, a rise in arterial blood pressure leads to a sudden increased afterload of the left ventricle (LV), which sequentially results in elevated LV systolic pressure (LVSP) and wall stress (Evans et al., 1995). Enhanced LV wall stress induces PVC, VT and VF within 10 s in isolated heart models using Langendorff technique (James & Jones, 1990;Salmon et al., 1997). As proved by previous studies, aortic occlusion increases LVSP and shortens action potential duration among in situ porcine (Horner et al., 1994) and human (Taggart et al., 1992) hearts. ...
Background
Patients with dilated cardiomyopathy, increased ventricular volume, pressure overload or dysynergistic ventricular contraction and relaxation are susceptible to develop serious ventricular arrhythmias (VA). These phenomena are primarily based on a theory of mechanoelectric feedback, which reflects mechanical changes that produce alterations in electrical activity. However, very few systematic studies have provided evidence of the preventive effects of artemisinin (ART) on VA in response to left ventricle (LV) afterload increases. MicroRNAs (miRNAs) are endogenous small non-coding RNAs that regulate expression of multiple genes by suppressing mRNAs post-transcriptionally.
Aims
The aims of this study were to investigate preventive effects of ART on mechanical VA and the underling molecular mechanisms of differentially expressed miRNAs (DEMs).
Methods
For the study, 70 male Wistar rats were randomly divided into seven groups: group 1 was a control group (sham surgery); group 2 was a model group that underwent transverse aortic constriction (TAC) surgery; groups 3, 4, 5 and 6 were administered ART 75, 150, 300 and 600 mg/kg before TAC surgery, respectively; and group 7 was administered verapamil (VER) 1 mg/kg before TAC surgery. A ventricular arrhythmia score (VAS) was calculated to evaluate preventive effects of ART and VER on mechanical VA. The high throughput sequencing-based approach provided DEMs that were altered by ART pretreatment between group 2 and group 4. All predicted mRNAs of DEMs were enriched by gene ontology (GO) and Kyoto Encyclopedia annotation of Genes and Genomes (KEGG) databases. These DEMs were validated by a real time quantitative polymerase chain reaction (RT-qPCR).
Results
The average VASs of groups 3, 4, 5, 6 and 7 were significantly reduced compared with those of group 2 (2.70 ± 0.48, 1.70 ± 0.95, 2.80 ± 0.79, 2.60 ± 0.97, 1.40 ± 0.52, vs 3.70 ± 0.67, p < 0.01, respectively). The three top GO terms were neuron projection, organ morphogenesis and protein domain specific binding. KEGG enrichment of the 16 DEMs revealed that MAPK, Wnt and Hippo signaling pathways were likely to play a substantial role in the preventive effects of ART on mechanical VA in response to LV afterload increases. All candidate DEMs with the exception of rno-miR-370-3p, rno-miR-6319, rno-miR-21-3p and rno-miR-204-5p showed high expression levels validated by RT-qPCR.
Conclusions
Artemisinin could prevent mechanical VA in response to LV afterload increases. Validated DEMs could be biomarkers and therapeutic targets of ART regarding its prevention of VA induced by pressure overload. The KEGG pathway and GO annotation analyses of the target mRNAs could indicate the potential functions of candidate DEMs. These results will help to elucidate the functional and regulatory roles of candidate DEMs associated with antiarrhythmic effects of ART.
... Streptomycin (SM), a blocker of SACs, has been known to inhibit the activities of SACs in single cell, tissue, and intact preparations in various species (Lab, 2006). It can decrease the occurrence of arrhythmias induced by changes in ventricular wall-stress in isolated rat hearts (Salmon et al., 1997). Eckardt et al. (2001) studied Langendorff-perfused rabbit hearts and found that SM (200 μmol/L) could inhibit the shortened repolarization induced by stretching. ...
Objective:
To investigate stretch-induced electrophysiological changes in chronically infarcted hearts and the effect of streptomycin (SM) on these changes in vivo.
Methods:
Sixty Wistar rats were divided randomly into four groups: a control group (n=15), an SM group (n=15), a myocardial infarction (MI) group (n=15), and an MI+SM group (n=15). Chronic MI was obtained by ligating the left anterior descending branch (LAD) of rat hearts for eight weeks. The in vivo blockade of stretch-activated ion channels (SACs) was achieved by intramuscular injection of SM (180 mg/(kg∙d)) for seven days after operation. The hearts were stretched for 5 s by occlusion of the aortic arch. Suction electrodes were placed on the anterior wall of left ventricle to record the monophasic action potential (MAP). The effect of stretching was examined by assessing the 90% monophasic action potential duration (MAPD90), premature ventricular beats (PVBs), and ventricular tachycardia (VT).
Results:
The MAPD90 decreased during stretching in both the control (from (50.27±5.61) ms to (46.27±4.51) ms, P<0.05) and MI groups (from (65.47±6.38) ms to (57.47±5.76 ms), P<0.01). SM inhibited the decrease in MAPD90 during inflation ((46.27±4.51) ms vs. (49.53±3.52) ms, P<0.05 in normal hearts; (57.47±5.76) ms vs. (61.87±5.33) ms, P<0.05 in MI hearts). The occurrence of PVBs and VT in the MI group increased compared with that in the control group (PVB: 7.93±1.66 vs. 1.80±0.86, P<0.01; VT: 7 vs. 1, P<0.05). SM decreased the occurrence of PVBs in both normal and MI hearts (0.93±0.59 vs. 1.80±0.86 in normal hearts, P<0.05; 5.40±1.18 vs. 7.93±1.66 in MI hearts, P<0.01).
Conclusions:
Stretch-induced MAPD90 changes and arrhythmias were observed in chronically infarcted myocardium. The use of SM in vivo decreased the incidence of PVBs but not of VT. This suggests that SACs may be involved in mechanoelectric feedback (MEF), but that there might be other mechanisms involved in causing VT in chronic MI.
... MEC is now commonly observed when a cardiac catheter touches the endocardium and generates extra systoles. Mechanical stimulation induces arrhythmias in many, if not all, species including humans (Taggart et al., 1992;Taggart & Lab, 2008), pigs (Madias et al., 2008), dogs (Hansen et al., 1990), rabbits (Zabel et al., 1996;Bode et al., 2006), lambs (Chen et al., 2004), and rats (Salmon et al., 1997). MEC has been studied in a variety of preparations including the whole organ (in vivo or in vitro) (Franz et al., 1992;Link, 2003), bundles of muscle (papillary muscle (Nicolosi et al., 2004), trabecula (Wilhelm et al., 2006), etc.), isolated cells (Zeng et al., 2000;Niu & Sachs, 2003;Nishimura et al., 2006;Liu et al., 2008) and cultured cell sheets (Kong et al., 2005). ...
Transient stretching of the ventricle can trigger arrhythmias and evoke ventricular fibrillation, especially when the stimulation occurs in the vulnerable period. To explore the sensitivity of small hearts we used a commercial pressure servo to study the kinetic relationship of left ventricular pressure to excitability and arrhythmias in the rat heart. Stimulation protocols were readily composed on the computer and programmed to vary the stimulus amplitude and timing relative to pacing. The pressure-induced premature ventricular excitations were similar to those observed in larger hearts, but the convenience of using small hearts allows the use of inexpensive transgenic animals to explore the molecular basis of transduction.
... 90,91 TrpC1 is a st ret ch-act ivat ed channel in heart , 92 and st ret ch is associat ed wit h arrhyt hmia. 93,94 Furt hermore, TrpC channels, by causing localized Ca 2þ increases in t he subsarcolemmal region, might int e rfere wit h t he funct ioning of t he volt age-regulat ed Ca 2þ channels or NCX1, as proposed earlier for Ins(1, 4, 5)P 3 -induced Ca 2þ release. TrpC3 prot ein is expressed in t he surface membrane of single pacemaker cells from mouse heart and a st ore-operat e d Ca 2þ influx in pacemaker t issue has been demonst rat ed. ...
Arrhythmias arise from a complex interaction between structural changes in the myocardium and changes in cellular electrophysiology. Electrophysiological balance requires precise control of sarcolemmal ion channels and exchangers, many of which are regulated by phospholipid, phosphatidylinositol(4,5)bisphosphate. Phosphatidylinositol(4,5)bisphosphate is the immediate precursor of inositol(1,4,5)trisphosphate, a regulator of intracellular Ca2+ signalling and, therefore, a potential contributor to arrhythmogenesis by altering Ca2+ homeostasis. The aim of the present review is to outline current evidence that this signalling pathway can be a player in the initiation or maintenance of arrhythmias.
... 1. Gd 3+ (Yang and Sachs, 1989;Hansen et al., 1991;Suleymanian et al., 1995;Tavi et al., 1996), 2. streptomycin (Kent et al., 1989;Gannier et al., 1994;Salmon et al., 1997) and 3. venom from a spider generally called Grammastola spatulata (Hu and Sachs, 1996) with the systematic name Phrixotrichus spatulatus (Perez-Miles et al., 1996). ...
... Occurrence of ventricular premature beats due to increased left ventricular pressure was reduced by streptomycin, another compound known to block SACs. 29 Afterdepolarizations produced by constant stretch in atrial tissue were suppressed by Gd 3ϩ . 14 SAC blockade by Gd 3ϩ was also reported to inhibit the release of atrial natriuretic peptide, 17 a substance that might contribute to action potential shortening during atrial stretch. ...
Atrial fibrillation (AF) is frequently associated with atrial dilatation caused by pressure or volume overload. Stretch-activated channels (SACs) have been found in myocardial cells and may promote AF in dilated atria. To prove this hypothesis, we investigated the effect of the SAC blocker gadolinium (Gd(3+)) on AF propensity in the isolated rabbit heart during atrial stretch.
In 16 isolated Langendorff-perfused rabbit hearts, the interatrial septum was perforated to equalize biatrial pressures. Caval and pulmonary veins were occluded. Intra-atrial pressure (IAP) was increased in steps of 2 to 3 cm H(2)O by increasing the pulmonary outflow fluid column. Vulnerability to AF was evaluated by 15-second burst pacing at each IAP level. At baseline, IAP needed to be raised to 8.8+/-0.2 cm H(2)O (mean+/-SEM) to induce AF. A dose-dependent decrease in AF vulnerability was observed after Gd(3+) 12.5, 25, and 50 micromol/L was added. AF threshold increased to 19.0+/-0.5 cm H(2)O with Gd(3+) 50 micromol/L (P<0.001 versus baseline). Spontaneous runs of AF occurred in 5 hearts on a rise of IAP to 13.8+/-3.3 cm H(2)O at baseline but never during Gd(3+). Atrial effective refractory period shortened progressively from 78+/-3 ms at 0.5 cm H(2)O to 52+/-3 ms at 20 cm H(2)O (P<0.05). Gd(3+) 50 micromol/L had no significant effect on effective refractory period.
Acute atrial stretch significantly enhances the vulnerability to AF. Gd(3+) reduces the stretch-induced vulnerability to AF in a dose-dependent manner. Block of SAC might represent a novel antiarrhythmic approach to AF under conditions of elevated atrial pressure or volume.
... In multicellular cardiac preparations it has been reported to block stretch-activated changes in electrical activity (e.g. Salmon et al. 1997;Eckardt et al. 2000;Babuty & Lab, 2001), and stretch-induced increases in [Ca 2+ ] i in single myocytes (Gannier et al. 1994). In addition streptomycin can block [Ca 2+ ] i transients and contraction in unstretched preparations (e.g. ...
We tested the hypothesis that both stretch-activated channels (SACs) and intracellular calcium ([Ca(2+)](i)) are important in the electrical response of single guinea-pig ventricular myocytes to axial stretch. Myocytes were attached to carbon fibre transducers and stretched, sarcomere length increased by approximately 9 %, and there was a prolongation of the action potential duration. Streptomycin, a blocker of SACs, had no effect upon the shortening, [Ca(2+)](i) transients or action potentials of electrically stimulated, unstretched myocytes, at a concentration of 50 microM, but at 40 microM, prevented any stretch-induced increase in action potential duration. Under action potential clamp, stretch elicited a current with a linear current-voltage relationship that was inward at membrane potentials negative to its reversal potential of -30 mV, in 10 of 24 cells tested, and was consistent with the activation of non-specific, cationic SACs. This current was not seen in any stretched cells that were exposed to 40 microM streptomycin. However, exposure of cells to 5 microM BAPTA-AM, in order to reduce [Ca(2+)](i) transients, also abolished stretch-induced prolongation of the action potential. We conclude that both SACs and [Ca(2+)](i) are important in the electrical response of cardiac myocytes to stretch, and propose that stretch-induced changes in electrical activity and [Ca(2+)](i) may be linked by inter-dependent mechanisms.
... Amiloride and its derivatives block mechano-sensitive channels in molluscan neurones (Small and Morris, 1995), Xenopus oocytes (Hamill et al., 1992), coronary smooth muscle cells (Wu and Davis, 2001), whereas in cardiomyocytes these drugs are used to block transporters as the Na + , Ca 2+ exchanger or Na + , H + exchanger (Kaczorowski et al., 1989). The cationic aminoglycoside antibiotics also block stretch-activated channels (Salmon et al., 1997), but l-type Ca 2+ channels are blocked as well (Haws et al., 1996). Streptomycin (IC 50 o40 mM) can suppress stretch-induced arrhythmias in guinea pig hearts (Gannier et al., 1994). ...
The mechanical state of the heart feeds back to modify cardiac rate and rhythm. Mechanical stretch of myocardial tissue causes immediate and chronic responses that lead to the common end point of arrhythmia. This review provides a brief summary of the author's personal choice of contributions that she considers have fostered our understanding of the role of mechano-electric feedback in arrhythmogenesis. Acute mechanical stretch reversibly depolarises the cell membrane and shortens the action potential duration. These electrophysiological changes are related to the activation of mechano-sensitive ion channels. Several different ion channels are involved in the sensing of stretch, among them K(+)-selective, Cl(-)-selective, non-selective, and ATP-sensitive K(+) channels. Sodium and Ca(2+) entering the cells via non-selective ion channels are thought to contribute to the genesis of stretch-induced arrhythmia. Mechano-sensitive channels have been cloned from non-vertebrate and vertebrate species. Chronic stress on the heart activates gene expression in cardiomyocytes and non-myocytes. The signal transduction involves atrial natriuretic peptides and growth factors that initiate remodelling processes leading to hypertrophy which in turn may contribute to the electrical instability of the heart by increasing the responsiveness of mechano-sensitive channels. Selective block of these channels could provide some new form of treatment of mechanically induced arrhythmias, although at present there are no drugs available with sufficient selectivity. Detailed understanding of how mechanical strain on myocardial cells is translated into channel activation will allow to identify new targets for putative antiarrhythmic drugs.
... 5,15 The aminoglycoside antibiotic streptomycin has been reported to block SACs and to reduce mechanically induced arrhythmias. 17,18 The diastolic mechanics of the intact ventricle are of particular interest in relation to mechanoelectric feedback because studies have indicated that diastolic stretch may govern some of the electrophysiologic changes. 5,19 Regional twodimensional and three-dimensional strains have been measured during passive loading in a variety of species, and results from these investigations have indicated that passive strains in the LV are multiaxial and nonhomogeneous. ...
Mechanical stimulation can induce electrophysiologic changes in cardiac myocytes, but how mechanoelectric feedback in the intact heart affects action potential propagation remains unclear.
Changes in action potential propagation and repolarization with increased left ventricular end-diastolic pressure from 0 to 30 mmHg were investigated using optical mapping in isolated perfused rabbit hearts. With respect to 0 mmHg, epicardial strain at 30 mmHg in the anterior left ventricle averaged 0.040 +/- 0.004 in the muscle fiber direction and 0.032 +/- 0.006 in the cross-fiber direction. An increase in ventricular loading increased average epicardial activation time by 25%+/- 3% (P < 0.0001) and correspondingly decreased average apparent surface conduction velocity by 16%+/- 7% (P = 0.007). Ventricular loading did not significantly alter action potential duration at 20% repolarization (APD20) but did at 80% repolarization (APD80), from 179 +/- 7 msec to 207 +/- 5 msec (P < 0.0001). The dispersion of APD20 was decreased with loading from 19 +/- 2 msec to 13 +/- 2 msec (P = 0.024), whereas the dispersion of APD80 was not significantly changed. These electrophysiologic changes with ventricular loading were not affected by the nonspecific stretch-activated channel blocker streptomycin (200 microM) and were not attributable to changes in myocardial perfusion or the presence of an electromechanical decoupling agent (butanedione monoxime) during optical mapping.
Acute loading of the left ventricle of the isolated rabbit heart decreased apparent epicardial conduction velocity and increased action potential duration by a load-dependent mechanism that may not involve stretch-activated channels.
... In microorganisms, including prokaryotes, MSCs serve as sensory transducers for various processes including volume regulation (Martinac, 2001; Sachs, 2002) A traditional approach for ion channel function is the use of specific blockers. For mechanosensitive channels there have only been non-specific agents such as gadolinium (Caldwell et al., 1998; Yang and Sachs, 1989) and streptomycin (Gannier et al., 1994; Salmon et al., 1997). Recently, we isolated a peptide from the venom of Grammostola spatulata that specifically inhibits the mechanosensitive channels of astrocytes and kidney cells, opening the way for new biochemical and electrophysiological studies of MSCs (Suchyna et al., 2000). ...
The peptide GsMTx4 from the tarantula venom (Grammostola spatulata) inhibits mechanosensitive ion channels. In this work, we report the cDNA sequence encoding GsMTx4. The gene is translated as a precursor protein of 80 amino acids. The first 21 amino acids are a predicted signal sequence and the C-terminal residues are a signal for amidation. An arginine residue adjacent to the N-terminal glycine of GsMTx4 is the cleavage site for release. The resulting peptide is 34 amino acids in length with a C-terminal phenylalanine and not a serine-alanine previously identified [J. Gen. Physiol. 115 (2000) 583]. We chemically synthesized this peptide and folded it in 0.1 M Tris, pH 7.9 with oxidized/reduced glutathione (1/10). Properties of the synthetic peptide were identical to the wild type for high performance liquid chromatography (HPLC), mass spectrometry, CD, and NMR. We also cloned GsMTx4 in a thioredoxin fusion protein system containing six histidines. Nickel affinity columns allowed rapid purification and folding occurred in conditions described above with 0.5 M guanidiniumHCl present. Thrombin cleavage liberated GsMTx4 with three extra amino acids at the N-terminus. The retention time in HPLC analysis and the CD spectrum was similar to wild type. Both the synthetic and cloned peptides were active in the patch clamp assay.
... Salmon et al. found that 200 µM (but not 50 µM) streptomycin causes a significant reduction in wall-stress induced arrhythmias in working rat heart. 21 In contrast, Sung et al. saw no effect of 200 µM streptomycin on acute load-induced changes in conduction velocity and AP duration in rabbit whole heart preparations. 22 In our superfused SAN tissue preparations, high concentrations of streptomycin (100-500 µM) caused a reduction in control BR (and contractility), suggesting an inhibitory effect on the L-type Ca 2+ channels. ...
Acute dilation of the right atrium (e.g., via increased venous return) raises spontaneous beating rate (BR) of the heart in many species. Neural mechanisms contribute to this behavior in vivo, but a positive chronotropic response to stretch can also be observed in isolated right atrial tissue preparations and even at the level of single sino-atrial node (SAN) cells. The underlying mechanism has previously been reported to be compatible with stretch-activation of cation nonselective ion channels (SAC). This review reports species peculiarities in the chronotropic response of isolated SAN tissue strips to stretch: in contrast to guinea pig, murine SAN preparations respond to distension with a reduction in spontaneous BR. This differential response need not necessarily involve disparate (sub-)cellular mechanisms, as SAC activation would occur against the background of very different SAN electrophysiology in the two species. On the basis of single SAN cell action potential recordings, this review illustrates how this may give rise to potentially opposing effects on spontaneous BR. Interestingly, streptomycin (a useful SAC blocker in isolated cells) has no effect on stretch-induced chronotropy in situ, and this is interpreted as an indication of protection of SAC, in native tissue, from interaction with the drug.
... 10 Using the blockers of SACs, streptomycin (SM), many studies have demonstrated that SACs were involved in the changes of stretch-induced myocardial electrophysiological properties in normal cardiac tissues. 11,12 On the basis of the data from clinical observation, there is a high incidence of cardiac arrhythmias and sudden death in patients with diseased myocardium, especially left ventricular hypertrophy, 13 heart failure, 14 myocardial ischaemia, 15 and hearts transplanted by autologous skeletal muscle cell. 16 Furthermore, above all that had been mentioned existed the ventricular asynchrony that produced the stretch of myocardium and subsequent MEF during cardiac cycle. ...
Aims To explore whether the stretch of ischaemic myocardium could modulate the electrophysiological characteristics, especially repolarization via mechanoelectric feedback (MEF), as well as the effect of streptomycin (SM) on these changes.
Methods Thirty-six wistar rats were randomly divided into four groups: control group ( n = 9), SM group ( n = 9), myocardial infarction (MI) group ( n = 9), and MI + SM group ( n = 9). After perfused on Langendorff, the isolated hearts were stretched for 5s by a ballon inflation of 0.2mL. After being stretched, the effect of the stretch was observed for 30s, including the 20, 20–70, 70, and 90% monophasic action potential duration (MAPD), i.e. MAPD 20 , MAPD 20–70 , MAPD 70 , and MAPD 90 , respectively, premature ventricular beats (PVB), and ventricular tachycardia (VT).
Results The stretch caused a decrease in MAPD 20–70 (both P < 0.01) and an increase in MAPD 90 (both P < 0.01) in both control and MI groups. Moreover, the MAPD 90 in the MI group had increased more significantly than that in the control group ( P < 0.05). A concentration of 200 µmol/L of SM had no influence on both MAPD 20–70 and MAPD 90 of basic state ( P > 0.05, except MAPD 20–70 between the control and SM groups, P < 0.01), whereas it had reduced the length of MAPD 90 ( P < 0.05) and inhibited the decrease in MAPD 20–70 induced by the inflation. There was a decrease in the tendency of MAPD 70 after the stretch ( P = NS) and SM had reversed the tendency, whereas MAPD 20 had no obvious changes after inflation. The incidence rate of PVB and VT in the MI group was higher than that in the control group after inflation ( P < 0.01). The 200 µmol/L SM reduced the incidence rate of PVB, and obviously inhibited the occurrence of VT ( P < 0.01).
Conclusions Stretch could alter the electrophysiological activities of myocardium via MEF, which could enhance in acute myocardial infarction and facilitate the generation and maintenance of malignant arrhythmias. SM could significantly inhibit the occurrence of arrhythmias, which may correlate with the effect on blocking stretch-activated ion channels.
Background:
Beat-to-beat variability in ventricular repolarization (BVR) associates with increased arrhythmic risk. Proarrhythmic remodeling in the dog with chronic AV-block (CAVB) compromises repolarization reserve and associates with increased BVR, which further increases upon dofetilide infusion and correlates with Torsade de Pointes (TdP) arrhythmias. It was hypothesized that these pro-arrhythmia-associated increases in BVR are induced by beat-to-beat variability in preload.Methods and Results:Left ventricular monophasic action potential duration (LVMAPD) was recorded in acute (AAVB) and CAVB dogs, before and after dofetilide infusion. BVR was quantified as short-term variability of LVMAPD. The PQ-interval was controlled by pacing: either a constant or an alternating preload pattern was established, verified by PV-loop. The effect of the stretch-activated channel blocker, streptomycin, on BVR was evaluated in a second CAVB group. At alternating preload only, BVR was increased after proarrhythmic remodeling (0.45±0.14 ms AAVB vs. 2.2±1.1 ms CAVB, P<0.01). At CAVB, but not at AAVB, dofetilide induced significant proarrhythmia. Preload variability augmented the dofetilide-induced BVR increase at CAVB (+1.5±0.8 ms vs. +0.9±0.9 ms, P=0.058). In the second group, the increase in baseline BVR by alternating preload (0.3±0.03 ms to 1.0±0.8 ms, P<0.01) was abolished by streptomycin (0.5±0.2 ms, P<0.05).
Conclusions:
In CAVB dogs, the inverse relation between BVR and repolarization reserve originates from an augmented sensitivity of ventricular repolarization to beat-to-beat preload changes. Stretch-activated channels appear to be involved in the mechanism of BVR.
This chapter explores natural pest control agents of practical or historical importance. Natural products have been used to control animal pests, plant diseases, and weeds since ancient times. The biological activity spectrum of natural pesticides is extremely variable, and the selectivity is often achieved by targeted application, often relying on the behavior of the target pest. However, the biological property of a chemical is a function of its structure rather than its origin. The biological activity exerted by a given dose of the compound under given circumstances, especially as related to safety, depends on the way in which the chemical is used. Natural substances used for pest and disease control are generally nonpersistent under field conditions. Most of these often-complex molecules are readily transformed abiotically by light and/or oxygen into less toxic products. There are many natural insect control agents, most of which were discovered by empirical screening of plants and, recently, other natural sources. Moreover, research on insect physiology and behavior made possible the commercialization of chemicals that can be used to manipulate insect development or behavior.
The cardiac late sodium current (INa,L) has been in the focus of research in the recent decade. The first reports on the sustained component of voltage activated sodium current date back to the seventies, but early studies interpreted this tiny current as a product of a few channels that fail to inactivate, having neither physiologic nor pathologic implications. Recently, the cardiac INa,L has emerged as a potentially major arrhythmogenic mechanism in various heart diseases, attracting the attention of clinicians and researchers. Research activity on INa,L has exponentially increased since Ranolazine, an FDA-approved antianginal drug was shown to successfully suppress cardiac arrhythmias by inhibiting INa,L. This review aims to summarize and discuss a series of papers focusing on the cardiac late sodium current and its regulation under physiological and pathological conditions. We will discuss critical evidences implicating INa,L as a potential target for treating myocardial dysfunction and cardiac arrhythmias.
We have compared the effects of streptomycin sulphate, gentamicin sulphate and neomycin sulphate on cell shortening (our index of contractility) and intracellular Ca2+ ([Ca2+]i) transients of rat ventricular myocytes. All three agents abolished shortening and [Ca2+]i, transients but streptomycin was significantly less potent than the other agents. The IC50 of streptomycin was 0.37 mM for shortening and 0.78 mM for [Ca2+]i, approximately an order of magnitude greater than equivalent values for gentamicin and neomycin. Gentamicin and streptomycin shortened the action potential duration of most cells but prolonged the action potential duration of others. We therefore conclude that multiple ionic mechanisms affecting action potential duration are modulated by these antibiotics. Our observations are consistent with the negative inotropic effect of antibiotics being caused by a decrease in Ca2+ influx causing a reduction in the [Ca2+]i transient.
The aim of this study was to determine whether the changes in myocardial activation pattern resulting from acute stretching during ventricular fibrillation can becounteracted by administering a compound that blocks receptors sensitive to stretch. The study involved 16 isolated rabbit hearts, in which refractoriness and activation frequency during ventricular fibrillation were measured before, during and after localized acute stretching of the left ventricular free wall, either without (series A, n=8) or with (series B, n=8) the presence of streptomycin, 200 μmol. At baseline and during and after stretching, ventricular fibrillation was slower with streptomycin perfusion in series B than in series A (dominant frequency at baseline, 13±2 Hz vs. 16±2 Hz, respectively; P<.005; dominant frequency with stretching, 14±2 Hz vs. 19±3 Hz, respectively; P<.005). Streptomycin attenuated the electrophysiological changes produced by stretching and had a direct effect on refractoriness and activation frequency during ventricular fibrillation.
Ventricular Dilatation and d,l‐Sotalol/Flecainide in Isolated Rabbit Heart. Introduction : The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l‐sotalol and flecainide.
Methods and Results: The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid‐filled balloon placed in the left ventricle. Pacing was performed alternately using epi‐ and endocardial monophasic action potential (MAP)‐pacing catheters at cycle lengths from 1,000 to 300 msec. d,l‐Sotalol (10 μM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5%± 3.1%), MAP duration (+14.9%± 3.2%), and QT interval (+15.5%± 4.1%) (mean ± SEM at 1,000 msec). These effects had a reverse rate‐dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of WD, d,l‐sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2μM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate‐dependent increase of pacing thresholds (+47.6%± 8.2%), prolongation of QRS (+48.8%± 5.6%), and conduction time (+78.6%± 8.6%) (mean ± SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate‐dependent prolongation of QRS (+16.7%± 5.5%) and conduction time (+17.1%± 4.3%).
Conclusion: Our data demonstrate that, during AVD, the Class III effect of d,l‐sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide‐related proarrhythmia in certain clinical situations.
Cardiac “mechanotransduction” involves various physiological and biophysical phenomena in which mechanical energy is transduced
to changes in function of cardiac myocytes and of the whole heart. In this chapter different manifestations of mechanotransduction
are reviewed, with special emphasis on the “mechano-electric” feedback aspect. The chapter covers both physiological and pathological
roles of mechanical stimulation of heart tissue.
Only in the past five decades have we begun to understand, on the chemical level, what and how mechanosensitive signaling
molecules are involved in the physiological regulation of downstream events in response to mechanical stimulation in health
and disease. Currently, the forefront of mechanotransduction is focused on mapping an enormous number of signalling and regulatory
signalling pathways that are responsible for sensing and transducing mechanical forces. Mechanosensitive ion channels (MSC)
are one of the major classes of molecules involved in mechanosensitive signal transduction. MSC have been described in a wide
variety of cell types in many different organisms, ranging from bacteria to mammals. MSC participate in several physiological
processes such as touch and pain sensation, salt and fluid balance, blood pressure control, cell volume regulation, and turgor
control. Abnormal regulation of the structure and function of MSC may contribute to the pathogenesis of quite a few diseases
including neuronal degeneration, muscular dystrophy, cardiac arrhythmias, hypertension, kidney disease, and glioma. Accumulating
evidence from our and other groups suggests that MSC may play an important role in the activation of several intracellular
mechanosensitive signaling pathways. This review summarizes the recent developments and the state of the current thinking
regarding the role of MSC in the regulation of different mechanosensitive signaling proteins and signaling pathways
Perturbations in ventricular mechanical loading can be arrhythmogenic and have been associated with sudden cardiac death in
patients suffering from congestive heart failure, dilated cardiomyopathy, or ventricular volume overload (1–3). Stretch-induced
changes in action potential propagation or repolarization could provide a mechanism for mechanically induced arrhythmias.
However, there is a paucity of information regarding the effects of altered load on conduction velocity. The few existing
reports present an unclear picture; some of the discrepancies may be due to the varying techniques used.
Die Dilatation des Vorhofs wirkt proarrhythmisch. Untersucht wurde der Einfluß von Antiarrhythmika und der Inhibitoren dehnungsabhängiger Ionenkanäle Gadolinium und Streptomycin auf die atriale Flimmerschwelle (AFS), Refraktärzeit (ERP) und Aktionspotentialdauer(APD). Nur Chinidin und die Kombination Verapamil/Chinidin bewirkten eine Verlängerung der ERP und APD (p<0,05) am undilatierten Vorhof. AFS,ERP und APD nahmen bei atrialer Dilatation ab, lagen aber im Vergleich zur Kontrolle ohne Substanzen signifikant höher. Gadolinium und Streptomycin verhinderten unter Dilatation jegliche signifikante Änderungen. Eine Verlängerung der ERP schwächt also die proarrhythmische Wirkung der Dilatation ab. Durch Blockade der dehnungsabhängigen Ionenkanäle werden die Dehnungseffekte vollständig aufgehoben.
Cardiac memory refers to long-lasting T-wave changes that follow an episode of altered ventricular activation sequence. Memory-induced alterations in repolarizing ion channel activity have been characterized. However, the mechanism by which changes in activation sequence produce these effects is unknown. We hypothesized that cardiac memory is mediated by the response of stretch-activated receptors (SARs) to a change in mechanical activation sequence.
In anesthetized, closed-chest dogs, coronary sinus leads were used to pace the posterolateral left ventricle (LV) continuously for 1 hour at a rate of 120 bpm. The surface vectorcardiogram was used to quantify cardiac memory by measuring T-wave displacement after pacing. Streptomycin, which has been shown to block SARs, was given at a dose of 4 g intramuscularly 1 hour before experimental LV pacing sessions. T-wave displacement after control sessions of LV pacing in the absence of drug (n = 12) was compared to that produced by pacing after streptomycin administration (n = 10 sessions).
There was a distinct and consistent cardiac memory seen after 1 hour of LV pacing under control conditions, with T-wave displacement of 1.28 +/- 0.43 mV (P < 0.001 vs baseline). Pretreatment with streptomycin had no direct effect on the electrocardiogram or hemodynamics, but decreased pacing-induced T-wave displacement to 0.50 +/- 0.28 mV (P < 0.001 vs control sessions).
Streptomycin, a SAR blocker, dramatically attenuated the development of cardiac memory following epicardial pacing. These data suggest that SARs are a critical link between mechanical sequence of activation and regional modulation of action potential duration that is responsible for cardiac memory.
The interaction between acute ventricular dilatation (AVD) as one aspect of ventricular dysfunction and Class I and III antiarrhythmic drugs is uncertain. We therefore investigated the effects of AVD on the electrophysiologic properties of d,l-sotalol and flecainide.
The isolated rabbit heart was used as a model of AVD. The ventricular size and, therefore, the diastolic pressure were modified by sudden volume changes of a fluid-filled balloon placed in the left ventricle. Pacing was performed alternately using epi- and endocardial monophasic action potential (MAP)-pacing catheters at cycle lengths from 1,000 to 300 msec. d,l-Sotalol (10 microM) resulted in a significant (P < 0.05) lengthening of refractoriness (+13.5% +/- 3.1%), MAP duration (+14.9% +/- 3.2%), and QT interval (+15.5% +/- 4.1%) (mean +/- SEM at 1,000 msec). These effects had a reverse rate-dependence. AVD to a diastolic pressure of 30 mmHg reduced refractoriness and left ventricular MAP duration. In comparison with the control group with the same extent of AVD, d,l-sotalol still led to a significant prolongation of repolarization for all cycle lengths except 300 msec, so that its effects were not absolutely but relatively preserved. In contrast, flecainide (2 microM) had no significant effects on refractoriness or MAP duration. It led to a significant, rate-dependent increase of pacing thresholds (+47.6% +/- 8.2%), prolongation of QRS (+48.8% +/- 5.6%), and conduction time (+78.6% +/- 8.6%) (mean +/- SEM at 300 msec). In the flecainide group, AVD significantly increased the normal rate-dependent prolongation of QRS (+16.7% +/- 5.5%) and conduction time (+17.1% +/- 4.3%).
Our data demonstrate that, during AVD, the Class III effect of d,l-sotalol is preserved, whereas flecainide's effect of slowing conduction is exaggerated. This may contribute to flecainide-related proarrhythmia in certain clinical situations.
There is now substantial evidence that Ins(1,4,5)P3, by virtue of its role in Ca2+ release, can initiate electro-physiological disturbances which develop into cardiac arryhtmias, and in addition appears to be involved in the progression of the cardiomyocytes to apoptosis. Therapeutic regimens targeting the generation or the actions of Ins(1,4,5)P3 may thus prove beneficial in treating or preventing a number of different cardiac pathologies.
1. The present review focuses on the role of the Ca2+- releasing second messenger inositol 1,4,5-trisphosphate (IP3) in initiating arrhythmias during early reperfusion following a period of myocardial ischaemia.
2. Evidence for an arrhythmogenic action of IP3 was provided by studies showing a correlation between the extent of the increase in IP3 and the incidence of arrhythmias in early reperfusion. In addition, phospholipase C inhibitors selective for thrombin receptor stimulation were anti-arrhythmic only when arrhythmias were thrombin initiated.
3. Mechanisms by which IP3 could initiate arrhythmias are discussed, with particular emphasis on the role of slow and unscheduled Ca2+ release.
4. The reperfusion-induced IP3 and arrhythmogenic responses can be initiated through either α1-adrenoceptors or thrombin receptors, but endothelin receptor stimulation was ineffective. Further studies have provided evidence that the noradrenaline-mediated response was mediated by α1A-receptors, while the α1B-adrenoceptor subtype appeared to be protective.
5. Reperfusion-induced IP3 responses could be inhibited by procedures known to reduce the incidence of arrhythmias under these conditions, including preconditioning, inhibiting Na+/H+ exchange or by dietary supplementation with n-3 polyunsaturated fatty acids.
6. Inositol 1,4,5-trisphosphate generation in cardiomyocytes can be facilitated by raising intracellular Ca2+ and it seems likely that the rise in Ca2+ in ischaemia and reperfusion is responsible for the generation of IP3, which will, in turn, further exacerbate Ca2+ overload.
Epidemiological evidence in humans suggests that intrauterine growth retardation is associated with an increased risk of hypertension and coronary heart disease in later life. To begin to understand the mechanisms involved, we developed and exploited a rat model of intrauterine growth retardation to assess predisposition to arrhythmias and resting blood pressure levels at defined ages from 4 to 18 months.
Isolated working heart experiments were carried out on rats that had been subjected to intrauterine growth retardation by prenatal protein deprivation and age-matched male Wistar controls to measure susceptibility to wall stress-induced arrhythmias. In addition, resting systolic and diastolic blood pressures were measured in conscious rats via an indwelling arterial catheter.
Hearts from intrauterine growth retarded animals showed significantly more ventricular premature beats and more episodes of ventricular tachycardia at all ages examined (4, 9 and 18 months), and at 4 and 18 months, a reduction in coronary blood flow. Diastolic pressure was significantly raised by intrauterine growth retardation in both groups examined (4 and 9 months).
Protein malnutrition during the intrauterine period results in profound intrauterine growth retardation that is associated with a raised diastolic blood pressure and an increased predisposition to cardiac arrhythmias in later life. These results are consistent with epidemiological observations made in human populations, and as similar pathophysiological changes may operate in both situations, intrauterine protein deprivation may be a useful model to help define some of the mechanisms involved.
The exact mechanism of mechano-electrical feedback and stretch-induced arrhythmias is unknown, but the role of stretch-activated ion channels and specific calcium channels has been proposed. The aim of the present study was to test the hypothesis that stretch-activated ion channels and not calcium channels contribute to stretch-related alterations of repolarization and that these effects can be neutralized by stretch-activated channel block. We studied the interaction of acute ventricular dilatation and the stretch-activated channel blocker streptomycin and the specific calcium channel blocker verapamil in an isolated retrogradely perfused rabbit heart model in which the left ventricular size is modified by abruptly changing the volume of a fluid-filled balloon placed in the left ventricle. Acute ventricular dilatation led to a rate-dependent decrease in repolarization. The mean effective refractory period (ERP) and monophasic action potential duration (MAP90) for cycle lengths between 300 and 1,000 ms decreased from 174.2+/-9 ms and 178.9+/-7 ms to 161.6+/-11 ms and 169.7+/-5 ms, respectively. Streptomycin (80 microM) inhibited this stretch-related shortening of repolarization (ERP: 175.4+/-8 ms; MAP90: 179.7+/-8 ms, p < 0.05) but had almost no effect on already dilated ventricles. Counteraction of the observed electrophysiologic changes could only be achieved by increasing the streptomycin concentration to 200 microM. Streptomycin nearly completely suppressed stretch-related ectopic ventricular complexes. In contrast, verapamil (1 microM) had no effect on stretch-related changes in repolarization and stretch-induced arrhythmias. The present study indirectly implicates stretch-activated ion channels in the genesis of stretch-related changes in repolarization and arrhythmias. The electrophysiologic changes after ventricular dilatation to a degree that increases left ventricular pressure in a clinically relevant range can be influenced by the stretch-activated channel blocker streptomycin but not by specific calcium channel block. This may have clinically important implications for the development of new antiarrhythmic drugs.
Chronic enlargement of atrium is common in atrial fibrillation, and the effects of stretch on atrial action potentials seem inconsistent. As atrial muscle is heterogeneous, we suggest that atrial stretch induces a variable electrophysiologic response and that the effects of stretch are only partially mediated by stretch-activated channels.
Sixteen guinea pig hearts were perfused by the Langendorff method using Kreb's solution with and without 80 microM streptomycin, which is a stretch-activated channel blocker. Suction electrodes were used to record monophasic action potentials (MAPs) of the left atrium. Left auricular pressure was monitored by a balloon. We determined the MAP duration at 50% and 90% repolarization (MAPD50 and MAPD90) in basal conditions and after a slow onset but sustained stretch of the atrium in the absence and presence of streptomycin. Stretch induced no overall consistent or significant change in mean MAPD50 and MAPD90. The individual responses were markedly variable. The most frequent response (about 50%) was a decrease in MAPD50 and MAPD90. In 25% of cases, there was no change, and in 25% we observed increases in MAPD50 and MAPD90. Streptomycin did not affect MAPD50 and MAPD90, but it dramatically modified the distribution of MAPD changes induced by stretch. In particular, streptomycin removed stretch-induced shortening of MAPD50 and MAPD90, whereas it did not affect stretch-induced lengthening.
Sustained stretch of atrium induces variable modifications of MAPD that are only partially inhibited by streptomycin. This suggests the participation of ionic channels other than specific stretch-activated channels in the response of atrial myocardium to sustained stretch.
In single guinea pig ventricular myocytes, streptomycin sulphate (streptomycin) reduced intracellular Ca(2+) transients (IC(50) 1.9 mM) and contractility (IC(50) 1.0 mM), 2 mM streptomycin prolonged the action potential. Under switch voltage clamp, 2 mM streptomycin reduced the L-type Ca(2+) current (I(CaL)) amplitude and (Ca(2+)-dependent) relative inactivation at positive membrane potentials and reduced the rapid and slow components of the delayed rectifier current (I(K)). This latter effect seemed Ca(2+)-dependent, not being seen when nifedipine and BAPTA were used to reduce intracellular Ca(2+). Fifty micromolars of streptomycin had no significant effects on any parameter studied. We conclude that the negative inotropic effect of streptomycin results from blockade of I(CaL), and thus, a reduction of intracellular Ca(2+) transients, while prolongation of the action potential is more consistent with effects on I(K). These observations link mechanical and electrical effects of streptomycin that may be important, for example, when streptomycin is used to block stretch-activated events in cardiac muscle.
Myocardial stretch induces several electrophysiological changes and arrhythmias, but little is known on its possible role in triggering ventricular fibrillation (VF) during acute coronary occlusion. In thiopental-anesthetized, open-chest pigs submitted to a 40-min ligation of the left anterior descending coronary artery, the association between the early increase in end-diastolic length (measured by means of ultrasonic crystals) in the ischemic region and subsequent VF was analyzed. Animals received no treatment (n = 35) or intravenous nitroglycerin (2.5 microg/kg/min for 20 min, starting 10 min after coronary occlusion, n = 8) or Gd(3+) (80 microM/kg for 35 min, starting 5 min before occlusion, n = 15). Twenty-four animals (41 %) had VF, 16 to 39 min after coronary occlusion. The magnitude of ischemic dilation and the incidence of VF were similar among groups. End-diastolic length in the ischemic region 15 min after coronary occlusion was 115.7 +/- 1.2 % of baseline in animals with VF and 111.4 +/- 0.9 % in those without (P = 0.007), and was the strongest predictor of this arrhythmia (P = 0.003) after adjusting for treatment and other possible confounding variables. Thus, the dilation of the ischemic region is closely and independently associated with VF following coronary occlusion. Although the interventions tested in the present study failed to protect against this arrhythmia, the results strongly suggest an influence of ischemic dilation on VF.
We investigated the relation between morning blood pressure (BP) variations, sympathetic activity, and QT intervals in 156 never-treated subjects with essential hypertension and different patterns of morning BP increase. The morning BP peak (MP) was defined as a rise in systolic BP >or=50 mm Hg and/or diastolic BP >or=22 mm Hg during early morning (6:00 to 10:00 AM) compared with mean BP during the night. Clinical characteristics of patients with morning BP peak (MP+, n= 69, morning systolic BP=+54+/-4, diastolic BP=+32+/-5 mm Hg) did not differ from patients without BP peak (MP-, n= 87, morning systolic BP=+24+/-5, diastolic BP=+19+/-3 mm Hg). The daytime (10:00 AM to 10:00 PM) and the nighttime (10:00 PM to 6:00 AM) BP profile did not differ between the two groups. During daytime and nighttime ECG monitoring, the corrected QT (QTc) interval, and QTc dispersion did not differ significantly between the two groups, whereas during the morning period the QT values were significantly broader in the MP+ group compared with the MP- group (P<or=0.001). Morning LF/HF ratio was significantly higher in MP+ patients than in MP- patients (P<or=0.02). Both systolic and diastolic morning BP, in combination with ratio LF/HF power, were significant predictors of QTc dispersion (adjusted R(2)=0.59, P<or=0.01) and QTc interval (adjusted R(2)=0.41, P<or=0.01), whereas inclusion of physical activity and echocardiographic parameters did not add explanatory information. The prolongation of cardiac repolarization times and morning sympathetic overactivity coexist in hypertensive patients with morning BP peaks, and they might contribute to raised cardiovascular risk in these patients.
Deaths secondary to low-energy impacts to the precordium in young individuals (commotio cordis) have been reported with increasing frequency. In a swine model, baseball impacts induce ventricular fibrillation when directed at the center of the left ventricle during the vulnerable portion of repolarization just prior to the T-wave peak. It has been hypothesized that activation of stretch-sensitive channels could be crucial for this electrophysiological phenomenon. In this study, a nonselective stretch-activated cation channel was pharmacologically blocked prior to chest blows to determine whether this channel represents a possible pathway by which commotio cordis events occur.
In a randomized and blinded experiment, 12 swine (mean 17.1 +/- 2.5 kg) received either 2-g streptomycin intramuscularly (mean serum concentration 115 +/- 18 muM) or sterile water prior to chest impact. Each animal received six precordial impacts with a baseball propelled at 40 mph.
There was no significant difference in the frequency of induced VF in the animals administered streptomycin (10 of 19 impacts: 53%) compared to those control animals receiving only sterile water (10 of 31: 32%) (P = 0.15). However, the magnitude of ST segment elevation was less in the streptomycin-treated animals (19 +/- 19 mV) versus controls (61 +/- 46 mV) (P = 0.015).
Streptomycin did not alter the frequency of ventricular fibrillation in our commotio cordis model, indicating that the stretch-activated channel is not implicated in the genesis of chest blow-induced cardiac arrest. However, streptomycin did reduce ST elevation following impact suggesting that the stretch-activated channel may play a role in ST segment elevation following chest wall blows.
Mechanosensitivity is a property common to many cell types. Because channels gated by mechanical stimuli (mechanosensitive channels, MSCs) are implicated in many normal and pathological cellular responses, they present a valid target for therapeutic agents. However the process of mechanotranduction, the structure, function and pharmacology of eukaryotic MSCs are not well understood and matching experimental and in vivo stimuli is difficult. With respect to the pharmacology of these channels, a further complication arises because agents that modulate the activity of MSCs may not even bind to the channel itself, but may cause their effects by changing the properties of the tension-sensing lipid bilayer and/or cytoskeleton. MSCs in the myocardium are discussed in the context of their probable role in the generation of stretch-activated arrhythmias. The actions of the three most prominent agents used to study MSCs in the heart, the lanthanide gadolinium, the aminoglycosidic antibiotic streptomycin, and a peptide toxin isolated from tarantula venom, GsMTx-4, are compared. While all three can prevent mechanically-induced cardiac arrhythmias in experimental situations, only GsMTx-4 seems to have the potential as a novel therapeutic agent for the targeting of arrhythmias provoked by MSCs.
The aim of this study was to determine whether the changes in myocardial activation pattern resulting from acute stretching during ventricular fibrillation can be counteracted by administering a compound that blocks receptors sensitive to stretch. The study involved 16 isolated rabbit hearts, in which refractoriness and activation frequency during ventricular fibrillation were measured before, during and after localized acute stretching of the left ventricular free wall, either without (series A, n=8) or with (series B, n=8) the presence of streptomycin, 200 micromol. At baseline and during and after stretching, ventricular fibrillation was slower with streptomycin perfusion in series B than in series A (dominant frequency at baseline, 13+/-2 Hz vs. 16+/-2 Hz, respectively; P< .005; dominant frequency with stretching, 14+/-2 Hz vs. 19+/-3 Hz, respectively; P< .005). Streptomycin attenuated the electrophysiological changes produced by stretching and had a direct effect on refractoriness and activation frequency during ventricular fibrillation.
The incidence of atrial fibrillation correlates with increasing atrial size. The electrical consequences of atrial stretch contribute to both the initiation and maintenance of atrial fibrillation. It is suggested that altered calcium handling and stretch-activated channel activity could explain the experimental findings of stretch-induced depolarisation, shortened refractoriness, slowed conduction and increased heterogeneity of refractoriness and conduction. Stretch-activated channel blocking agents protect against these pro-arrhythmic effects. Gadolinium, GsMTx-4 toxin and streptomycin prevent the stretch-related vulnerability to atrial fibrillation without altering the drop in refractory period associated with stretch. Changes the activity of two-pore K+ channels, which are sensitive to stretch and pH but not gadolinium, could underlie the drop in refractoriness. Intracellular acidosis induced with propionate amplified the change in refractoriness with stretch in the isolated rabbit heart model in keeping with the clinical observation of increased propensity to atrial fibrillation with acidosis. We propose that activation of non-specific cation stretch-activated channels provides the triggers for acute atrial fibrillation with high atrial pressure while activation of atrial two-pore K+ channels shortens atrial refractory period and increases heterogeneity of refractoriness, providing the substrate for atrial fibrillation to be sustained. Stretch-activated channel blockade represents an exciting target for future antiarrhythmic drugs.
Velocity of electrical conduction in cardiac tissue is a function of mechanical strain. Although strain-modulated velocity is a well established finding in experimental cardiology, its underlying mechanisms are not well understood. In this work, we summarized potential factors contributing to strain–velocity relationships and reviewed related experimental and computational studies. We presented results from our experimental studies on rabbit papillary muscle, which supported a biphasic relationship of strain and velocity under uni-axial straining conditions. In the low strain range, the strain–velocity relationship was positive. Conduction velocity peaked with 0.59 m/s at 100% strain corresponding to maximal force development. In the high strain range, the relationship was negative. Conduction was reversibly blocked at strain. Reversible block occurred also in the presence of streptomycin. Furthermore, our studies revealed a moderate hysteresis of conduction velocity, which was reduced by streptomycin. We reconstructed several features of the strain–velocity relationship in a computational study with a myocyte strand. The modeling included strain-modulation of intracellular conductivity and stretch-activated cation non-selective ion channels. The computational study supported our hypotheses, that the positive strain–velocity relationship at low strain is caused by strain-modulation of intracellular conductivity and the negative relationship at high strain results from activity of stretch-activated channels. Conduction block was not reconstructed in our computational studies. We concluded this work by sketching a hypothesis for strain-modulation of conduction and conduction block in papillary muscle. We suggest that this hypothesis can also explain uni-axially measured strain–conduction velocity relationships in other types of cardiac tissue, but apparently necessitates adjustments to reconstruct pressure or volume related changes of velocity in atria and ventricles.
In this study, we describe a bioreactor system to deliver controlled stretch protocols to bioengineered heart muscle (BEHMs) and test the system when streptomycin (an aminoglycoside antibiotic, which blocks stretch-activated channels) is either added to or excluded from the culture medium. Streptomycin is a very commonly used component of cell culture antibiotic-antimycotic media additives, so its effects on muscle development and functional response to mechanical signals in vitro is worthy of investigation. Our hypothesis is that BEHMs will not adapt to the applied mechanical stretch protocol when streptomycin is present in the culture medium, but will do so when streptomycin is excluded. Bioengineered heart muscles were formed by culturing primary neonatal cardiac myocytes in a fibrin gel using a method previously developed in our laboratory. A custom bioreactor system was designed using SolidWorks and structural components manufactured using fusion deposition modeling. We utilized a stretch protocol of 1 Hz, 10% strain for 7 d. BEHMs were stretched in the presence and absence of streptomycin. As controls, BEHMs were maintained in a cell culture incubator with and without streptomycin. The contractile properties of all BEHMs were evaluated to determine the active force. We were able to demonstrate compatibility of the bioreactor system with BEHMs and were able to stretch 58 constructs with zero incidence of failure. When the BEHMs were stretched in the absence of streptomycin, the active force increased from a mean value of 51.7 +/- 5.6 (N = 10) to 102.4 +/- 16.3 microN (N = 10), with p < 0.05. However, BEHMs that were stretched in the presence of streptomycin did not show any significant increase in active force generation. The average active force of BEHMs increased from a mean value of 57.6 +/- 10.2 (N = 10) to 91.4 +/- 19.8 microN (N = 10) when stretched in the presence of streptomycin. In this study, we demonstrate compatibility of the a bioreactor system with BEHMs, stability of the BEHMs in response to stretch protocols, and significant functional improvement in response to controlled stretch only when streptomycin is excluded from the culture medium, supporting our hypothesis.
The contribution of intracellular calcium to ventricular fibrillation (VF) was investigated using chronically instrumented dogs with healed myocardial infarctions. A 2-minute coronary occlusion was initiated during the last minute of exercise. Fourteen animals developed ventricular fibrillation (susceptible) whereas the remaining 12 did not (resistant) during this exercise plus ischemia test. The test was then repeated for the susceptible animals after pretreatment with the intracellular calcium chelator BAPTA-AM (1.0 mg/kg). BAPTA-AM significantly reduced left ventricular dp/dt max and prevented VF in 8 of 12 susceptible animals. Conversely, myocardial cytosolic calcium levels were increased in resistant animals using the calcium channel agonist Bay K 8644 (30 micrograms/kg) or phenylephrine (10 micrograms.kg-1.min-1 3-5 min before occlusion). Bay K 8644 induced VF in all 5 resistant animals tested whereas phenylephrine induced VF in 8 of 12 resistant animals. BAPTA-AM pretreatment attenuated the hemodynamic effects of Bay K 8644 or phenylephrine and prevented VF in five of five Bay K 8644- and four of seven phenylephrine-treated animals. Finally, the endogenous level of calcium/calmodulin (Ca-CaM)-dependent phosphorylation of 170- and 55-kDa substrate proteins was measured (as an index of intracellular free calcium concentration). In the susceptible dog heart, the endogenous level of Ca-CaM-dependent phosphorylation was estimated to be two- to threefold higher than that observed in resistant dog heart. Treatment of resistant dog tissue with the calcium ionophore A23187 increased the level of Ca-CaM-dependent phosphorylation of these two proteins to the level observed in susceptible dog heart. These data suggest that elevated cytosolic calcium facilitates development of malignant arrhythmias and that elevated cytosolic calcium levels may be present in animals particularly susceptible to ventricular fibrillation.
Patch-clamp recordings from ventricular myocytes of neonatal rats identified ionic channels that open in response to membrane stretch caused by negative pressures (1 to 6 cm Hg) in the electrode. The stretch response, consisting of markedly increased channel opening frequency, was maintained, with some variability, during long (greater than 40 seconds) stretch applications. The channels have a conductance averaging 120pS in isotonic KCl, have a mean reversal potential 31 mV depolarized from resting membrane potential, and do not require external Ca++ for activation. The channels appear to be relatively non-selective for cations. Since they are gated by physiological levels of tension, stretch-activated channels may represent a cellular control system wherein beat-to-beat tension and/or osmotic balance modulate a portion of membrane conductance.
The membrane of tissue-cultured chick pectoral muscle contains an ionic channel which is activated by membrane stretch. Nicotinic channels and Ca2+-activated K+ channels are not affected by stretch. In 150 mM-external K+ and 150 mM-internal Na+ the channel has a conductance of 70 pS, linear current-voltage relationship between -50 and -140 mV and a reversal potential of +30 mV. Kinetic analysis of single-channel records indicates that there are one open (O) and three closed (C) states. The data can be fitted by the reaction scheme: C1-C2-C3-O. Only the rate constant that governs the C1-C2 transition (k1,2) is stretch-sensitive. None of the rates are voltage-sensitive. The rate constant k1,2 varies with the square of the tension as k1, 2 = k0 X e alpha T2, where alpha is a constant describing the sensitivity to stretch and T is the tension. A typical value of alpha is 0.08 (dyn cm-1)-2. Following exposure to cytochalasin B the channel becomes more sensitive to stretch. The stretch-sensitivity constant, alpha, increases from 0.08 to 2.4 (dyn cm-1)-2. The probability of the channel being open is strongly dependent upon the extracellular K+ concentration. With a suction of 2 cmHg the probability increases from 0.004 in normal saline (5 mM-K+) to 0.26 in 150 mM-K+. The channel appears to gather force from a large area of membrane (greater than 3 X 10(5) A2), probably by a cytochalasin-resistant cytoskeletal network.
1. An improved perfusion system for the isolated rat heart is described. It is based on the isolated working heart of Neely, Liebermeister, Battersby & Morgan (1967) (Am. J. Physiol. 212, 804-814) and allows the measurement of metabolic rates and cardiac performance at a near-physiological workload. The main improvements concern better oxygenation of the perfusion medium and greater versatility of the apparatus. Near-physiological performance (cardiac output and aortic pressure) was maintained for nearly 2 h as compared with 30 min or less in the preparations of earlier work. 2. The rates of energy release (O2 uptake and substrate utilization) were 40-100% higher than those obtained by previous investigators, who used hearts at subphysiological workloads. 3. Values are given for the rates of utilization of glucose, lactate, oleate, acetate and ketone bodies, for O2 consumption and for the relative contributions of various fuels to the energy supply of the heart. Glucose can be replaced to a large extent by lactate, oleate or acetate, but not by ketone bodies. 4. Apart from quantitative differences there were also major qualitative differences between the present and previous preparations. Thus insulin was not required for maximal rates of glucose consumption at near-physiological, in contrast with subphysiological, workloads when glucose was the sole added substrate. When glucose oxidation was suppressed by the addition of other oxidizable substrates (lactate, acetate or acetoacetate), insulin increased the contribution of glucose as fuel for cardiac energy production at high workload. 5. In view of the major effects of workload on cardiac metabolism, experimentation on hearts performing subphysiologically or unphysiologically is of limited value to the situation in vivo.
Triggered activity in cardiac tissue is caused by afterdepolarizations. These are transient depolarizations of the cell membrane that occur before an action potential has fully repolarized (early afterdepolarizations; EADs) or after its complete repolarization (delayed afterdepolarizations; DADs). In recent years, much new information has been discovered about the mechanisms that cause EADs and DADs. DADs arise in Ca2+-loaded cells and result from an oscillatory increase of intracellular Ca2+ from focal regions of the sarcoplasmic reticulum, thereby activating a Ca2+-sensitive transient inward current. In contrast, EADS, at least those arising near the plateau, result from the interplay of surface membrane currents, and do not depend on Ca2+ overload. Lengthening of the action potential duration appears to lead to the time- and voltage-dependent recovery of L-type Ca2+ current to carry the depolarizing charge for this EAD. The mechanism causing a second type of EAD, the phase 3 EAD, remains to be explained. In this report, the cellular mechanisms thought to underlie the different types of triggered activity and their possible roles in generation of triggered arrhythmias are considered.
(C) Lippincott-Raven Publishers.
1. Voltage-clamp experiments were carried out in calf Purkinje fibres to determine the basis of transient depolarizations (TDs) associated with digitalis-induced arrhythmias. 2. Under the influence of strophanthidin, depolarizing clamp pulses were followed by a transient inward current (TI) which was small or absent in untreated preparations. The TI also appeared in the wake of a train of action potentials. 8. The TI can help generate spontaneous depolarizations in preparations showing the low voltage oscillations which often occur with advanced digitalis toxicity. 8. The TI can help generate spontaneous depolarizations in preparations showing the 'low voltage oscillation' which often occur with advanced digitais toxicity. It was designated TI because its magnitude and timing were appropriate to account for the TD. 3. Longitudinal voltage non-uniformity during the TI was determined with two voltage-recording micro-electrodes. Although the non-uniformity was not severe, the TI wave form was observed when the voltage difference signal was used to measure membrane current density. 4. Over the diastolic range of potential, the strophanthidin-induced TI appeared superimposed upon the normal pace-maker mechanism, the decay of a potassium current, iK2. The TI could be dissociated from iK2, however, by means of its unusual kinetic properties. 5. TIs could also be recorded at holding potentials positive to -55 mV, i.e. outside the range where iK2 deactivation occurs. 6. The TI amplitude showed a slow and strongly sigmoid dependence on the duration of the preceding depolarizing pulse. Stronger depolarizing reduced the TI amplitude, while slowing and exaggerating the sigmoid time-dependence. 7. Two clamp pulses in close succession gave additive effects in evoking a subsequent TI. This finding and the sigmoid time-dependence fit with previous observations that TDs are most prominent following a series of closely spaced action potentials.
1. Stretch-activated anion currents were studied in sino-atrial and atrial cells using the whole-cell patch clamp technique. With continuous application of positive pressure (5-15 cmH2O) through the patch clamp electrode, the cell was inflated and the membrane conductance was increased. 2. Voltage clamp steps revealed that the stretch-activated currents had time-independent characteristics. The increase in membrane conductance was reversible on subsequent application of negative pressure to the electrode. 3. The reversal potential of the stretch-activated currents was shifted by 60 mV for a 10-fold change in intracellular Cl- concentration, while it was unaffected by replacement of Na+ in the extracellular solution by N-methyl-D-glucamine. Cell superfusion with Cl(-)-deficient solution (10 mM Cl-) reduced the amplitude of outward current. These findings indicate that the stretch-activated conductance is Cl- selective. 4. The sequence of anion permeability through the stretch-activated conductance was determined to be I-(1.7) > NO3-(1.5) > Br-(1.2) > Cl-(1.0) > and F-(0.6). SCN- appeared to be more permeant than I-. 5. The stretch-activated conductance was reduced by the Cl- channel blockers, 4,4'-dinitrostilbene-2,2'-disulphonic acid disodium salt, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid or anthracene-9-carboxylate (9-AC). Administration of furosemide or bumetanide had no effect. 6. The stretch-activated Cl- current was recorded even though intracellular Ca2+ ions were chelated by including 10 mM EGTA in the pipette solution. Neither the specific peptide inhibitor of cyclic AMP-dependent protein kinase (50 microM), nor the non-selective blocker of protein kinases, H-7 (20 microM), was effective in reducing the stretch-activated Cl- current, suggesting that the stretch-activated Cl- current is a novel type of cardiac Cl- current, which shows a different modulatory mechanism from that of other cardiac Cl- currents.
Cardiac cell swelling occurs under abnormal conditions. Currents through volume-sensitive channels, if present in heart, will affect the cardiac electrical activity. Single canine ventricular myocytes were voltage clamped under conditions that largely suppressed Na, K, and Ca channel currents and currents generated by electrogenic transport systems. Cell width and membrane conductance were monitored continuously. Swelling was induced by increasing the osmolarity of the pipette solution or by decreasing the osmolarity of the external solution. During cell swelling, the cell widened and membrane conductance increased. This increase in membrane conductance was sensitive to Cl channel blockers and to external Cl removal, suggesting that a major component was provided by a Cl channel. The current-voltage relationship of the swelling-induced current displayed an outward rectification, with an average zero-current voltage of -60 mV. The activation of the swelling-induced current did not seem to depend on external or internal Ca and was not sensitive to a protein kinase inhibitor (H-8). Shape-altering agents chlorpromazine decreased while dipyridamole and trinitrophenol increased the membrane conductance without osmotic perturbations, suggesting that changes in tension in the cell membrane may play a role in opening and closing of the swelling-induced channels.
The Cai oscillation generated by the sarcoplasmic reticulum in response to an action potential occurs relatively synchronously within and among cells. The sarcoplasmic reticulum can also generate spontaneous Cai oscillations (S-CaOs), ie, not triggered by sarcolemmal depolarisation. The local increase in Cai due to S-CaOs is equivalent to that induced by an action potential. Heterogeneity of diastolic Cai among cells within myocardial tissue caused by asynchronous S-CaOs leads to heterogeneous myofilament activation, the summation of which produces a Ca2+ dependent component to diastolic tone. The local increases in Cai due to S-CaOs also cause oscillatory sarcolemmal depolarisations due to Ca2+ modulation of the Na-Ca exchanger and of non-specific cation channels. When local S-CaOs within a myocardial cell is sufficiently synchronised the resultant depolarisation summates and can be sufficient to trigger a spontaneous action potential. Inhomogeneous levels of diastolic Cai among cells may lead to heterogeneity in cell coupling and thus may also affect the impulse conduction in myocardial tissue. The magnitude of the S-CaOs induced diastolic tonus and depolarisation varies with the extent to which S-CaOs are synchronised; partially synchronised S-CaOs following an action potential induced Ca2+ release produce an aftercontraction and afterdepolarisation. Inhomogeneity of diastolic sarcoplasmic reticulum Ca2+ loading and sarcomere lengths within individual cardiac cells due to S-CaOs leads to inhomogeneous systolic Cai levels and sarcomere inhomogeneities in response to a subsequent action potential, which compromise the systolic contraction amplitude. Heterogeneity of systolic Cai among cells due to diastolic S-CaOs also leads to heterogeneity of action potential repolarisation times, due to heterogeneous Cai modulation of the Na-Ca exchanger, the non-specific cation channel, the L type Ca2+ channel and, depending upon species, Ca2+ activated K+ channels. S-CaOs occurrence during a long action potential plateau may also modulate the removal of voltage inactivation of L type Ca2+ channels, and affects the likelihood of the occurrence of "early afterdepolarisations." Thus, as a single entity, S-CaOs may be implicated in diverse manifestation of heart failure--impaired systolic performance, increased diastolic tonus, and an increased probability for the occurrence of arrhythmias.
1. The effect of different levels of ventricular pressure upon the prevalence of ventricular arrhythmias has been studied in 42 rat hearts by using the isolated working heart model.
2. The results have shown that there is an increased prevalence of arrhythmia at the highest levels of pressure tested in hearts from both normal and hypertensive rats. In hearts from normal rats total ectopic counts were 304 at 140 mmHg and 150 at 60 mmHg (P < 0.05). In hearts from hypertensive rats total ectopic counts were 4217 at 180 mmHg and 2179 at 100 mmHg (P < 0.05).
3. Hearts from hypertensive rats demonstrated significantly more ectopic activity than hearts from normal rats at all test pressures. Median values for ectopic counts for each study period were 68 in hearts from hypertensive rats and seven in hearts from normotensive rats (P < 0.001).
4. There was evidence that the increased arrhythmia observed was not due to ischaemia or fatigue, and it seems likely that these effects of raised pressure are due to its effect on ventricular wall stress. The differences between various pressures were relatively small, but do suggest that a sustained increase in ventricular wall stress is arrhythmogenic.
5. This small arrhythmogenic effect over the relatively short period of these studies may be of much greater significance when considered in the context of the prolonged periods for which the diseased heart may be subjected to raised wall stress.
Alterations in loading conditions and muscle length influence the electrophysiology of ventricular myocardium and may play a role in arrhythmogenesis in globally dilated or dyskinetic ventricles. To test the hypothesis that stretch can initiate arrhythmias in normal myocardium, the response to graded mechanical stretch was studied in seven isolated blood-perfused canine ventricles. After eight conditioning contractions produced by His bundle pacing (2 Hz), global stretch of the ventricle was produced by a servocontrolled pump that abruptly increased ventricular volume by a precise amount (delta V) during early diastole and then returned ventricular volume to the initial holding volume (Vi). Ventricular premature contractions were readily produced; ventricular couplets and short runs of ventricular tachycardia were occasionally elicited. The probability of a stretch-induced arrhythmia was determined from multiple alternating sequences in which a stretch of known amplitude (delta V) or no stretch was delivered. As delta V was increased, the probability of a stretch-induced arrhythmia was low initially, increased sharply after a threshold was exceeded, and approaching 100% with physiological volumes. With Vi set to a standard value of 20 ml, corresponding to end-diastolic pressure of 5.3 +/- 5.2 mm Hg (mean +/- SD), the delta V resulting in a 50% chance of a stretch-induced arrhythmia (delta V50) was 15.0 +/- 1.6 ml. A decline in delta V50 was consistently observed when Vi was increased. While delta V50 values were remarkably similar (10.7% coefficient of variation), the pressure at the time the ventricular premature depolarization was triggered was highly variable for different ventricles; this finding suggests that myocardial strain is more important than absolute level of wall stress in the initiation of these arrhythmias. These results demonstrate that myocardial stretch predictably initiates arrhythmias and that the susceptibility to stretch-induced arrhythmias is enhanced by ventricular dilatation. Thus, ventricular ectopy in patients with regionally or globally dilated hearts may arise, in part, by a mechanism of myocardial stretch.
Stretch-activated channels have been proposed as the transduction mechanism between load and protein synthesis in cardiac hypertrophy. Under this hypothesis, cardiac deformation is linked to an increased sodium (Na) influx, which, in turn, increases protein synthesis. We have tested whether stretch actually increases Na influx by applying patch-clamp techniques to cultured chick embryo cardiac myocytes and to freshly isolated adult guinea pig cardiomyocytes. Our experiments, in excised and cell-attached patches, revealed the existence of ionic channels that opened, or increased their frequency of opening, upon the application of negative pressures to the lumen of the patch-clamp pipettes. These stretch-sensitive channels allowed the passage of the major monovalent physiological cations, Na and potassium (K), and, to a much lesser extent, the major divalent cations calcium (Ca) and magnesium (Mg). Under normal conditions, the channels had a high open channel noise that prevented the customary, straightforward statistical analysis of single channel data. However, when one of the major monovalent cations was iso-osmotically replaced by sucrose, the open channel noise decreased significantly and permitted a good delineation of the open and closed channel states and, therefore, application of standard patch-clamp, statistical analysis techniques. Under these "sucrose," "monoionic" conditions, the reversal potential was, as one should expect, close to the equilibrium potential for the major monovalent cation present. When high extracellular K solution was used to minimize the cell resting potential, the reversal potential for these stretch-activated currents was estimated to be around -40 mV. Therefore, under normal conditions, stretch should induce an inward, depolarizing current, carried mostly by Na ions.(ABSTRACT TRUNCATED AT 250 WORDS)
The initiating cause of the first ectopic beat and its precipitation of sustained lethal arrhythmia in acute myocardial ischemia is not clear. Comparable uncertainties surround sudden death in myocardial failure. Progress in control of ventricular fibrillation has been slow, perhaps because diagnosis and treatment have been based on the premise that ischemic biochemical changes solely cause the alterations in electrophysiological behavior. Alternative approaches need exploration. Evidence that mechanical changes can initiate electrophysiological changes by a process sometimes referred to as "mechanoelectric feedback" is accumulating. It operates when any primary mechanical change in ventricular muscle, e.g., contraction produces a change in its electrical properties. Mechanical changes are prevalent in ischemia and cardiac failure. If mechanoelectric feedback operates here, it is not surprising that arrhythmias in some of these pathologies parallel the degree of mechanical myocardial dysfunction rather than electrophysiological changes, independent of etiology. This mechanoelectric feedback system may exist as an intrinsic property of normal myocardium, providing a feedback control of activation processes at both the cellular and gross levels. Its disruption during pathological events produces instability in the system and thus ventricular arrhythmia. This concept provides a new potential avenue for arrhythmia therapy.
Transient diastolic dilatation of the isolated canine left ventricle predictably elicits arrhythmias. To test the hypothesis that such arrhythmias may be mediated by sarcolemmal stretch-activated channels, we attempted to inhibit stretch-induced arrhythmias with gadolinium (Gd3+), a potent stretch-activated channel blocker. In experiments with six isolated canine hearts, left ventricular volume was increased for 50 msec during early diastole and then returned to initial volume by a computerized servopump. The stretch volume was adjusted to yield a probability of eliciting a stretch-induced arrhythmia of 95 +/- 2% before treatment with Gd3+. When Gd3+ (1-10 microM) was administered, dose-dependent suppression of stretch-induced arrhythmias was observed. The probability of a stretch-induced arrhythmia was reduced to 13 +/- 10% (p less than 0.05) with 10 microM Gd3+. Washout of Gd3+ completely reversed this effect. Since Gd3+ is known to be a calcium channel antagonist, we compared the effect of Gd3+ on stretch-induced arrhythmias with that of verapamil and nifedipine. These calcium channel blockers produced no demonstrable inhibition of stretch-induced arrhythmias when administered at concentrations (1 microM) that substantially depressed left ventricular pressure development. Thus, our results indirectly implicate stretch-activated channels in the genesis of stretch-induced arrhythmias and provide preliminary evidence for a potential new mode of antiarrhythmic drug action--blockade of stretch-activated channels.
1. The effect of a sudden acute change in blood pressure upon arrhythmia provocation has been studied in an isolated working heart model from the Wistar—Kyoto strain of rat. Twenty-four hearts were studied.
2. They were perfused with two different, modified, Krebs—Henseleit solutions at a fixed left atrial pressure.
3. Acute changes in pressure, both increases and decreases, were arrhythmogenic. Whilst ectopic activity was more predictably produced by pressure reductions, this consisted of simple ventricular ectopics only. Pressure increases, in contrast, were capable of provoking more complex and sustained arrhythmias.
4. The effect of pressure changes were highly dependent upon electrolyte concentrations in the perfusate. Low potassium and magnesium concentrations increased the amount of arrhythmia provoked by pressure increases but tended to reduce that provoked by pressure reductions.
5. We conclude that the direct effect of an acute change in pressure upon the myocardium is arrhythmogenic. However, the myocardial response to a pressure change is interdependent upon prevailing electrolyte concentrations.
The possibility, that a GTP-binding protein is involved in the transducing mechanism leading to the formation of inositol trisphosphate (InsP3) in heart was explored in rat heart ventricles. Accordingly, a crude membrane fraction was isolated from 3[H] inositol prelabelled rat heart ventricles. When incubated with the non-hydrolysable GTP analogues GTP gamma S and GMP-PNP, it produced InsP3 in a time- and concentration-dependent manner. GDP beta S and the aminoglycoside antibiotic neomycin were effective inhibitors of this activation. In the absence of GTP gamma S or GMP-PNP, no such formation occurred with Ca2+ concentration from 10 nM to 1 microM but formation tripled in relation to the control level when Ca2+ concentration was raised from 1 microM to 100 microM. GTP gamma S increased the Ca2+ sensitivity of InsP3 production towards more physiologically relevant concentrations occurring during diastole (100 nM). These findings strongly suggest the presence in heart of a particulate Ca2(+)-dependent phospholipase C, whose activity is regulated by guanine nucleotides. This Ca2(+)-dependent phospholipase C observed in a cell free system was evidenced also in a multicellular system when altering the free Ca2+ concentrations around the physiological range. The results support the possibility that the enzyme might be activated during each cardiac cycle and thus produce two potential activators of cardiac contraction, namely InsP3 and diglycerides.
Many inotropic maneuvers act by increasing the intracellular calcium concentration [( Ca2+]i). The present report illustrates this with respect to the positive inotropic effects of cardiac glycosides and catecholamines. It is shown that the increased contractility produced by cardiac glycosides is accompanied by an increase in intracellular Na concentration and, furthermore, that the relationship between contraction and Na is very steep. This steep dependence, which may result from a Na-Ca exchange which exchanges several Na ions per Ca, means that maneuvers that have only small effects on Na will have significant effects on contraction. Cardiac glycosides also produce abnormal pacemaker activity and cardiac arrhythmias. These originate from a transient inward current activated by oscillations of [Ca2+]i, which result from spontaneous oscillatory release of Ca ions from the sarcoplasmic reticulum. The local anesthetic group of antiarrhythmic agents abolishes the transient inward current. Catecholamines also increase systolic [Ca2+]i and, in high enough concentrations, can produce oscillations of [Ca2+]i. This tendency of glycosides and catecholamines to produce arrhythmogenic oscillations of [Ca2+]i is a major limitation to their use. Therefore, inotropic agents that act by means other than increasing [Ca2+]i may be of great efficacy.
Various mechanisms have been suggested to explain the high prevalence of ventricular arrhythmia in patients with heart failure, but as yet there is no unifying theory. There is growing evidence that changes in myocardial mechanical properties may directly alter cardiac electrophysiology by a process of mechanoelectric feedback. Moreover, when changes in cardiac loading similar to those seen in heart failure are produced experimentally in normal heart, there is a greater tendency to arrhythmogenesis. The intimate relation between changes in mechanical function and arrhythmia in heart failure could account for the lack of effect of most conventional antiarrhythmic drugs on arrhythmogenesis, and the beneficial effect of peripheral vasodilators. This paper argues that mechanically induced changes in electrophysiology are very important in the development of arrhythmia in cardiac failure; there may be no need to implicate other mechanisms, such as relative ischaemia, metabolic changes, or changes in sympathetic tone.
Properties of a mechano-electrical transduction channel were studied in enzymatically dissociated chick vestibular hair cells by using a whole-cell recording variation of the patch voltage-clamp technique. The apical hair bundle was stimulated by a glass rod which moved along a one-dimensional axis when stimulated by either a triangular or a trapezoidal command voltage. The motion of the glass rod was monitored optically using a photodiode. In response to triangular stimuli, the hair cell generated a current of triangular wave form with occasional step-like spiky or zigzag-appearing events. Control experiments confirmed that the current was generated only when the hair bundle was displaced towards the tallest stereocilium. The mechano-sensitive current was blocked by streptomycin and by neomycin. The blockage by streptomycin was clearly voltage dependent: the reduction of the current became larger with hyperpolarization of the membrane. This suggests that the positively charged antibiotic molecules plug the mechanically gated channels. From the evidence presented in 3 and 4 above, the mechano-sensitive current recorded here was identified as the mechano-electrical transduction (m-e.t.) current. The permeability of the m-e.t. channel to various monovalent cations was determined from reversal potential measurements. Since a CsCl-EGTA intracellular medium was used, all the permeabilities were calculated relative to PCs. The sequence of permeabilities was Li greater than Na greater than or equal to K greater than or equal to Rb greater than Cs greater than choline greater than TMA greater than TEA. External Ca ions were indispensable for the recording of transduction current and Sr ions could replace Ca ions without loss of the transduction activity. The minimum [Ca]o for stable generation of the m-e.t. current was 20 microM in Cs saline. The addition of 50-200 microM-Ca to the isotonic Ba saline could maintain the m-e.t. current. The m-e.t. current was observed in isotonic Ca and in Sr salines. Isotonic Ba, Mg and Mn salines were enriched with 1-2 mM-Ca in order to generate the m-e.t. current. The permeabilities of the divalent cations relative to Cs were calculated from the reversal potentials, and the sequence of permeabilities among divalent cations was Ca greater than Sr greater than Ba greater than Mn greater than Mg. Step-like m-e.t. currents were observed in Cs saline. The smallest step amplitude with clear resolution had a conductance of 49.7 +/- 4.5 pS (mean +/- S.D., n = 7 cells). This is likely to be an elementary m-e.t. channel conductance.(ABSTRACT TRUNCATED AT 400 WORDS)
Cardiac arrhythmias are generated as the result of disorders of automaticity or impulse conduction. Regardless of the mechanism, however, calcium is likely to be involved. The rate of Ca2+ flux across the membrane of automatic cells alters their firing rate. Myocardial cells that do not ordinarily initiate action potentials can do so when they are partially depolarized. Low extracellular Ca2+ concentrations or Ca2+ channel-blocking drugs can reduce or abolish such ectopic firing. Early afterdepolarizations are also induced in cardiac cells by partial depolarization, whereas delayed afterdepolarizations are induced by Ca2+ overloading. Early afterdepolarizations and delayed afterdepolarizations can both be suppressed by low external Ca2+ concentrations or by Ca2+ channel blockers. With respect to arrhythmias ascribable to disorders of conduction, Ca2+ channel blockers can aggravate conduction disturbances in the sinoatrial node or atrioventricular junction. Furthermore, these drugs can abolish those reentrant arrhythmias in which a cardiac impulse rotates around a loop in which nodal tissue is an integral element. Ca2+ channel blockers can also reduce the susceptibility for ventricular fibrillation to supervene in ischemic hearts, especially when the sympathetic nervous system is overactive.
There is evidence to suggest that hypertensive patients are at increased risk of sudden death. However, to date this evidence is restricted to treated hypertensives and it is not clear whether this risk is due to the hypertension itself, its treatment, or its long-term consequences. This study has investigated, by 24-h ambulatory electrocardiography, a total of 58 newly diagnosed hypertensive patients who have never previously been treated and compared the first 50 of them with 50 matched control subjects. The results show that there is an increased prevalence of ventricular arrhythmia in hypertensive patients, compared with normal controls, and that this arrhythmia is present from the outset and thus not dependent on treatment or the development of long-term complications. Multivariate analysis showed that age (positive correlation) and potassium (negative correlation) made the most important independent contributions to the prevalence of arrhythmia in these patients, but even so only accounted for about 33% of the observed arrhythmia. This suggests that the precursors of arrhythmia in hypertension are multifactorial. One other factor appeared to make an important contribution to ventricular arrhythmia in these patients. In contrast to recent evidence from the study of treated hypertensives, the prevalence of ventricular arrhythmia was significantly lower in patients exhibiting electrocardiographic evidence of left ventricular hypertrophy. However, left ventricular hypertrophy did also appear to sensitize the myocardium to the arrhythmogenic effect of low serum potassium levels. These findings may help to explain some of the previous confusion surrounding treatment, hypertension and ventricular arrhythmia.
The role of inositol 1,4,5-trisphosphate (InsP3) in excitation-contraction coupling in skeletal muscle was investigated by several methods. The following results were obtained. InsP3 is released by electrical stimulation of muscles. Exogenous InsP3 releases calcium from skinned muscle fibers at relatively high doses under normal conditions but does so at very low concentrations when blockers of the InsP3 5-phosphatase are present. Blockers of InsP3 release are effective blockers of calcium transients elicited by electrical stimulation of muscle fibers. It is proposed that InsP3 acts as a chemical second messenger between transverse (T)-tubular membrane depolarization and calcium release from the sarcoplasmic reticulum in skeletal muscle.
The cellular mechanisms that cause cardiac arrythmias are of immense importance and are the object of intense investigation. One mechanism postulated to cause cardiac arrhythmias, and possibly conduction disturbances, is delayed afterdepolarizations (also called late afterdepolarizations, oscillatory afterpotentials, or transient depolarizations). These depolarizations are induced by Ca2+ overload of the cardiac cell. In the last decade, several review articles have been published on the subjects of delayed afterdepolarizations and of mechanisms of arrhythmias (16, 28, 86). The purpose of this review is to summarize recent experimental evidence pertinent to the mechanisms responsible for delayed afterdepolarizations and their relation to Ca2+, and to gain insight into their clinical relevance.
During 30 years of follow-up, there were 183 sudden deaths in men and 77 in women ages 35 to 94 years who participated in the Framingham Study. Risk of sudden death was increased threefold in hypertensive persons but only if there was no previously diagnosed coronary heart disease. Men receiving antihypertensive treatment had more than twice the risk of sudden death compared with those who were untreated, whether or not they had prior manifestations of coronary heart disease. More than twice as many men who died suddenly were receiving antihypertensive therapy compared with those in the population at risk of the same age. In those with overt coronary heart disease, 34% of those dying suddenly were on antihypertensive treatment compared with 18% of those of the same age in the general population. Multivariate analysis taking into account the level of blood pressure, electrocardiographic abnormalities, and previously diagnosed coronary heart disease and cardiac failure, all of which are predisposing factors for sudden death, indicated a persistent increased risk of sudden death in association with antihypertensive treatment. Tests of interaction indicate that the excess sudden death risk was not confined to those with electrocardiographic abnormalities. In women, it may be associated with diabetes. These data suggest that some feature of antihypertensive treatment as practiced in the general population may contribute to sudden death incidence in an ill-defined subgroup of hypertensive persons.
We studied the effect of three aminoglycoside antiobiotics which have been shown to replace Ca from lipid monolayers on the superficially bound Ca of isolated beating left atria from guinea‐pigs.
The cellular Ca content was determined by means of ⁴⁵ Ca after having attained complete exchange.
The antibiotics dibekacin, sisomicin, and gentamicin, all reduced the cellular Ca content by 10–20% in a dose‐dependent manner. The loss of superficially bound Ca was accompanied by a decline of the contractile force by 40–90%.
It is concluded that in isolated atrial muscle it is the amount of Ca bound to the outer surface of the cardiac plasmalemma, rather than the extracellular Ca ²⁺ concentration, that determines contraction height.
All aminoglycoside antibiotics in clinical use are ototoxic in that they can cause permanent malfunctioning of the cochlea and vestibule1-3. It is generally assumed that these ototoxic antibiotics primarily affect the sensory hair cells in the inner ear and that degeneration of secretory and neural elements comes later2-5. Most experimental work on aminoglycoside antibiotic ototoxicity has concentrated on the irreversible effects on inner ear functioning and hair cell morphology2,3. Little is known about their mode of action on the hair cells. Interference with hair cell protein synthesis, in analogy with the antimicrobial mechanism, has been ruled out as the primary cause of the high selective6 toxicity of the aminoglycosides3,5. Recent evidence indicates that these antibiotics may interfere with membrane phos-pholipid metabolism7. Morphological studies have demonstrated that early on in the ototoxic process the membrane of the sensory hairs is already damaged4. The aminoglycoside antibiotics are reported to have a direct and reversible8,9 effect on hair cell functioning in organs of the acoustico-lateralis system of lower vertebrates8-11. Hence, the primary site of action is probably the hair cell membrane. We have now therefore examined reversible effects of ototoxic antibiotics on single fibre afferent nerve activity and extracellular receptor potentials in the lateral-line organ of Xenopus laevis. Our results show that the antibiotics, in concentrations similar to those measured in the cochlear fluids3, have a dual action on the hair cells. They not only increase the spontaneous afferent nerve activity, probably by an effect on the hair cell membrane, but also severely impair the mechano-electric transduction process, resulting in a large phase lag of the receptor potentials. The latter effect, which is antagonised by Ca2+ may be due to interference of the antibiotics with the mechanical properties of the sensory hairs.
Ventricular arrhythmias can be initiated by a mechanism of transient diastolic dilation. To test the hypothesis that Ca2+ release from sarcoplasmic reticulum (SR) is important in initiation of such stretch-induced arrhythmias (SIAs), we studied effects of ryanodine in an isolated canine heart model. Arrhythmias were induced by a computerized ventricular volume servo-pump system that transiently increased left ventricular volume by precise amounts (delta V) during diastole. The probability of eliciting an SIA (PSIA) was compared at the minimum delta V that resulted in PSIA of > or = 90% under baseline conditions. Block of SR Ca2+ release with 10(-5) M ryanodine in 11 ventricles produced mild inhibition of SIAs, reducing PSIA by 19.4% (P = 0.039). Because ryanodine produces leakage of SR Ca2+ at low concentration and block of SR Ca2+ release at high concentration, ryanodine concentration was varied from 10(-9) to 10(-5) M in six ventricles. Ryanodine had minimal effect on PSIA over this concentration range. In six ventricles with elevated intracellular Ca2+ produced by pretreatment with 0.1-0.3 microM strophanthidin, 10(-5) M ryanodine did not significantly reduce PSIA. Probability of inducing ventricular pairs or nonsustained ventricular tachycardia was greater in strophanthidin-treated ventricles than in controls, but induction of these repetitive ventricular beats in the strophanthidin group was virtually abolished by addition of 10(-5) M ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)
1. EMD 57033 produces a positive inotropic effect by increasing the sensitivity of cardiac muscle myofilaments to calcium. Since the elevation in intracellular calcium produced by conventional inotropic compounds is thought to be arrhythmogenic, it is hoped that compounds such as EMD 57033 may increase cardiac output without exacerbating arrhythmias in patients with cardiac failure. This is the first study to examine whether EMD 57033 influences the susceptibility of the heart to ventricular arrhythmias.
2. We used the isolated working rat heart to investigate the effect of EMD 57033 on wall-stress-induced ventricular arrhythmia. Arrhythmias were induced by increases in ventricular afterload, and the effect of 2 μmol/l EMD 57033 on ventricular arrhythmias was investigated. The effect of 2 μmol/l EMD 57033 on contractility and arrhythmias was also assessed in the presence of different levels of perfusate calcium.
3. EMD 57033 was positively inotropic in the working rat heart, but it also produced a reversible increase in wall-stress-induced ventricular arrhythmia. The incidence of both ventricular ectopics and complex arrhythmias such as ventricular tachycardia were significantly increased by EMD 57033. Arrhythmias increased progressively as the level of perfusate calcium was raised within the physiological range.
4. The mechanism by which EMD 57033 increases wall-stress-induced arrhythmia is unclear, but it seems unlikely to be directly due to elevation of intracellular calcium. Further studies of the arrhythmogenic profile of this novel compound are required in a variety of models to assess its suitability and safety as a potentially therapeutic compound in heart failure.
Reperfusion following myocardial ischemia causes a rapid and transient release of inositol (1,4,5)triphosphate [Ins(1,4,5)P3]. The aim of this study was to test whether this increased Ins(1,4,5)P3 release was important for the development of ventricular arrhythmias and whether agents that inhibit this signal transduction pathway, such as aminoglycoside antibiotics, suppress arrhythmias.
In perfused rat hearts, ventricular tachycardia (VT), ventricular fibrillation (VF), and accumulation of Ins(1,4,5)P3 were measured during early reperfusion. A number of different compounds, including neomycin, gentamicin, streptomycin, spermine, reserpine, and prazosin, were effective in inhibiting the reperfusion-induced Ins(1,4,5)P3 release and the onset of VT and VF in parallel. A strong correlation existed between Ins(1,4,5)P3 content, measured at 2 minutes of reperfusion, and the incidence of reperfusion VT and VF. In addition, intravenous gentamicin suppressed the onset of arrhythmias under ischemic and reperfusion conditions in vivo.
Our results are consistent with the view that Ins(1,4,5)P3 release plays a pivotal role in mediating arrhythmias during early reperfusion. Agents inhibiting Ins(1,4,5)P3 release are antiarrhythmic and may have potential use clinically.
Gadolinium-sensitive stretch-activated channels have been implicated in the process of mechanotransduction signaling of ventricular myocardium. Such channels nonspecifically transport Na+ and Ca2+ in the inward direction. We tested the hypothesis that Na+ and Ca2+ influx are important in the genesis of stretch-induced arrhythmias (SIAs) in an isolated, blood-perfused canine ventricle. To elicit SIAs, left ventricular volume was transiently increased in early diastole using a computerized servo-pump system. Monophasic action potential recordings revealed stretch-induced depolarizations (SIDs) that preceded the arrhythmias. In five ventricles, raising the perfusate Ca2+ concentration from 1 to 3 mM increased ventricular sensitivity to SIAs, manifested by a decrease in the volume change required to precipitate an arrhythmia 50% of the time (delta V50) from 19.5 +/- 2.7 to 15.2 +/- 1.9 ml (P < 0.05). When the perfusate Na+ concentration was decreased from 150 to 90 mM in seven ventricles, delta V50 greatly increased (31.1 +/- 14.4 vs. 17.7 +/- 5.3 ml, P < 0.05), and SID amplitude decreased by 47% (P = 0.002). The suppression of SIAs with low extracellular Na+ is unlikely to be mediated by voltage-gated Na+ channels because lidocaine (5 mg/dl) did not alter SID amplitude. Thus the transsarcolemmal Na+ gradient (and probably that of Ca2+) modulates the amplitude of SIDs, which, in turn, initiate SIAs. These data provide initial evidence that Na+ and Ca2+ help mediate the mechanotransduction processes that underly the genesis of SIAs.
The aim was to investigate the effect of lowering external potassium on the sensitivity of the normal and hypertrophied rat heart to arrhythmias induced by increases in ventricular wall stress.
The isolated working heart model was used to compare hypertrophied hearts from the spontaneously hypertensive rat (SHR) with hearts from normotensive control rats (NCR) from the Wistar and Wistar-Kyoto strains. Young animals [131.5(SEM 0.64) days] were used to ensure uncomplicated left ventricular hypertrophy. Arrhythmias were induced by 20 s increases in ventricular wall stress. The ECG was recorded and the arrhythmic response of each heart was compared during perfusion with Tyrode solutions containing [K] 6, 4.8, 3.6, and 2.4 mM.
Hypertrophied SHR hearts showed a significantly greater arrhythmic response than control hearts at all levels of afterload increase when perfused with [K] 3.6 and 2.4 mM (t test P < 0.05 and P < 0.01). Both the number and complexity of arrhythmias were increased in the SHR hearts; ventricular tachycardia occurred in 10/12 compared with 4/12 control hearts whereas ventricular fibrillation occurred in 5/12 hearts but in none of the control hearts.
At higher levels of [K] the sensitivity of SHR and normal hearts to wall stress induced arrhythmias is similar. However, as [K] is lowered to 3.6 mM or below, hypertrophied hearts show a greatly enhanced response to increases in ventricular wall stress. They develop a larger number of ventricular ectopics and more complex ventricular arrhythmias when compared to normal hearts. This may be of relevance to arrhythmic sudden death in hypertensive patients in whom left ventricular hypertrophy, potassium depletion, and blood pressure lability is common. Excessive fluctuations in systolic pressure and therefore ventricular wall stress could provide a powerful arrhythmic stimulus in hypertensive patients with left ventricular hypertrophy, even before ischaemia, cardiac failure, or extensive extracellular fibrosis have developed.
The aim was to test the hypothesis that in single guinea pig ventricular myocytes a large stretch induced increase in resting calcium was sensitive to the mechanosensitive channel blocker streptomycin.
Carbon fibres were used to stretch cells loaded with the fluorescent calcium indicator indo-1. Force, sarcomere length, and internal calcium activity ([Ca2+]i) were measured.
In approximately 60% of the cells studied, a stretch which increased sarcomere length by approximately 6% caused a large increase in [Ca2+]i (up to 60% of the size of a [Ca2+]i transient at 0.25 Hz). When a mixture of antibiotics (streptomycin-penicillin) was used in solutions to isolate and store cells, this phenomenon was never observed (n = 19 cells). Direct application of physiological saline solution (PSS) could not reverse the increase in [Ca2+]i within 60 s of application (n = 7 cells). Direct application of penicillin [1000 IU per 50 ml (40 microM)] reversed the increase in [Ca2+]i within 60 s of application in only 3/7 cells. In contrast direct application of the aminoglycoside antibiotic streptomycin (40 microM) rapidly reversed the large increase in [Ca2+]i induced by stretch in each of 13 cells [within 18(SD 10) s of application]. Acute application of 40 microM streptomycin did not modify L-type Ca2+ currents measured under whole cell patch clamp conditions. Measurement of the resting tension--sarcomere length curves in cells stored in solution containing streptomycin and penicillin revealed two populations of cells on the basis of their stiffness.
This stretch induced increase in [Ca2+]i may be associated with stretch activated arrhythmias in the heart. The effects of streptomycin are consistent with its reported inhibitory action on stretch activated channels.
We show that gadolinium (Gd3+) is a potent calcium channel blocker in guinea-pig isolated ventricular myocytes. A dose-dependent inhibition of ICaL was found with an EC50 of 1.4 microM and a complete inhibition at 10 microM Gd3+. When compared with Cd2+, it appeared that the blockade of ICaL is a complex phenomenon probably involving more than one site of interaction (a Hill coefficient of 1.6 was found for Gd3+ vs. 1.0 for Cd2+). It is concluded that Gd3+ ions completely block ICaL at concentrations used to block stretch-activated channels (SAC), rendering its use as a specific SAC inhibitor problematic.
Ca2+ overload is known to play a major role in cell dysfunctioning in ischaemia/reperfusion and in cardiac glycoside intoxication. Suppression of Ca2+ overload or its consequences may therefore improve cellular function in these pathological conditions. Recent evidence suggests that Ca2+ overload occurs secondary to Na+ overload. Both depressed efflux and increased influx mechanisms have been mentioned as factors contributing to Na+ load. Prevention of this initial Na+ overload, without interfering with the normal Na+ current during the action potential, may therefore represent a novel pharmacological approach in the management of Ca2+ overload. The new cardioprotective drug R 56865 potently protects the heart against Ca2+ overload: ischaemia induced and ouabain induced arrhythmias and cell death are prevented in the absence of negative inotropism (no L-type Ca2+ channel blockade). At least three interactions at the cellular level may be held responsible for protection in these conditions. First, excessive Na+ entry into myocardial cells due to non-inactivating Na+ channels in depolarised cells is inhibited at concentrations that do not affect action potential configuration or contractile force. This leads to prevention of Na+ overload and subsequent Ca2+ overload and cell death. Second, R 56865 inhibits the transient inward current in Ca(2+)-(over)loaded cells, thus effectively preventing after-depolarisations and triggered propagated contractions. It has been proposed that R 56865, independent of its action on Na+ loading, might reduce oscillatory Ca2+ release from the intracellular Ca2+ stores, without interfering with the normal release mechanisms. Third, the drug attenuates K+ efflux in Na+ and Ca2+ loaded cells. In this way, R 56865 may contribute to prevention of action potential shortening and inhomogeneous repolarisation.(ABSTRACT TRUNCATED AT 250 WORDS)
Anion conductance is known to be activated by mechanical stimulation, such as osmotic cell swelling or cell inflation via the patch pipette, of canine or rabbit cardiac myocytes. The effects of mechanical stimulation on time-dependent currents, however, remain unsettled. Using the whole-cell voltage-clamp method, we have found that mechanical stimuli enhance the L-type Ca2+ current (ICa,L) in rabbit cardiac myocytes. At every membrane potential, ICa,L was reversibly increased by osmotic cell swelling and by cell inflation caused by applying a positive pressure of 10 to 15 cm H2O via the patch pipette. ICa,L was increased during cell inflation by 37 +/- 21% (mean +/- SD, n = 17) in atrial cells and by 37 +/ -8% (n = 7) in sinoatrial node cells in solution containing 2 mmol/L Ca2+. The current-voltage relationship, the inactivation time constant, the steady state inactivation curve, and the conductance properties of ICa,L were all virtually unaffected by mechanical stimulation except for the open probability, which appears to increase. The increase in ICa,L was not dependent on protein kinase A, since an inhibitor peptide of cAMP-dependent protein kinase failed to prevent the increase in ICa,L during mechanical stimuli (n=5). The increase in ICa,L caused by cell inflation was unaffected by the chelation of intracellular Ca2+ by the addition of 10 mmol/L EGTA or 10 mmol/L BAPTA to the pipette solution, suggesting that the effect was not mediated by changes in intracellular Ca2+. Thus, mechanical stimulation due to cell swelling or inflation may itself directly increase ICa,L in rabbit cardiac myocytes.
To investigate the influence of external magnesium on wall stress-induced arrhythmias and to establish whether early cardiac hypertrophy affects the sensitivity of the heart to these arrhythmias.
We used a modified isolated working heart model to compare hypertrophied hearts from the spontaneously hypertensive rat (SHR) with hearts from normotensive control rats from the Wistar and Wistar-Kyoto strains. Young rats (aged 137.9 +/- 2.04 days mean +/- SEM) were used to ensure left ventricular hypertrophy uncomplicated by fibrosis and cardiac failure. Arrhythmias were induced by 20 s increases in ventricular afterload during which the electrocardiogram was recorded.
In control experiments SHR hearts showed a significantly greater arrhythmic response than did normotensive control rat hearts during perfusion with low-potassium (2.4 mmol/l) solutions. Concomitant low magnesium (0.4 mmol/l) perfusion significantly increased the arrhythmic response to 2.4 mmol/l potassium in SHR hearts but had no effect on arrhythmias in normotensive control rat hearts. With low magnesium, the number of ventricular premature beats was increased in SHR hearts compared with that in normotensive control rat hearts during perfusion with 4.2 and 2.4 mmol/l potassium. In addition, perfusion with low magnesium and 2.4 mmol/l potassium led to an increase in the complexity of arrhythmias, ventricular tachycardia occurring in nine of 10 SHR hearts but in only two of six normotensive control rat hearts.
There is a synergistic effect of low magnesium and low potassium on wall-stress-induced arrhythmia in the SHR heart. An analogous effect might be important in patients with hypertension and left ventricular hypertrophy, in whom systolic blood pressure is frequently labile, leading to wide fluctuations in ventricular wall stress; combined potassium and magnesium depletion is a common complication of antihypertensive treatment.
Prior studies have shown that the performance of the left ventricle on any one beat is influenced by the mechanical events of the previous beat, a phenomenon called "previous-beat contraction history". This previous-beat contraction history, which appears to be an interplay between the mechanical events of one contraction and the activation state of the next contraction, could depend, at least in part, on mechanosensitive ion channels. The purpose of this study, therefore, was to test the hypothesis that mechanosensitive ion channels contribute to previous-beat contraction history: If previous-beat contraction history depends on mechanosensitive ion channels, the magnitude of its effect should be decreased by blocking mechanosensitive ion channels.
We performed experiments in buffer-perfused isolated rabbit hearts in which left ventricular pressure and volume were controlled with a servo-motor system. We evaluated the pulse interval-dependent expression of previous-beat contraction history under control conditions (no drug) and in the presence of 100 and 500 microM streptomycin, a blocker of mechanosensitive ion channels.
Under control conditions, previous-beat contraction history nor its dependence on pulse interval was influenced significantly by either concentration of streptomycin.
Mechanosensitive ion channels do not play a role in the expression of previous-beat contraction history in the left ventricle of the isolated rabbit heart.