Cellular uncoupling during ischemia in hypertrophied and failing rabbit ventricular myocardium: effects of preconditioning

Effects of Rotigaptide, a Gap Junction Modifier, on Defibrillation Energy and Resuscitation From Cardiac Arrest in Rabbits

The gap junction modifier Rotigaptide (ZP123), which promotes cellular coupling, was hypothesized to decrease defibrillation thresholds during prolonged ventricular fibrillation (VF). Thirty-two New Zealand white rabbits were randomized to receive saline (control, n = 16) or Rotigaptide (n = 16). Following 4 min of untreated VF, biphasic defibrillation shocks were applied through chest wall patches, starting either at 300 volts (V) (n = 16) or 500 V (n = 16), with 200 V increasing steps to 900 V in case of shock failure. Rotigaptide significantly decreased defibrillation voltage requirements (average cumulative voltage of all shocks: 1206 ± 709 V in control group vs. 844 ± 546 V in treated group, P = .002). Rotigaptide had no effect on heart rate, QRS duration, QT interval, ventricular effective refractory period, monophasic action potential duration or on connexin 43 density using immunofluorescence. Rotigaptide improves the ability to defibrillate after untreated VF.

A new approach for resolution of complex tissue impedance spectra in hearts

This study was designed to test the feasibility of using sinusoidal approximation in combination with a new instrumentation approach to resolve complex impedance (uCI) spectra from heart preparations. To assess that feasibility, we applied stimuli in the 10-4000 Hz range and recorded potential differences (uPDs) in a four-electrode configuration that allowed identification of probe constants (Kp) during calibration that were in turn used to measure total tissue resistivity ρt from rabbit ventricular epicardium. Simultaneous acquisition of a signal proportional to the supplied current (Vstim) with uPD allowed identification of the V- I ratio needed for ρt measurement, as well as the phase shift from Vstim to uPD needed for uCI spectra resolution. Performance with components integrated to reduce noise in cardiac electrophysiologic experiments, in particular, and provide accurate electrometer-based measurements, in general, was first characterized in tests using passive loads. Load tests showed accurate uCI recovery with mean uPD SNRs between 10 (1) and 10 (3) measured with supplied currents as low as 10 nA. Comparable performance characteristics were identified during calibration of nine arrays built with 250 μm Ag/AgCl electrodes, with uCIs that matched analytic predictions and no apparent effect of frequency ( F = 0.12, P = 0.99). The potential ability of parasitic capacitance in the presence of the electrode-electrolyte interface associated with the small sensors to influence the uCI spectra was therefore limited by the instrumentation. Resolution of uCI spectra in rabbit ventricle allowed measurement of ρt = 134 ± 53 Ω· cm. The rapid identification available with this strategy provides an opportunity for new interpretations of the uCI spectra to improve quantification of disease-, region-, tissue-, and species-dependent intercellular uncoupling in hearts.

Effects of ischemic preconditioning on ischemia/reperfusion-induced arrhythmias by upregulatation of connexin 43 expression

BACKGROUND

The susceptibility of hypertrophied myocardium to ischemia-reperfusion injury is associated with increased risk of postoperative arrhythmias. We investigate the effects of ischemic preconditioning (IP) on post-ischemic reperfusion arrhythmias in hypertrophic rabbit hearts.

METHODS

Thirty-three rabbit models of myocardial hypertrophy were randomly divided into three groups of 11 each: non-ischemia-reperfusion group (group A), ischemia-reperfusion group (group B), and ischemic preconditioning group (group C). Another ten healthy rabbits with normal myocardium served as the healthy control group. Rabbit models of myocardial hypertrophy were induced by abdominal aortic banding. Surface electrocardiogram (ECG) was recorded and Curtis-Ravingerova score was used for arrhythmia quantification. Connexin 43 (Cx43) expression was assessed by immunohistochemistry.

RESULTS

Ratios of heart weight to body weight and left ventricular weight to body weight increase significantly in the three groups compared with the healthy control group (p < 0.05). Arrhythmia incidence in group C is significantly lower than group B (p < 0.05). Curtis-Ravingerova score in group C is lower than group B (p < 0.05). Cx43 expression area in group A is smaller by comparison with the healthy control group (p < 0.05). Cx43 expression area and fluorescence intensity in group B are reduced by 60.9% and 23.9%, respectively, compared with group A (p < 0.05). In group C, Cx43 expression area increases by 32.5% compared with group B (p < 0.05), and decreases by 54.8% compared with group A (p < 0.05).

CONCLUSIONS

The incidence of ischemia/reperfusion-induced arrhythmias in hypertrophic rabbit hearts decreases after IP, which plays an important protecting role on the electrophysiology of hypertrophied myocardium by up-regulating the expression of Cx43.

The persistent sodium current blocker riluzole is antiarrhythmic and anti-ischaemic in a pig model of acute myocardial infarction

BACKGROUND

The potential of the cardiac persistent sodium current as a target for protection of the myocardium from ischaemia and reperfusion injury is gaining increasing interest. We have investigated the anti-ischaemic and antiarrhythmic effects of riluzole, a selective INaP blocker, in an open chest pig model of infarction.

METHODS AND PRINCIPAL FINDINGS

The left anterior descending coronary artery (LAD) was ligated in 27 anesthetised pigs (landrace or large white, either sex, 20-35 kg) which had received riluzole (8 mg/kg IP; n = 6), lidocaine (2.5-12 mg/kg bolus plus 0.05-0.24 mg/kg/min; n = 11) or vehicle (n = 10) 50 min prior. Arrhythmias could be delineated into phase 1a (0 to 20 min), phase 1b (20 to 50 min) and phase 2 (from 50 min to termination at 180 min) and were classified as premature ventricular contractions (PVCs), non-sustained ventricular tachycardia (VT) or ventricular fibrillation (VF) (spontaneously reverting within 15 s) or sustained VT or VF (ie. requiring cardioversion at 15 s). Riluzole reduced the average number of all arrhythmias in Phase 2 (PVCs from 484+/-119 to 32+/-13; non sustained arrhythmias from 8.9+/-4.4 to 0.7+/-0.5; sustained arrhythmias from 3.9+/-2.2 to 0.5+/-0.4); lidocaine reduced the average number of non-sustained and sustained arrhythmias (to 0.4+/-0.3 and 0.4+/-0.3 respectively) but not PVCs (to 390+/-234). Riluzole and lidocaine reduced the average number of sustained arrhythmias in phase 1b (from 1.8+/-0.4 to 0.17+/-0.13 (p<0.02) and to 0.55+/-0.26 (p = ns) respectively). Neither lidocaine or riluzole changed the ECG intervals: there was no statistical significance between groups at time zero (just before ligation) for any ECG measure. During the course of the 3 hour period of the ischaemia R-R, and P-R intervals shortened slightly in control and riluzole groups (not significantly different from each other) but not in the lidocaine group (significantly different from control). QRS and QTc did not change appreciably in any group Riluzole reduced the degree of histopathological tissue damage across the infarct zone considerably more than did lidocaine.

CONCLUSIONS

At the doses used, riluzole was at least as effective as lidocaine at reducing the number of episodes of ischaemic VT or VF in pigs, and much more effective at reducing the number of PVCs. We propose that this is related to the ability of riluzole to block cardiac persistent sodium current.

Acute ischemia of canine interventricular septum produces asymmetric suppression of conduction

BACKGROUND

Acute ischemia depresses tissue excitability more rapidly in the epicardium than in the endocardium of the canine left ventricular (LV) free wall. However, the effects of acute ischemia on conduction in the interventricular septum (IVS), which is composed of right ventricular (RV) and LV endocardium and midmyocardium without epicardium, are less known.

OBJECTIVE

The purpose of this study was to evaluate the hypothesis that the IVS exhibits transseptal differences in local tissue response to acute ischemia.

METHODS

Isolated canine IVS preparations were perfused through the septal branch of the anterior descending coronary artery, and conduction on the cut-exposed transseptal surfaces was optically mapped before and after two sequential episodes of 8 minutes of global ischemia (separated by >60 minutes of reperfusion). The preparations were paced alternately between the RV endocardium and LV endocardium at cycle lengths of 250, 300, and 1,500 ms.

RESULTS

Prior to ischemia, transseptal conduction was radial and symmetric during either RV endocardial or LV endocardial pacing at all cycle lengths. Eight minutes of ischemia depressed conduction velocity more in the RV half than in the LV half of the IVS and caused local conduction block in the sub-RV endocardium, especially during rapid pacing. The K(ATP) channel blocker glibenclamide (10 micromol/L) prevented development of this transseptal asymmetry and conduction block during ischemia.

CONCLUSION

Acute global ischemia increased transseptal heterogeneity and induced sub-RV endocardial block of conduction via activation of the ATP-sensitive potassium current. Such changes could contribute to initiation of arrhythmia in patients with septal infarction.

Ischemic preconditioning: from molecular mechanisms to therapeutic opportunities

Ischemia/reperfusion (I/R) injury is a major contributory factor to cardiac dysfunction and infarct size that determines patient prognosis after acute myocardial infarction. Considerable interest exists in harnessing the heart's endogenous capacity to resist I/R injury, known as ischemic preconditioning (IPC). The IPC research has contributed to uncovering the pathophysiology of I/R injury on a molecular and cellular basis and to invent potential therapeutic means to combat such damage. However, the translation of basic research findings learned from IPC into clinical practice has often been inadequate because the majority of basic research findings have stemmed from young and healthy animals. Few if any successful implementations of IPC have occurred in the diseased hearts that are the primary target of viable therapies activating cardioprotective mechanisms to limit cardiac dysfunction and infarct size. Therefore, the first purpose of this review is to facilitate understanding of pathophysiology of I/R injury and the mechanisms of cardioprotection afforded by IPC in the normal heart. Then I focus on the problems and opportunities for successful bench-to-bedside translation of IPC in the diseased hearts.

Interaction of cardiovascular risk factors with myocardial ischemia/reperfusion injury, preconditioning, and postconditioning

Therapeutic strategies to protect the ischemic myocardium have been studied extensively. Reperfusion is the definitive treatment for acute coronary syndromes, especially acute myocardial infarction; however, reperfusion has the potential to exacerbate lethal tissue injury, a process termed "reperfusion injury." Ischemia/reperfusion injury may lead to myocardial infarction, cardiac arrhythmias, and contractile dysfunction. Ischemic preconditioning of myocardium is a well described adaptive response in which brief exposure to ischemia/reperfusion before sustained ischemia markedly enhances the ability of the heart to withstand a subsequent ischemic insult. Additionally, the application of brief repetitive episodes of ischemia/reperfusion at the immediate onset of reperfusion, which has been termed "postconditioning," reduces the extent of reperfusion injury. Ischemic pre- and postconditioning share some but not all parts of the proposed signal transduction cascade, including the activation of survival protein kinase pathways. Most experimental studies on cardioprotection have been undertaken in animal models, in which ischemia/reperfusion is imposed in the absence of other disease processes. However, ischemic heart disease in humans is a complex disorder caused by or associated with known cardiovascular risk factors including hypertension, hyperlipidemia, diabetes, insulin resistance, atherosclerosis, and heart failure; additionally, aging is an important modifying condition. In these diseases and aging, the pathological processes are associated with fundamental molecular alterations that can potentially affect the development of ischemia/reperfusion injury per se and responses to cardioprotective interventions. Among many other possible mechanisms, for example, in hyperlipidemia and diabetes, the pathological increase in reactive oxygen and nitrogen species and the use of the ATP-sensitive potassium channel inhibitor insulin secretagogue antidiabetic drugs and, in aging, the reduced expression of connexin-43 and signal transducer and activator of transcription 3 may disrupt major cytoprotective signaling pathways thereby significantly interfering with the cardioprotective effect of pre- and postconditioning. The aim of this review is to show the potential for developing cardioprotective drugs on the basis of endogenous cardioprotection by pre- and postconditioning (i.e., drug applied as trigger or to activate signaling pathways associated with endogenous cardioprotection) and to review the evidence that comorbidities and aging accompanying coronary disease modify responses to ischemia/reperfusion and the cardioprotection conferred by preconditioning and postconditioning. We emphasize the critical need for more detailed and mechanistic preclinical studies that examine car-dioprotection specifically in relation to complicating disease states. These are now essential to maximize the likelihood of successful development of rational approaches to therapeutic protection for the majority of patients with ischemic heart disease who are aged and/or have modifying comorbid conditions.

Protection of the abnormal heart

Myocardial ischemia and reperfusion injury have been extensively investigated in the laboratory mainly in healthy tissues. However, in clinical settings, ischemic heart disease coexists with certain illnesses, which could potentially influence the response of the myocardium to ischemia and reperfusion. Recent research has revealed that the abnormal heart may not be always vulnerable to ischemic injury. Furthermore, the effect of powerful means of protection, such as ischemic preconditioning, may not be in operation under certain pathological conditions. With this evidence in mind, the present review will focus on the response of the abnormal heart to ischemia and reperfusion, the possible underlying mechanisms, and potential cardioprotective strategies.

Reduction of ST elevation in repeated coronary occlusion model depends on both altered metabolic response and conduction property

The aim of this study was to elucidate the mechanisms of altered electrical response to ischemia in repeated coronary occlusion model. To test its dependence on metabolic response, extracellular K+ concentration (eKC), myocardial pH and PCO2 were simultaneously measured with epicardial ECG during three consecutive 4 min of left anterior descending coronary artery (LAD) occlusion separated by 15 min of reperfusion in canine hearts. ECG changes induced by infusion of high K+-buffer (10 mM) into the coronary arterial bed via carotid artery-LAD bypass (referred to as high K+-challenges: HKC) were also tested prior to (the first HKC), and during each reperfusion period (the second to the fourth HKC). ST elevation was significantly reduced in subsequent occlusions (3.14 +/- 0.48 and 2.98 +/- 0.47 mV in the second and third occlusion, both P<0.05, compared to 4.91 +/- 0.78 mV in the first). This was accompanied by significant attenuation of the changes in eKC, tissue pH and PCO2. ST elevation induced by HKC also significantly reduced after repeated occlusion (4.09 +/- 0.79 mV in the fourth HKC vs. 5.64 +/- 0.68 mV in the first, P<0.05) in spite of the identical changes in eKC during HKC. This progressive decrease in ST changes by HKC was rather consistent with augmented conduction delay (86.4 +/- 7.1% increase in activation time in the fourth vs. 54.3 +/- 3.4% in the first, P<0.01). These findings indicate that repeated ischemia induces altered electrical response to subsequent ischemia based on both attenuated metabolic response and altered conduction property.

Preconditioning in intact and previously diseased myocardium: laboratory or clinical dilemma?

We studied the effects of various cycles of preconditioning (PC) (one cycle, 1 x PC; two cycles, 2 x PC; three cycles, 3 x PC; and four cycles, 4 x PC) on cardiac function, infarct size, and the incidence of reperfusion-induced arrhythmias in isolated hearts obtained from rabbits with hypercholesterolemia. After 8 weeks of hypercholesterolemia, hearts were subjected to 30 min of ischemia followed by 120 min of reperfusion. Various cycles of PC resulted in a "cycle-dependent" reduction in infarct size in the age-matched nonhypercholesterolemic group. In the 8-week hypercholesterolemic group, increasing cycles of PC resulted in a significant increase in infarct size from their nonpreconditioned ischemic/reperfused control value of 44 +/- 5% to 45 +/- 6%, 49 +/- 5%, 59 +/- 6% (p < 0.05), and 58 +/- 5% (p < 0.05), respectively. PC increased the vulnerability of the myocardium to reperfusion-induced arrhythmias in hypercholesterolemics indicating that PC may be an "intact heart" phenomenon. The effects of PC appear currently to be a dilemma in laboratories and clinics. The solution to the problem of PC in intact and diseased myocardium requires further data from two different sources: (a) previously "diseased" animals, and (b) diseased human myocardium from clinics. Once these data are available, then the effects under which PC will be beneficial rather than harmful could be established and the dilemma solved.

Behavior of ectopic surface: effects of beta-adrenergic stimulation and uncoupling

By using both experimental and theoretical means, we have addressed the progression of ectopic activity from individual cardiac cells to a multicellular two-dimensional network. Experimental conditions that favor ectopic activity have been created by local perfusion of a small area of cardiomyocyte network (I-zone) with an isoproterenol-heptanol containing solution. The application of this solution initially slowed down and then fully blocked wave propagation inside the I-zone. After a brief lag period, ectopically active cells appeared in the I-zone, followed by evolution of the ectopic clusters into slowly propagating waves. The changing pattern of colliding and expanding ectopic waves confined to the I-zone persisted for as long as the isoproterenol-heptanol environment was present. On restoration of the control environment, the ectopic waves from the I-zone broke out into the surrounding network causing arrhythmias. The observed sequence of events was also modeled by FitzHugh-Nagumo equations and included a cell's arrangement of two adjacent square regions of 20 x 20 cells. The control zone consisted of well-connected, excitable cells, and the I-zone was made of weakly coupled cells (heptanol effect), which became spontaneously active as time evolved (isoproterenol effect). The dynamic events in the system have been studied numerically with the use of a finite difference method. Together, our experimental and computational data have revealed that the combination of low coupling, increased excitability, and spatial heterogeneity can lead to the development of ectopic waves confined to the injured network. This transient condition appears to serve as an essential step for the ectopic activity to "mature" before escaping into the surrounding control network.

Transmural reentry triggered by epicardial stimulation during acute ischemia in canine ventricular muscle

Ischemia depresses tissue excitability more rapidly in the ventricular epicardium than in the endocardium. We hypothesized that this would provide the substrate for transmural reentry originating in the epicardium. We mapped transmural conduction in isolated and perfused wedges taken from canine left ventricles during global ischemia while pacing alternately between the epicardium and endocardium. Ischemia reduced conduction velocity more in the epicardium than in the endocardium. We observed that the epicardial-initiated activation penetrated the ventricular wall transmurally while failing to conduct laterally along the epicardium, then conducted laterally along the endocardium and midmyocardium, and reentered the epicardium in 9 of 16 wedges during epicardial stimulation after 600 +/- 182 s of ischemia. Endocardial stimulation applied immediately before or after the epicardial stimulation initiated activation that spread quickly along the endocardium and then transmurally to the epicardium without reentry in six of the nine wedges. The transmural asymmetric conduction was not observed in four separate wedges after the endocardium was removed. Therefore, ischemia-induced transmural gradient of excitability provided the substrate for reentry during epicardial stimulation.

Origin of ischemia-induced phase 1b ventricular arrhythmias in pig hearts

OBJECTIVES

The goal of this study was to establish the role of ventricular filling on the 1b phase of ischemia-induced arrhythmias.

BACKGROUND

Ischemia-induced ventricular arrhythmias occur in two phases. The mechanism of the initiation of delayed (1b) arrhythmias is unknown. The 1b arrhythmias (15 to 60 min of ischemia) are abundant in in situ hearts but scarce in isolated perfused hearts (with drained ventricles).

METHODS

Left ventricular (LV) epicardial mapping (11 x 11 matrix, 5 mm interelectrode distance) of the initiation of delayed arrhythmias was performed in open-chested pigs (group A, n = 7) and isolated pig hearts without (group B, n = 8) and with a filled intraventricular balloon (group C, n = 5).

RESULTS

There were no differences in ischemic zone size between groups. The ischemia-induced rise in tissue impedance was similar in groups A and B. Arrhythmias were less frequent and less severe in group B than in groups A or C, with no differences between groups A and C. An epicardial focal origin was detected in 26% of all first beats, significantly more from the ischemic border than from elsewhere. During a pacing protocol with a long pause (a separate group of four isolated hearts with a balloon), more premature beats occurred in the first postpause interval than in any other interval.

CONCLUSIONS

In isolated hearts 1b arrhythmias were less frequent and less severe than in working preparations. Focal activity was documented in 26% of arrhythmias and emerged from the ischemic border. Postpause contractile potentiation was associated with more arrhythmias. Our study suggests that the initiation of ischemia-induced 1b arrhythmias is related to LV wall stress.

Transmural reentry during acute global ischemia and reperfusion in canine ventricular muscle

Coronary occlusion and reperfusion produce tachyarrhythmias. We tested the hypothesis that variations in transmural activation after global ischemia and reperfusion were responsible for arrhythmias. We arterially perfused 36 isolated transmural wedges from canine left ventricular free walls. After ≥100 min of stabilization, the artery was occluded for 25 min, followed by reperfusion at various flow rates. We recorded 256 channels of fluorescent action potentials on transmural surfaces from preocclusion to >15 min after reperfusion. During endocardial pacing at 300 ms, ischemia of ≥570 ± 165 s ( n = 34) produced 1:1 endocardial conduction and then 2:1 and 4:1 block as the wave fronts conducted toward epicardium. Transmural reentry appeared after 535 ± 146 s of ischemia ( n = 31). Further ischemia caused epicardial inactivation and eliminated reentry ( n = 24). During reperfusion, tissues progressed through sequences of epicardial inactivation and reappearance of activation with 1:1, 2:1, and 4:1 conduction; both sustained and nonsustained reentry occurred. We conclude that heterogeneous activation responses to endocardial pacing during acute ischemia provide the substrate for initiating reentry, suppressed reentry during further ischemia, and caused reentry during reperfusion.

Failure to precondition pathological human myocardium

OBJECTIVES

We investigated the effects of ischemic preconditioning (PC) on diabetic and failing human myocardium and the role of mitochondrial KATP channels on the response in these diseased tissues.

BACKGROUND

There is conflicting evidence to suggest that PC is a healthy heart phenomenon.

METHODS

Right atrial appendages were obtained from seven different groups of patients: nondiabetics, diet-controlled diabetics, noninsulin-dependent diabetics (NIDD) receiving KATP channel blockers, insulin-dependent diabetics (IDD), and patients with left ventricular ejection fraction (LVEF) >50%, LVEF between 30% and 50% and LVEF <30%. After stabilization, the muscle slices were randomized into five experimental groups (n = 6/group): 1) aerobic control-incubated in oxygenated buffer for 210 min, 2) ischemia alone-90 min ischemia followed by 120 min reoxygenation, 3) preconditioning by 5 min ischemia/5 min reoxygenation before 90 min ischemia/120 min reoxygenation, 4) diazoxide (Mito KATP opener, 0.1 mm)-for 10 min before the 90 min ischemia/120 min reoxygenation and 5) glibenclamide (10 microm)-10 min exposure prior to PC (only in the diabetic patient groups). Creatine kinase leakage into the medium (CK, U/g wet wt) and MTT reduction (OD/mg wet wt), an index of cell viability, were assessed at the end of the experiment.

RESULTS

Ischemia caused similar injury in both normal and diseased tissue. Preconditioning prevented the effects of ischemia in all groups except NIDD, IDD and poor cardiac function (<30%). In the diazoxide-treated groups, protection was mimicked in all groups except the NIDD and IDD groups. Interestingly, glybenclamide abolished protection in nondiabetic and diet-controlled NIDD groups and did not affect NIDD groups receiving KATP channel blockers or IDD groups.

CONCLUSIONS

These results show that failure to precondition the diabetic heart is due to dysfunction of the mitochondrial KATP channels and that the mechanism of failure in the diabetic heart lies in elements of the signal transduction pathway different from the mitochondrial KATP channels.

Animal models of chronic heart failure

Chronic heart failure is associated with multiple pathophysiological alterations and adaptations, such as marked anatomic and biochemical changes of the myocardium, left ventricular dysfunction and dilatation, increased systemic vascular resistance, and activation of neurohumoral and cytokine systems. The use of animal models has provided a new insight into the complex pathogenesis of this syndrome and supplemented clinical experience. However, all of the animal models used have advantages and limitations, and the transfer from experimental to human heart failure needs critical evaluation. The current review will focus upon new aspects of rat and rabbit models of heart failure.

Depressed tolerance to fluorocarbon-simulated ischemia in failing myocardium due to impaired [Ca(2+)](i) modulation

The aim of this study was to investigate the tolerance of failing myocardium from postinfarction rats to simulated ischemia. Myocardial infarction (MI) was induced by ligation of the left coronary artery in male Wistar rats. Isometric force and free intracellular Ca(2+) concentration ([Ca(2+)](i)) were measured in isolated left ventricular papillary muscles from sham-operated and post-MI animals 6 wk after surgery. Ischemia was simulated by using fluorocarbon immersion with hypoxia. Results showed that mechanical performance was depressed during the period of hypoxia in physiological salt solution (44 +/- 7% of baseline in sham vs. 30 +/- 6% of baseline in MI, P < 0.05) or ischemia (16 +/- 2% of baseline in sham vs. 9 +/- 1% of baseline in MI, P < 0.01) accompanied by no corresponding decrease of peak [Ca(2+)](i) (hypoxia: 51 +/- 8% of baseline in sham vs. 46 +/- 7% of baseline in MI, P = NS; ischemia: 47 +/- 5% of baseline in sham, 39 +/- 7% of baseline in MI, P = NS). After reoxygenation, [Ca(2+)](i) rapidly returned to near preischemic basal levels, whereas developed tension in fluorocarbon remained significantly lower. This dissociation between peak [Ca(2+)](i) and isometric contractility was more pronounced in the failing myocardium from postinfarction rats. In conclusion, more severe impairment of [Ca(2+)](i) homeostasis in the failing myocardium from postinfarction rats increases susceptibility to ischemia-reperfusion injury.

Functional role of c-Src in gap junctions of the cardiomyopathic heart

Given the essential role played by gap junctions in the coordination of cardiac muscle contraction, it is plausible that down-regulation of gap junctional conduction is in part responsible for the contractile dysfunction observed in hypertrophied and failing hearts. In the present study, we analyzed the expression and function of the gap junction protein, connexin43, in the ventricular myocardium of hereditary cardiomyopathic, Syrian BIO 14.6 hamsters. Immunoprecipitation and immunoblot analyses revealed that levels of tyrosine phosphorylated connexin43 were increased in BIO 14.6 hamsters at the late stage of congestive heart failure. Furthermore, the increased tyrosine phosphorylation was correlated with increased c-Src activity. The functional consequences of tyrosine phosphorylation of connexin43 in gap junction were assessed using transfected cells expressing constitutively active c-Src. It was found that constitutively active c-Src diminished propagation of Ca(2+) waves in HEK293 cells and reduced gap junctional conductance between pairs of cardiac myocytes. We, therefore, conclude that during the progression of cardiac dysfunction in the cardiomyopathic heart, gap junctional communication is reduced via c-Src-mediated tyrosine phosphorylation of connexin43.