Decreased intercellular coupling after prolonged rapid stimulation in rabbit atrial muscle

Ischemic, genetic and pharmacological origins of cardiac arrhythmias: the contribution of the Quebec Heart Institute

Research in the field of basic electrophysiology at the Quebec Heart Institute (Laval Hospital, Quebec City, Quebec) has evolved since its beginning in the 1990s. Interests were focused on cardiac arrhythmias induced by drugs, allelic variants and metabolic factors produced during ischemia. The results have contributed to the creation of new standards in drug development, more specifically, testing all new drugs for their potential effects on cardiac potassium currents, which could produce life-threatening proarrhythmic effects. In a French-Canadian population, three heterozygous single nucleotide polymorphisms in hK(v)1.5, a gene encoding for a major atrial repolarizing current, were found. These variants affect the expression level of the hK(v)1.5 channel and change the inactivation process in the presence of its accessory beta subunit. Because these effects could shorten atrial action potential, their presence was tested in postcoronary bypass patients and a higher prevalence was found in patients with postoperative atrial fibrillation. Finally, three potentially proarrhythmic factors characteristic of ischemia were identified: pH decrease; oxygen free radicals, which both increase the flow of K(+) ions through human ether-a-go-go-related gene and hK(v)1.5, producing a reduction in action potential duration, frequently leading to cardiac arrhythmias; and lysophosphatidylcholine, a metabolite involved in the production of cardiac arrhythmias early during ischemia that was shown to be a major cause of electrical uncoupling. Over the past decade, the Quebec Heart Institute has provided a significant amount of original data in the field of basic cardiac electrophysiology, specifically concerning arrhythmias originating from pharmacological agents, genetic background and cardiac ischemia.

Influence of anisotropic conduction properties in the propagation of the cardiac action potential

Anisotropy, the property of being directionally dependent, is ubiquitous in nature. Propagation of the electrical impulse in cardiac tissue is anisotropic, a property that is determined by molecular, cellular, and histological determinants. The properties and spatial arrangement of connexin molecules, the cell size and geometry, and the fiber orientation and arrangement are examples of structural determinants of anisotropy. Anisotropy is not a static property but is subject to dynamic functional regulation, mediated by modulation of gap junctional conductance. Tissue repolarization is also anisotropic. The relevance of anisotropy extends beyond normal propagation and has important implications in pathological states, as a potential substrate for abnormal rhythms and reentry.

Electrophysiologic properties and ventricular fibrillation in normal and myopathic hearts

This study tests the hypothesis that moderate myocardial dysfunction is associated with altered myocardial anisotropic properties and structurally altered ventricular fibrillation (VF). Mongrel dogs were randomized to either a control group or a group that was rapidly paced at 250 beats/min until the left ventricular ejection fraction was [Formula: see text] 40%. Changes in anisotropic properties and the electrical characteristics of VF associated with the development of moderate myocardial dysfunction were assessed by microminiature epicardial mapping studies. In vivo conduction, refractory periods, and repolarization times were prolonged in both longitudinal and transverse directions in myopathic animals versus controls. VF was different in myopathic versus control animals. There were significantly more conducted deflections during VF in normal hearts compared with myopathic hearts. Propagated deflection-to-deflection intervals during VF were significantly longer in myopathic hearts compared with controls (125.5 ± 49.06 versus 103.4 ± 32.9 ms, p = 0.009). There were no abnormalities in cell size, cell shape, or the number of intercellular gap junctions and there was no detectable change in the expression of the gap junction proteins Cx43 and Cx45. Moderate myocardial dysfunction is associated with significant electrophysiological abnormalities in the absence of changes in myocardial cell morphology or intercellular connections, suggesting a functional abnormality in cell-to-cell communication.Key words: cardiomyopathy, anisotropy, fibrillation, defibrillation.

The protective effect of D-sotalol against hypoxia-induced myocardial uncoupling

The effects of D-sotalol on intercellular electrical coupling and ultrastructure under hypoxic conditions were investigated in myocardial samples from eight young (1-2 months) and four older (10-12 months) guinea pigs. A right ventricular muscle strip was kept simultaneously in two divided chambers and superfused with normoxic and/or hypoxic (97% N2+ 3% Co2) Krebs solution. Hypoxia caused shortening of action potential duration (APD) and electrical cell-to-cell uncoupling. If the uncoupling appeared after short-term hypoxia (less than 30 min), administration of 3.10(-7)M of D-sotalol to the hypoxic perfusate led to a recovery of electrical coupling. Transmission electron microscopy revealed moderate reversible ultrastructural alterations of the cardiomyocytes. No apparent changes in intercellular junctions were observed. The recoupling effect of sotalol decreased with the time of hypoxia as the ultrastructural damage progressed. After prolonged hypoxia (more than 30 min), cardiomyocytes were markedly injured, intercellular junctions were severely affected, and gap junctions occurred less frequently. In these cases, administration of D-sotalol caused only transient recoupling. After 1 h of hypoxia, no recoupling was observed. Pretreatment with D-sotalol prevented hypoxia-induced electrical uncoupling and markedly attenuated ultrastructural damage, although shortening of APD still persisted. Our results indicate that the cardioprotective effect of D-sotalol on electrical intercellular coupling is closely associated with sotalol-induced prevention of the ultrastructural damage. Considering previous results, we suggest that this protective effect of D-sotalol may be related to its ability to increase intracellular cyclic adenosine monophosphate and, thereby, to decrease cytosolic free Ca. These effects can explain the antiarrhythmic and defibrillating properties of D-sotalol.

Is cyclic AMP involved in the defibrillating effect of sotalol?

Ventricular fibrillation induced in animals pretreated with sotalol, a class III antiarrhythmic agent, would spontaneously terminate and revert into a sinus rhythm. This phenomenon has been attributed to the class III action of this drug, i.e., prolongation of myocardial action potential duration and effective refractory period. Since various observations suggested that these alone cannot explain the defibrillating phenomenon, we hypothesised that sotalol affected ventricular intercellular synchronization by increasing intercellular coupling. Our recent experimental studies have shown that sotalol antagonized the cellular decoupling to guinea pig ventricular muscle strip caused by perfusion with either a hypoxic normal Tyrode's solution or an oxygenated high Ca2+ Tyrode's solution. We assumed that the most likely mechanism for the restoration of intercellular coupling would be increasing intracellular cAMP concentration. In order to test this hypothesis, we studied the modification of this sotalol-induced recoupling by a cAMP dependent protein kinase inhibitor. The results clearly supported our assumption since the addition of Arg-Gly-Tyr-Ala-Leu- Gly (pure A- kinase inhibitor) prevented the aforementioned cellular recoupling action of sotalol in a dose-dependent manner. It can thus be concluded that changes in intracellular cAMP level are involved in the synchronizing / defibrillating effect of sotalol.

Sotalol facilitates spontaneous ventricular defibrillation by enhancing intercellular coupling. An entirely new mechanism for its antiarrhythmic action

We have previously shown that sotalol, a class III antiarrhythmic agent, helps spontaneous ventricular defibrillation in various mammalian species. Since we hypothesized that self ventricular defibrillation depends on a high degree of intercellular synchronization, and since the major electrophysiological action of sotalol causing prolongation of action potential duration (APD), cannot fully explain its defibrillating property, we carried out a series of studies to examine the effect of sotalol on intercellular myocardial coupling. Guinea pig right ventricular muscle preparations were superfused in a tissue bath and the spread of intracellularly injected fluorescent dye (Lucifer yellow CH) to the neighboring cells was studied under various conditions. When either the Ca2+ concentration of Tyrode's solution was elevated to 6 mM or the solution was made hypoxic by not bubbling O2 (n = 3 each), no spread of the injected dye was observed. The addition of 1 microM sotalol to the high Ca2+ solution or 0.5 microM to the hypoxic superfusate (n = 3 each) caused a wide spreading of the dye, thus strongly suggesting a marked improvement in the intercellular coupling. These results show an entirely new property of sotalol, i.e., enhancement of cellular synchronization, which may better explain its ability to cause spontaneous ventricular defibrillation than its class III action. Our previous demonstration of successful spontaneous ventricular defibrillation by several other agents that are known to enhance intercellular coupling but have contrasting actions on APD further substantiates our hypothesis.

How can we facilitate spontaneous termination of ventricular fibrillation and prevent sudden cardiac death? A working hypothesis

Ventricular fibrillation (VF) is one of the most life-threatening arrhythmias encountered in daily clinical practice. Its occurrence cannot be completely prevented by currently used antiarrhythmic drugs, and, in most instances, VF is sustained and leads to the patient's death unless a successful DC defibrillation is applied. However, spontaneous reversion of VF to sinus rhythm has been observed in various animals and occasionally even in man. Hence, facilitation of self-ventricular defibrillation must be explored as an alternative therapeutic approach. In experimental studies using several mammalian species, we have found that self ventricular defibrillation requires a good intercellular coupling and well synchronized electrical activity in the ventricles, which, in untreated animals, depend on their myocardial catecholamine content. It can then be hypothesized that any agent that elevates the catecholamine level during VF would facilitate spontaneous ventricular defibrillation, and drugs inhibiting extraneuronal catecholamine reuptake have indeed been shown to possess this ability. It is suggested that their effects are mediated by an increase in the intracellular cAMP level, and any compounds sharing this property could well prove efficacious in making VF transient and in reducing sudden cardiac death.

The role of catecholamines on intercellular coupling, myocardial cell synchronization and self ventricular defibrillation

Ventricular fibrillation (VF) is one of the most life threatening events. Although in humans VF is generally sustained (SVF) requiring artificial defibrillation, in various mammals and in some cases in humans VF terminates by itself, reverting spontaneously into sinus rhythm. Since VF is one of the main causes of sudden death, one of the important clinical problems today is if and how we can transform the fatal SVF into a self limited transient one (TVF). From electrophysiological studies carried out on anaesthetized open chest animals, we have found that TVF requires a high degree of intercellular coupling and synchronization. Cardiac myocytes are electrically coupled with adjacent cells. The intercellular coupling is a focus of low electrical resistance which allows rapid transmission of electrical impulses between cells. Any decrease in intercellular coupling decreases the ability of the heart for self defibrillation. The cell-to-cell coupling decreases with age, ischemia, VF and variations in physiological conditions probably due to an increase in intercellular resistance (Ri), widening in the internexal gaps, decrease in electrotonic space constant (lambda) etc. All of these factors are known to be affected by intracellular concentration of free Ca++ ([Ca++]). On the basis of studies carried out on various mammals at different ages, we hypothesized that the ability of the heart to defibrillate depends on the cardiac catecholamine level [CA], during VF. This hypothesis is supported by the facts, known from the literature, that increase in [CA] decreases intracellular free Ca++ concentration, decreases Ri and increases lambda.(ABSTRACT TRUNCATED AT 250 WORDS)

Pronounced increase in defibrillation threshold associated with pacing-induced cardiomyopathy in the dog

Progressive changes in myopathology after implantation of an automatic defibrillator could compromise device efficacy. The influence of heart failure development on the defibrillation threshold was evaluated by means of a rapid ventricular pacing model of heart failure in dogs. After transvenous pacemaker lead implantation, adult mongrel dogs were randomly assigned to either the control (n = 7) or rapidly paced group (240 beats/min, n = 6). Seventeen days after implantation, triplicate determinations of the defibrillation threshold were made with three epicardial electrodes. The average defibrillation threshold was four times higher in the rapidly paced group, 13.3 +/- 2.0 joules (mean +/- SEM), than in the control group, 3.3 +/- 0.7 joules (p < 0.01), and was significantly correlated with ventricular weight (r = 0.70, p < 0.01). Both defibrillation threshold energy per gram of ventricle and ventricular weight corrected for body weight were significantly higher in rapidly paced dogs compared with control dogs. It was concluded that myocardial hypertrophy and heart failure may profoundly increase defibrillation energy requirements.

A self-protecting servo-model for explanation of the mechanism involved in spontaneous ventricular defibrillation

Ventricular fibrillation (VF) is the most life-threatening arrhythmia. It has been suggested that VF in humans is always sustained. Recent publications indicated that VF can be either sustained (SVF) or transient (TVF), reverting spontaneously into sinus rhythm. In previous studies we have hypothesized that TVF requires, during VF, a high cardiac catecholamine level ([CA]). Since during VF sympathetic activity is enhanced, the question arises of why VF is sustained in the majority of cases. Looking on the living body as a self-protecting servo-mechanism, we propose a servo-model that on the one hand describes the mechanism involved in TVF and on the other proposes a therapeutic procedure which can help the heart in its effort to transform VF into TVF. Our model has been examined by various experimental studies. The results obtained strongly support our hypothesis.

Modulation of collision-induced changes in canine heart repolarization by cycle length

The possibility that cycle length modulates the electronic effect of activation sequence on repolarization was investigated in experiments using isolated canine cardiac Purkinje strands, in situ canine ventricular myocardium, and computer simulations. Action potential durations and refractory periods during one-way propagation were compared to those obtained during action potential collision. In both the computer simulations and the Purkinje strand experiments, collision decreased action potential duration more at long cycle lengths than at short cycle lengths. Comparably, collision of activation fronts in ventricular myocardium was associated with greater reductions in refractory period during pacing at long cycle lengths than at short cycle lengths. Theoretic considerations indicate that the magnitude of electrotonic effects of activation sequence on repolarization are directly related to action potential height and the square root of membrane resistance during repolarization and are inversely related to conduction velocity. In computer simulations and Purkinje strand experiments, changes in conduction velocity and action potential height elicited by decreasing cycle length could not fully account for the cycle length dependence of collision-induced changes in repolarization. Time-varying membrane resistance of a single cell was calculated in the simulations by briefly hyperpolarizing the membrane and determining the change in total ionic current. Membrane resistance during repolarization was less at short cycle lengths than at long cycle lengths. The results suggest the cycle length dependence of collision-induced changes in repolarization results largely from the effect of cycle length on membrane resistance during action potential repolarization, with changes in action potential height and conduction velocity playing a lesser role.

Potential distribution in three-dimensional periodic myocardium--Part II: Application to extracellular stimulation

Modeling potential distribution in the myocardium treated as a periodic structure implies that activation from high-current stimulation with extracellular electrodes is caused by the spatially oscillating components of the transmembrane potential. This hypothesis is tested by comparing the results of the model with experimental data. The conductivity, fiber orientation, the extent of the region, the location of the pacing site, and the stimulus strength determined from experiments are components of the model used to predict the distributions of potential, potential gradient, and the transmembrane potential throughout the region. Next, assuming that a specific value of the transmembrane potential is necessary and sufficient to activate fully repolarized myocardium, the model provides an analytical relation between large-scale field parameters, such as gradient and current density, and small-scale parameters, such as transmembrane potential. This relation is used to express the stimulation threshold in terms of gradient or current density components and to explain its dependence upon fiber orientation. The concept of stimulation threshold is generalized to three dimensions, and an excitability surface is constructed, which for cardiac muscle is approximately conical in shape. The numerical values of transmembrane potential and stimulation thresholds calculated using asymptotic analysis are in agreement with the results of animal experiments, confirming the validity of this approach to study the electrophysiology of periodic cardiac muscle.

The effect of heart rate on the termination of electrically induced ventricular fibrillation in the isolated perfused rat heart

Ventricular fibrillation (VF) which is normally sustained in large animals and humans, is transient in small animals. The purpose of the present study was to evaluate the possible effect of changing cardiac rate on spontaneous ventricular defibrillation. In isolated perfused rat heart, VF was electrically induced during normal spontaneous rhythm of the heart at normal rate and at various ventricular pacing rates. It was found that: 1) Electrically induced VF in isolated perfused, non-ischemic rat heart spontaneously terminated in 88% of the hearts; 2) Ventricular pacing rhythm of spontaneous rate plus 10% caused VF to be sustained in 26% of the hearts (which defibrillated spontaneously during normal rates); 3) Ventricular pacing at 200% of the basic rate led to sustained VF in about half the VF episodes (14 out of 33, p less than 0.005). In the remainder, which defibrillated spontaneously, a sustained VF could be achieved by further increase in ventricular pacing rate; 4) Slow pacing rate, as a result of the surgical production of atrioventricular (A-V) block, enhanced the probability of spontaneous defibrillation (21 of 21 episodes after slow pacing vs 24 of 34 episodes following pacing at previous normal sinus rhythm, p less than 0.05). Selective modulation of conduction velocity, refractory period or both, achieved by changes in ventricular pacing rate was assumed to play an important role in determining whether electrically-induced VF would be transient or sustained.

Overdrive suppression of conduction at the canine Purkinje-muscle junction

We have shown previously that overdrive suppression of conduction in depolarized His-Purkinje tissue requires conduction asymmetry. In this study we examined whether overdrive suppression of conduction can occur at the Purkinje-muscle junction, where natural asymmetry of conduction exists. Canine Purkinje-muscle preparations were superfused with hyperkalemic Tyrode's solution (KCl 8 to 12 mM), and action potentials were recorded from Purkinje, junctional, and muscle cells. Initially, the Purkinje fiber was paced at the shortest cycle length at which 1:1 anterograde Purkinje-muscle conduction occurred. The papillary muscle then was paced for 10 to 50 beats at shorter cycle lengths during which, because of conduction asymmetry at the Purkinje-muscle junction, 1:1 retrograde muscle-Purkinje conduction also occurred. After overdrive papillary muscle pacing, Purkinje fiber pacing at the same cycle length that previously resulted in 1:1 conduction now produced transient Purkinje-muscle conduction block (overdrive suppression of conduction). The degree and duration of overdrive suppression of conduction were proportional to the rate and duration of overdrive pacing. After overdrive pacing, Purkinje cell action potential amplitude and Vmax recovered within 300 msec, yet conduction block persisted for up to 7 sec. In contrast, excitability in papillary muscle cells near the Purkinje-muscle junction increased continuously after overdrive pacing. These data suggest that rapid activation of Purkinje cells during overdrive pacing was not required for overdrive suppression of conduction and that restoration of conduction after overdrive pacing was determined primarily by recovery of excitability in papillary muscle cells. Transient Purkinje-muscle conduction block after periods of rapid ventricular rates might account for overdrive-induced conduction disturbances normally attributed to bundle branch block.

The effect of calcium concentration on spontaneous ventricular defibrillation and VF threshold

In order to investigate the influence of the effective refractory period on spontaneous ventricular defibrillation, isolated rat hearts were perfused with Krebs-Henseleit solution containing 0.5, 2.7 and 5.1 mM calcium. After measuring the fibrillation threshold at spontaneous rate (SR), ventricular fibrillation (VF) was induced during basic ventricular pacing of 110% SR, or the highest rate permitting 1:1 electromechanical coupling. The VF threshold was significantly reduced from 13.6 +/- 3.5 to 7.9 +/- 5.3 and 5.1 +/- 3.4 mA at 0.5, 2.7 and 5.1 mM Ca++ concentrations, respectively. The incidence of spontaneous recovery from VF, induced during basic pacing, was 100%, 83% and 50% at calcium concentrations of 0.5, 2.7 and 5.1 mM, respectively, (p less than 0.01 for the incidences at 0.5 mM versus 5.1 mM Ca++). The incidence of spontaneous defibrillation decreased when the hearts were driven rapidly, with spontaneous recovery rates of 92%, 58% and 0% (p less than 0.0001] for corresponding increases in Ca++ concentration. Induced ventricular fibrillation of fine morphology was frequently observed at 5.1 mM Ca++. It appears that progressive impairment of spontaneous defibrillation is caused by an increase in calcium concentration, this effect being more pronounced at high ventricular rates. Variations in the effective refractory period, caused by alterations in extracellular calcium concentration and differences in intracellular Ca++ accumulation, may account for the above results.

Changes in passive electrical properties of guinea-pig ventricular muscle exposed to low K+ and high Ca2+ conditions

Changes in passive electrical properties of guinea-pig papillary muscle exposed to low K+, high Ca2+ conditions were examined using a single sucrose gap technique. While quiescent preparations exposed for 5 mins did not develop delayed afterdepolarizations, those placed in the test solution for 30 mins with or without stimulation developed afterdepolarizations. Changes occurring during a short exposure to low K+, high Ca2+ solution were increases in membrane resistance, membrane time constant and space constant by 47%, 83% and 17% compared with the control, respectively. There were no significant changes in internal longitudinal resistance and membrane capacity. During long exposure to the test solution (30 mins), delayed afterdepolarizations developed. There were similar increases in membrane resistance and in time constant as found during the short exposures. Internal longitudinal resistance was calculated to have increased by 24% during the long exposure. A 19% increase in membrane capacity was also found during the test condition. High Ca2+ or low K+ alone did not cause a significant increase in internal longitudinal resistance. The conduction velocity in the longitudinal direction decreased from 107 +/- 23 cm/s during the control to 80 +/- 7 cm/s during the test period for 30 mins. These results suggest that, in addition to the abnormal impulse formation based on afterdepolarizations, low K+, high Ca2+ solution changes the passive electrical properties of the fibers, resulting in a lower rate of impulse conduction.

Overdrive suppression of conduction in the canine His-Purkinje system after occlusion of the anterior septal artery

The purpose of these experiments was to determine whether overdrive suppression of conduction (OSC), i.e., transient worsening of conduction or development of atrioventricular block after cessation of rapid pacing, could be produced in the canine His-Purkinje system damaged by ligation of the anterior septal coronary artery and to investigate the responsible mechanism. We found that OSC occurred in vivo after rapid ventricular and His bundle pacing but not after atrial pacing, and that it occurred in vitro after rapid pacing from the left bundle branch but not after pacing from the proximal His bundle. OSC was related to the duration and cycle length of pacing. Lidocaine increased while verapamil reduced the duration of OSC in vivo. The mechanism responsible for the unidirectionality of OSC is not clear but is probably related to the geometry of the atrioventricular junction and the anterograde versus the retrograde activation sequence. Changes in regional myocardial blood flow, autonomic tone, hemodynamic variables, or ventricular function do not appear to be required to produce OSC, based on the demonstration of the phenomenon in vitro. The data suggest a time- and rate-dependent change in factors affecting conduction such as excitability or cell-to-cell coupling, possibly due to accumulation of intracellular cations such as calcium.

Cable analysis in quiescent and active sheep Purkinje fibres

Cable properties of sheep cardiac Purkinje fibres were studied under resting and paced conditions. Standard micro-electrode techniques were used to apply intracellular current pulses and record the resultant voltage changes at various distances from the current input. In a parallel set of experiments, fibre dimensions were measured after freezing and serial sectioning. Fibres selected on the basis of a cylindrical appearance had approximately uniform cross-sectional diameters which varied +/- 12% along their length. Electrotonic potentials recorded at rest and in diastole (under conditions that minimized diastolic depolarization) adhered quite closely to the behaviour expected for a unidimensional cable provided voltages were recorded greater than or equal to one fibre diameter from the current source. The unidimensional space constant, input resistance, and membrane time constant were significantly larger during quiescence than in diastole. These differences were accounted for by a 90% increase in membrane resistance at rest. There was no significant change in internal longitudinal resistance nor membrane capacitance associated with activity. The voltage distribution close to the current input (i.e. within one fibre diameter) strongly deviated from the theoretical three-dimensional voltage decay expected for a homogeneous cylinder. This finding suggests that the transverse resistance to current flow is much greater than the longitudinal resistance. The anisotropic behaviour within the cardiac Purkinje fibre may explain several previous observations: (i) the lack of a relationship between conduction velocity and fibre diameter; and (ii) the much shorter liminal length for excitation in Purkinje fibres than for point-stimulated squid axons.