The ionic mechanism of reperfusion-induced early afterdepolarizations in feline left ventricular hypertrophy.

Left ventricular hypertrophy (LVH) potentiates reperfusion-associated ventricular fibrillation. To study the mechanism responsible, patch-clamp techniques were used to evaluate transmembrane ionic currents during "reperfusion" after a CN(-)-induced metabolic surrogate for ischemia in isolated myocytes from a feline model of experimental LVH. Reperfusion caused the generation of early afterdepolarizations (EADs) from an average take-off potential of -33 mV in LVH cells but not in cells from normal hearts. 10 min after initiating reperfusion of normal cells, action potential duration (APD) at 50% repolarization (APD50) lengthened from 198 +/- 41 to 233 +/- 57 ms whereas in LVH cells APD50 lengthened from 262 +/- 84 to 349 +/- 131 ms (P < 0.05). Among the LVH cells, APD50 lengthening was significantly greater in the cells that had developed EADs. During reperfusion, steady state outward current in the voltage range of the action potential plateau (between -20 and +20 mV) was reduced from the control values in LVH cells but not in normal cells. Reperfusion-related reduction of steady state outward current in LVH cells was abolished under experimental conditions in which L-type Ca2+ current was isolated from other classes of currents whereas it was still observed under the condition in which pure K+ currents could be recorded. Thus, reduction of steady state outward current due to the reduction of outward K+ current over the action potential plateau voltage range appears to be responsible for an excessive prolongation of APD, leading to the development of EADs.

Cellular electrophysiologic changes and "arrhythmias" during experimental ischemia and reperfusion in isolated cat ventricular myocardium.

The cellular electrophysiologic consequences of both regional and global experimental ischemia and reperfusion were studied in the isolated cat myocardium, using conventional microelectrode techniques. Oxygenated Tyrode's solution was perfused through the left anterior descending and circumflex coronary arteries, while the preparation was superfused with Tyrode's solution gassed with 95% nitrogen and 5% carbon dioxide. Electrophysiologic characteristics of endocardial muscle cells were normal during coronary perfusion. When perfusion was discontinued for 30 minutes, resting membrane potential was decreased by 21.6 +/- 4.1%, action potential amplitude was decreased by 29.1 +/- 8.6% and action potential duration was decreased by 54.1 +/- 12.5% (p less than 0.001). Ectopic activity occurred after 5 to 10 minutes of ischemia and was more frequent in regional than in global ischemia (p less than 0.05). Rapid ventricular activity was observed in only 5 (17%) of 29 preparations during ischemia, whereas it occurred in 24 (83%) of 29 preparations during reperfusion. Rapid ventricular activity began 5 to 40 seconds (mean 19) after the start of reperfusion, stopped spontaneously after a mean of 113 +/- 211 seconds and occurred after both regional and global ischemia. The cellular electrophysiologic changes induced by ischemia returned to baseline values within the next 5 minutes. Repeated ischemia and reperfusion runs reproduced the same electrophysiologic changes and rapid ventricular activity. Coronary perfusion with procainamide (20 mg/liter) aggravated the ischemic depressions of action potential amplitude and action potential duration and increased conduction delay during ischemia, but it did not prevent rapid ventricular activity induced by reperfusion. In contrast, verapamil (1 mg/liter) perfusion did not affect the changes in action potential variables during ischemia but prevented reperfusion-induced rapid ventricular activity. Perfusion with calcium ion (Ca2+)-free Tyrode's solution just before ischemia and during reperfusion slowed or prevented reperfusion-induced rapid ventricular activity, without affecting the action potential changes during ischemia. It is concluded that, in these isolated perfused ventricular muscle preparations, different mechanisms may be operative in ischemic and reperfusion arrhythmias and Ca2+ may play an important role in the development of arrhythmias during the reperfusion phase of ischemia/reperfusion sequences.

Electrophysiologic consequences of chronic experimentally induced left ventricular pressure overload.

Cardiac electrophysiologic alterations were evaluated 1 to 8 months after partial supracoronary aortic constriction in cats. This procedure induced left ventricular systolic hypertension and hypertrophy with marked connective tissue infiltration. In situ, premature ventricular complexes were observed during or after vagal slowing of sinus rate in 8 (26%) of the 31 experimental animals, while an additional 3 of the 31 developed ventricular fibrillation. No arrhythmias were recorded in 31 normal or 7 sham-operated cats. In vitro, 29% of the left ventricular preparations from cats with pressure overload and 5% from control cats showed spontaneous ectopic activity. During stimulation at cycle lengths of 800 to 1,000 ms, multiple site impalements of subendocardial muscle cells within fibrotic regions revealed heterogeneous electrical abnormalities. These included short action potential duration, low amplitude action potentials generated from low resting potentials, split upstrokes and electrically silent areas. Impalements in nonfibrotic areas of the left ventricle showed prolongation of muscle action potential duration. Long-term disturbances in cellular electrophysiologic properties may favor the development of arrhythmias and thereby contribute to sudden cardiac death in left ventricular hypertension and hypertrophy.

Electrophysiological effects of alprenolol on depressed canine myocardium.

The electrophysiological effects of the beta-adrenergic antagonist, alprenolol, were compared in normal and depressed canine myocardium. Both (+) and (+/-)-alprenolol (5 x 10(-6) and 10(-5) mol.litre-1) decreased action potential amplitude and Vmax in Purkinje fibres superfused with Tyrode's solution in tissue bath. These concentrations shortened action potential duration and effective refractory period of Purkinje fibres but prolonged those of ventricular muscle. Alprenolol more markedly decreased Vmax and sometimes prevented action potential propagation in Purkinje fibres overlying infarcted regions. Similar depressant actions were noted in Purkinje fibres depolarised by exposure to 9 mmol.litre-1 K+. Depolarised and diseased myocardium is more sensitive to alprenolol and the drug's membrane depressant actions may be significant in terminating arrhythmias in such tissue.

Are calcium antagonists proarrhythmic?

Clinical and experimental studies demonstrate that calcium (Ca2+) overload in myocardial cells is an important factor in the genesis of various serious arrhythmias. Calcium antagonists block voltage-dependent channels and thus reduce entry of Ca2+ into heart cells. Because of their specificity for atrioventricular nodal cells, verapamil and diltiazem are used clinically to treat supraventricular arrhythmias involving transmission in the atrioventricular node. These two drugs and the dihydropyridine (DHP) calcium antagonists have been shown to prevent ventricular ischemic and reperfusion arrhythmias in the laboratory. Despite these data indicating that calcium antagonists are antiarrhythmic, a recent controversy has raised the possibility that certain calcium antagonists are unsafe to use, especially for patients with coronary heart disease. Proarrhythmia has been proposed to be a mechanism contributing to potentially adverse outcomes. Although excessive concentrations of verapamil and diltiazem may cause sino-atrial nodal asystole and varying degrees of atrioventricular block, there is little direct evidence that this contributes to significant proarrhythmia, for example, ventricular tachyarrhythmias. Nonetheless, although it appears paradoxical that agents which block the entry of Ca2+ into heart cells may be considered arrhythmogenic, there are circumstances under which dosage with certain calcium antagonists potentially leads to myocardial Ca2+ overload. For example, bouts of neurohormonal activation brought about by calcium antagonist-induced abrupt reductions in blood pressure may be accompanied each time by significant beta-adrenergic-enhanced influx of Ca2+ through the L-type cardiac calcium channels. This elevates the intracellular Ca2+ concentration and disturbs Ca2+ regulation, especially in diseased hearts whose intracellular Ca2+ regulation has already been compromised, and might induce alterations in cardiac electrical activity. In the present article, interactions among cardiac calcium channels, classes of calcium antagonists, and specific formulations of certain antagonists are considered with respect to directly induced ventricular arrhythmogenesis. Indirect potentially proarrhythmic actions of the calcium antagonists are also discussed. We outline some of the many questions that remain to be answered with respect to the actions of DHP on the heart including that of whether beta-adrenergic stimulation modifies the degree of cardiac Ca2+ channel inhibition by DHP-type calcium antagonists.

Effects of coronary bypass surgery on the electrical activity of revascularized myocardium. Immediate and early postoperativeobservations.

The effect of myocardial revascularization on bipolar epicardial electrograms was recorded with fixed wire electrodes from revascularized left ventricular sites and from control sites on the right ventricle. Studies were performed during and after surgery in 19 patients undergoing aorta-coronary bypass grafting for occlusive coronary artery disease and in 6 additional patients having aortic valve replacement for isolated aortic valve disease. In the latter 6 patients, neither left nor right ventricular electrogram voltage changed immediately following aortic valve replacement; however, left ventricular electrogram voltage gradually decreased for 5 days postoperatively. In the 19 patients with coronary artery disease, electrogram voltage in the revascularized area increased immediately following coronary bypass grafting (+40 to +300 per cent) in 13 patients (68 per cent) and immediately decreased (-20 to -70 per cent) in 6 patients (32 per cent). In 5 of the patients showing immediate increases, temporary occlusion of the bypass grafts for 3 minutes during surgery resulted in a decrease of electrogram voltage in the distribution of the occluded bypass, followed by return to preocclusion levels after release. Postoperative monitoring of electrogram voltage for 5 days in all patients with coronary artery disease revealed that the electrogram voltage in the revascularized area decreased to or below control levels in 16 patients (84 per cent) and remained increased in 3 patients (16 per cent). These observed changes did not correlate with preoperative hemodynamics, number of grafts, graft flow rate, aortic cross-clamp time, cardiopulmonary bypass time, and the early postoperative course. These preliminary observations suggest that coronary bypass grafting does affect the electrophysiological state of the revascularized myocardium. However, the mechanism by which it occurs and its clinical implications remain to be determined.

Caffeine and KC1 contracture in cat myocardium.

Caffeine increases the force of contracture induced in cat myocardium by exposure to 140 mM KC1 in isotonic Tyrode solution. The effect of caffeine does not require prior exposure to the drug, develops rapidly, and is partially antagonized by procaine but not by verapamil. These results suggest that caffeine acts on depolarized sarcolemma to release a sarcolemmal calcium pool.

Dihydropyridine sites separate from junctional sacroplasmic reticulum in hypertension.

Distribution patterns on linear sucrose gradients for dihydropyridine (DHP) binding sites and Mr 300K polypeptides marker for junctional sarcoplasmic reticulum were determined in left ventricular microsomes of control and chronically pressure-overloaded cat hearts. Major peak of DHP sites corresponded in position and shape to distribution of Mr 300K polypeptide in controls but appeared at a lighter buoyant density in hypertrophy, similar to junctional plasma membrane (T-tubules) isolated from normal microsomes by French press treatment of dyad fraction.

Depressant effects of phenoxygenzamine on potassium contracture in cat ventricular muscle.

Phenoxybenzamine has been shown to have a depressant action on K+-depolarization contracture in cat ventricular muscle. In the present study, we show that this depressant action is specific for phenoxybenzamine and occurs in the presence of beta-adrenergic receptor blockade with nadolol. K+-contracture is not depressed by phentolamine nor augmented by phenylephrine. Thus, the depressant action of phenoxybenzamine is not mediated by its effects on cardiac adrenergic receptors.

Cellular basis for arrhythmogenicity of ionophores with different cation selectivities.

In high concentrations, the ionophores salinomycin, monensin and X-537A cause cardiac arrhythmias in vivo. To determine if these arrhythmias result from a direct action of these ionophores on cardiac electrophysiology, we studied their effects on automaticity and transmembrane action potentials of isolated canine left ventricular Purkinje fibers. High concentrations of the ionophores suppressed automaticity and shortened action potential duration. These data suggest that high concentrations of the ionophores provoke cardiac arrhythmias in vivo by similar mechanisms despite their diverse cation transport selectivities.

Histamine attenuates the arrhythmogenic effects of norepinephrine in hearts of spontaneously hypertensive rats.

A potential physiological role for cardiac histamine and its interaction with norepinephrine were investigated in isolated left ventricles from spontaneously hypertensive rats (SHR). Prior to drug administration, left ventricle-to-body weight ratios and spontaneous firing rates (beats per min) were significantly increased in SHR ventricles vs. age- and sex-matched controls (WKY). Also, action potential duration was significantly prolonged in SHR at all levels of repolarization. In all hearts, norepinephrine (10(-7)-10(-4) M) increased spontaneous rate and the percent incidence of arrhythmias. The H2-receptor antagonist cimetidine (10(-5) M) potentiated the rate and arrhythmogenic effects of norepinephrine in SHR and, to a lesser extent, in WKY preparations; propranolol (10(-6) M) reduced them. Histamine (10(-7) M) also inhibited the norepinephrine-induced increase in arrhythmias in SHR, but not in WKY. The attenuation of adrenergically induced rhythm disturbances by histamine and their potentiation by cimetidine in hypertensive hearts support the hypothesis that histamine plays a role as a postjunctional modulator of adrenoceptor function in a setting of hypertension and myocardial hypertrophy.

Comparative study of the effects of salinomycin and monensin on electrophysiologic and contractile properties of canine myocardium.

The effects of the monocarboxylic ionophore, salinomycin (K+-selective), on isometric twitches, high K+-induced contracture and transmembrane action potentials were compared with those of the monocarboxylic ionophore, monensin (Na+-selective), in isolated canine right ventricular muscle. In a concentration (5 X 10(-6) M) which did not produce changes in resting force, salinomycin increased peak active force (Po, + 170 +/- 36%, mean % change from control +/- S.D., P less than 0.01), and maximal rates of force development (dP/dt max, + 123 +/- 33%, P less than 0.01) and relaxation (-dP/dt max, + 180 +/- 40%, P less than 0.01) of the isometric twitch. A similar response pattern was found for 5 X 10(-6) M monensin (Po, + 90 +/- 24%, P less than 0.01; dP/dt max, + 137 +/- 19%, P less than 0.01; -dP/dt max, + 145 +/- 20%, P less than 0.01). In contrast to their effects on isometric twitches, salinomycin reduced peak K+ contracture force (Pc, -35 +/- 14%, P less than 0.01) whereas monensin increased it (Pc, + 30 +/- 12%, P less than 0.02). Ventricular muscle action potential duration was shortened similarly by the ionophores. beta-Adrenergic receptor blockade with nadolol diminished salinomycin's effects on the isometric twitch and K+ contracture, but not its effect to shorten the action potential. Monensin's actions were unaffected by nadolol. These results suggest that salinomycin's effects arise from both a direct modulation of K+ movement and the release of endogenous catecholamine. In contrast, monensin may act to alter intracellular Na+ which in turn leads to Na+-Ca2+ exchange and Ca2+-mediated modulation of K+ movement.

Electrophysiologic effects of encainide on acutely ischemic rabbit myocardial cells.

The electrophysiologic effects of encainide were determined in normal and acutely ischemic (30 min) rabbit ventricular muscle cells. Encainide (10(-6), 5 X 10(-6) and 10(-5) M) had no effect on resting potential (RP); 10(-6) M encainide reduced overshoot and action potential (AP) amplitude of cells in normal left ventricles and cells in normal areas of ischemic ventricles. Encainide, 5 X 10(-6) M and 10(-5) M, depressed Vmax and prolonged AP duration of normal cells. Surviving cells within ischemic areas displayed AP with reduced RP, overshoot, AP amplitude, Vmax and shortened AP duration. All encainide concentrations reduced overshoot, AP amplitude and Vmax of depressed AP. Encainide's lengthening of AP duration was greater in cells within ischemic areas than in surrounding normal cells. Encainide (10(-6) M) prolonged effective refractory period and often blocked AP in ischemic cells. Encainide also caused depression in membrane responsiveness. Encainide's differential effect upon AP may significantly contribute to its antiarrhythmic activity in ischemic heart disease.

Regional sensitivity to verapamil in ventricular myocardium.

The effects of verapamil (0.1-4.0 micrograms/ml) on transmembrane action potential configuration was examined in various regions of the normal cat left ventricular endocardium. Surface electrograms showed that the sequence of activation of anterior papillary muscle fibers was from the base of the muscle to its apex, while overlying Purkinje fibers were activated in the opposite direction. Action potential duration in basal muscle fibers was relatively unaffected by verapamil, while cells at the tip of the papillary muscle showed increased slope in phase 2 and a more rapid rate of early repolarization. Regional sensitivity to verapamil may reflect disparate patterns of endocardial activation, and could influence the effectiveness of the drug against ventricular arrhythmias.

Myocardial infarction in the guinea pig: cellular electrophysiology.

The cellular electrophysiology of left ventricular preparations from guinea pig hearts was studied 1 hour, 24 hours, and 4-6 weeks after myocardial infarction produced by 6-8 single ties of the distal left coronary artery system or after sham operation. Microelectrode recordings were used to monitor cells from the endocardial surface of each preparation in tissue bath. All coronary ligated preparations displayed accelerated spontaneous activity compared to normal and sham operated preparations. Single and multiple premature ventricular depolarizations occurred frequently in coronary ligated and rarely in normal and sham operated preparations. Premature stimuli delivered to areas overlying and bordering the area of infarction, induced short bursts of self-terminating rapid repetitive ventricular activity in 4 of 8 (50%) acute (1-hour), 5 of 9 (55%) subacute (24-hour), and 14 of 20 (70%) healed (4-6-week) infarcted preparations. Such activity could not be induced in normal and sham operated preparations. The preparations with healed infarction were unique in that they demonstrated runs of self-terminating repetitive ventricular activity which occurred spontaneously or was inducible with premature stimulation. Recordings from multiple sites in acute, subacute, and healed preparations revealed a variety of transmembrane action potential abnormalities (i.e., reduced action potential amplitude and resting potential, decreased and increased action potential duration, and depressed maximum rates of phase 0 depolarization) in cells overlying and bordering areas of infarction. Only Purkinje fiber action potentials were recorded over the healed infarcts. These data demonstrate that a spectrum of electrophysiological alterations occur in response to ischemic injury and persist after healing of the injury in this new model of myocardial infarction utilizing the guinea pig.

Cellular electrophysiology of coronary artery ligation in chronic pressure overload.

We evaluated ischemia-induced cellular electrophysiologic abnormalities in chronic pressure overload ventricular myocardium in vitro. Left ventricular systolic hypertension was induced in cats via partial supracoronary aortic constriction (overload); at 1 1/2-3 months, resulting pressure overload was accompanied by ventricular hypertrophy (25-35% by weight) and patchy endocardial fibrosis. Two hours of subsequent acute myocardial ischemia (ischemia) was imposed on overload (ischemia/overload) via total occlusion of distal branches of the left coronary artery system. Spontaneous premature depolarizations in vitro were increased in ischemia/overload compared to control, ischemia or overload alone; bursts of spontaneous, repetitive depolarizations were also unique to these preparations. Multiple site recordings of endocardial transmembrane action potentials overlying the borders (interface) of fibrotic areas in ischemia/overload demonstrated numerous electrophysiologic abnormalities, including several not observed in control, ischemia or overload. Unique to the border areas of ischemia/overload preparations was the presence of maintained but depressed resting potential without action potentials; also, the incidence of depolarizations at the onset of the plateau phase was highest in these preparations. In non-fibrotic areas, electrophysiologic properties including resting potential and action potential amplitude and rate of rise were diminished in ischemia/overload compared to ischemia or overload preparations. These data demonstrate that acute myocardial ischemia in the setting of chronic pressure overload leads to additional cellular electrophysiologic abnormalities compared to ischemia or overload alone.