John C. Bailey

Alternans of action potential duration after abrupt shortening of cycle length: differences between dog Purkinje and ventricular muscle fibers

The purpose of this study was to determine whether the alternans of action potential duration (APD) occurring in Purkinje and ventricular muscle fibers after an abrupt shortening of cycle length can be explained by the two factors controlling the cycle length-dependent APD changes (i.e., restitution and memory effect). Action potentials were recorded simultaneously from dog Purkinje fibers and ventricular muscle fibers using conventional microelectrode techniques. APD change during alternans was dependent on the preceding diastolic interval in the same manner as during restitution in Purkinje fibers but not in ventricular muscle fibers. The course of memory change was not affected by the presence of alternans in either fiber type. In Purkinje fibers, APD alternans was attenuated by a Ca2+channel blocker, nisoldiplne (2 x 10-6 M), and augmented by a Ca2+ channel agonist, Bay K 8644 (3 x 10-6 M). These effects were attributed to the changes in the kinetics and the amplitude of restitution. In ventricular muscle fibers, APD alternans was always preceded and accompanied by alternans of action potential shape. Alternans of both action potential shape and APD was suppressed by nisoldipine (2 x 10-6 M) and attenuated by Bay K 8644 (3 x 10-3 M). These results show that in Purkinje fibers, APD during alternans can be explained by restitution and memory effect. However, in ventricular muscle fibers, the mechanism of APD alternans is linked to factors controlling action potential shape. These findings are compatible with the hypothesis that APD alternans in Purkinje fibers depends on the differences in the recovery of membrane currents generated by the preceding action potential and in ventricular muscle fibers on the differences in the concentration and/or handling of intracellular calcium.

Action potential duration alternans in dog purkinje and ventricular muscle fibers. Further evidence in support of two different mechanisms

An abrupt shortening of cycle length causes action potential duration (APD) alternation in both canine Purkinje (P) and ventricular (V) muscle fibers. Our recent study suggested that APD alternans is determined by the process controlling APD during electrical restitution in P but not in V fibers. In the latter, alternans was attributed to changes in the availability of intracellular calcium [Ca2+]i. We examined this hypothesis further with the following pharmacologic probes known to alter restitution or action of [Ca2+]i: tetradotoxin (0.5-3.0 microM), lidocaine HCl (2.0-12.0 micrograms/ml), sotalol (10 microM), nicorandil (10-20 microM), 4-amino-pyridine (0.5 microM), ryanodine (10 microM), caffeine (2 mM), and ARL 115 BS (100 microM). Alternans in P fibers persisted under all studied conditions but varied in magnitude depending on the time constant and amplitude of restitution. In V fibers, the magnitude of alternans did not correlate with APD changes during restitution, and APD alternans was associated with the alternans of action potential shape and alternans of developed tension. Alternans in V was suppressed by caffeine at 2.0 mM [Ca2+]o when tension was increased and by ryanodine at 1.0 mM [Ca2+]o when tension was decreased. Alternans in V was not altered by changes in [Ca2+]o within the range of 1.0-4.0 mM; by ARL 115 BS, a compound that increases myofibrillar sensitivity to calcium; or by any other pharmacologic probes. We concluded that in P fibers, APD alternans was determined by the factors controlling APD in the absence of alternans; V fibers posses an independent mechanism of alternans linked to alternans of tension and controlled by [Ca2+]i; in V fibers, alternans could be suppressed by both positive and negative inotropic interventions; and calcium released from sarcoplasmic reticulum plays an important role in the V alternans.

Effects of Acetylcholine on Automaticity and Conduction in the Proximal Portion of the His-Purkinje Specialized Conduction System of the Dog

Conventional intracellular recordings from the bundle of His and right bundle branch of the canine heart demonstrated that the slope of diastolic depolarization is markedly depressed by superfusion with relatively small concentrations (4−8 μg/ml) of acetylcholine. As the cells become less automatic, take-off potential increases, rise time of phase 0 is reduced, action potential amplitude increases, and conduction proceeds more rapidly.

Anticholinergic effects of disopyramide and quinidine on guinea pig myocardium. Mediation by direct muscarinic receptor blockade.

We studied the interaction of disopyramide, quinidine, and procainamide with cardiac muscarinic receptors. In electrophysiological experiments, the effects of disopyramide, quinidine, procainamide, and atropine were determined on spontaneously depolarizing guinea pig right atria (GPRA) both in the presence and absence of pharmacologically induced (physostigmine) cholinergic stimulation. All four agents demonstrated a concentration-dependent antagonism of the negative chronotropic effects of physostigmine. The order of anticholinergic potency was atropine » disopyr-amide > quinidine » procainamide. The ability of disopyramide to antagonize the physostigmine induced slowing was stereoselective, (+)disopyramide > (—)disopyramide. In contrast, the ability of quinidine to antagonize the negative chronotropic effects of physostigmine was non-stereoselective, quinidine = quinine. In parallel experiments, we studied the ability of disopyramide, quinidine, procainamide, and atropine to compete with the radiolabeled muscarinic receptor antagonist [3H] quinuclidinyl benzilate ([3H]QNB) for binding to muscarinic receptors in crude homogenates of GPRA and membrane vesicles from canine ventricular myocardium. All four agents inhibited [3H]QNB binding to muscarinic receptors. The order of anticholinergic potency determined by the receptor binding studies was identical to that determined by the physiological studies. The interaction of disopyramide with muscarinic receptors was stereoselective, (+)disopyramide > (−)disopyramide. Quinidine was only slightly more potent than quinine in inhibiting [3H]QNB binding to muscarinic receptors. Inter-action of antiarrhythmic drugs with muscarinic receptors satisfied criteria for a competitive interaction. The data from this study localize the anticholinergic effects of disopyramide and quinidine to the muscarinic receptor. Circ Res 47: 855-865, 1980

Acetylcholine antagonism of the electrophysiological effects of isoproterenol on canine cardiac Purkinje fibers.

The purpose of these experiments was to determine whether or not acetylcholine modulated the electrophysiological effects of isoproterenol on canine cardiac Purkinje fibers. Conventional microelectrode techniques were used. Predictably, isoproterenol produced shortening of action potential duration; acetylcholine significantly blunted this effect of isoproterenol. Isoproterenol restored excitability to fibers exposed to 22 mM potassium solutions, and acetylcholine abolished this isoproterenol-restored excitability. Both of these antagonistic effects of acetylcholine were blocked by atropine. Acetylcholine alone did not affect action potential duration in polarized fibers or excitability in potassium-depolarized fibers. Furthermore, acetylcholine had no effect on the decrease in action potential duration induced by premature electrical stimulation or by acetylstrophanthidin administration, or on excitability of fibers exposed to a zero sodium, high calcium superfusant-These data demonstrate a direct cellular basis for cholinergic antagonism of the electrophysiological effects of β-adrenergic stimulation of canine cardiac specialized intraventricular conducting tissue. cire Res 44: 378-383, 1979

Electrophysiological effects of ethanol, acetaldehyde, and acetate on cardiac tissues from dog and guinea pig.

Superfusion of isolated canine cardiac Purkinje fibers with ethanol (100-300 mg/ 100 ml) resulted in a concentration-dependent shortening of action potential duration. The effect occurred within 5 minutes of exposure to ethanol and was reversed completely after removal of the drug from the medium. Action potential amplitude, transmembrane resting potential, dV/dt of phase 0, and conduction time were not altered by these concentrations of ethanol. Superfusion with methanol, ethanol, l-butanol, n-butanol, and pentanol in a concentration of 200 mg/100 ml revealed that the magnitude of the alcohol-associated shortening of action potential duration was related directly to the water-octanol partition coefficient (lipophilic property) of the alcohol. Acetal-dehyde (0.08-0.80 DIM) prolonged the action potential duration of Purkinje fibers, but this effect was blocked by addition of the α-adrenergic blocking drug, phentolamine (10∽6 M). Acetate (1-5 mM) did not alter action potential configuration or conduction time of cardiac Purkinje fibers. Ethanol (100-300 mg/100 ml), acetaldehyde (0.08-0.80 mM), and acetate (1-5 mM) did not significantly change the rate of spontaneous depolarization of isolated guinea pig atria. These studies show that ethanol, in concentra-tions present in plasma during ethanol consumption, exerts direct reversible effects on cardiac Purkinje fibers. These effects probably result from a physical alteration of the sarcolemma secondary to interaction of ethanol with hydrophobic regions of the lipid bilayer. However, the metabolic by-products of ethanol do not exert direct electrophysiological effects on canine Purkinje fibers or guinea pig atria. Circ Res 47: 473-478, 1980

Parasympathetic effects on electrophysiologic properties of cardiac ventricular tissue

The physiologic importance of parasympathetic influence on the sinoatrial and atrioventricular nodes is well established, but the importance of parasympathetic modulation of ventricular function remains controversial. Recognized effects of muscarinic cholinergic stimulation on ventricular automaticity and ventricular repolarization, the ability of muscarinic cholinergic agonists to antagonize catecholamine effects in the ventricle and proposed mechanisms for these effects are described. Anatomic studies have demonstrated a great abundance of cholinergic nerve endings in association with the ventricular conducting system. Stimulation of the vagus nerve or addition of muscarinic cholinergic agonists suppresses ventricular automaticity in most species and antagonizes isoproterenol-induced action potential shortening and isoproterenol-restored slow response action potentials. In vivo, interactions between the parasympathetic and sympathetic nervous systems occur at multiple levels. Muscarinic cholinergic agonists inhibit release of norepinephrine from sympathetic nerve terminals, inhibit catecholamine-stimulated adenylate cyclase activity and alter cyclic guanosine monophosphate (GMP) and possibly cyclic adenosine monophosphate (AMP) levels. Evidence is also presented that, in vivo, parasympathetic effects on ventricular electrical function might influence the pathophysiologic milieu responsible for initiation or termination of certain ventricular arrhythmias. Vagal influences appear to be protective against certain digitalis-induced arrhythmias and protective in certain experimental acute myocardial infarctions. In human beings, there appears to be tonic vagal tone in the ventricle and vagal stimulation terminates certain types of ventricular tachycardia. The evidence presented supports a physiologic role of parasympathetic stimulation in altering ventricular electrical function.

Electrophysiological effects of the optical isomers of disopyramide and quinidine in the dog. Dependence on stereochemistry.

We studied the electrophysiological effects of the optical isomers of disopyramide and quinidine on canine cardiac Purkinje fibers. Conventional microelectrode techniques were employed to study the effects of racemic disopyramide, (+)-disopyramide, (−)-disopyramide, quinidine, and quinine. Racemic disopyramide, (+)-disopyramide, and quinidine prolonged action potential duration (APD) measured at 90% repolarization. In contrast, (−)-disopyramide and quinine shortened APD. These directionally opposite effects on repolarization were observed throughout 60 minutes exposure to drug and were concentration-dependent. All five compounds reduced dV/dt of phase 0, increased conduction time, and increased the current requirement for all-or-none depolarization. The effects of all five compounds on dV/dt, conduction time, and current requirements were time- and concentration-dependent. Our results indicate that the stereochemical configurations of disopyramide and quinidine determine their effects on repolarization of cardiac Purkinje fibers.

Dissociation between the electrophysiological properties and total tissue cyclic guanosine monophosphate content of guinea pig atria.

The purpose of this study was to investigate the role of cyclic guanosine monophosphate (cyclic GMP) in mediating the direct electrophysiological effects of acetylcholine in guinea pig atria. Acetylcholine significantly diminished spontaneous rate of right atria without increasing cyclic GMP content. Reductions in rate following acetylcholine were augmented by pretreatment with physostig- mine, but cyclic GMP levels remained unchanged. In left atria, acetylcholine significantly shortened action potential duration within 5 seconds (both with and without physostigmine pretreatment), but cyclic GMP content was not significantly elevated. Cyclic GMP levels in right atria were significantly increased in response to acetylcholine when the Ca+2 content of the buffer was elevated from 1.25 mu to 2.5 mM; however, reductions in automaticity in the right atria were not augmented in the high Ca2+buffer. Marked increases in cyclic GMP content were produced by Na nitroprusside superfusion without changing automaticity of right atria or action potential duration of left atria. Finally, both right and left atria were superfused with cyclic GMP analogs (8-bromo cyclic GMP and dibutyryl cyclic GMP) at high concentrations (10−4) for 15 minutes without producing significant effects on spontaneous rate or action potential duration. These results failed to show a correlation between total tissue cyclic GMP content and the electrophysiological effects of acetylcholine on guinea pig atria. The reasons for this are either that cyclic GMP does not mediate directly the electrophysiological effects of acetylcholine, or that small changes in cyclic GMP concentrations, undetectable when total tissue nucleotide levels are measured, occur in discrete effector pools of the cardiac cell to mediate the intracellular effects of the choline ester. Ore Res 45: 225-233, 1979

Potential biochemical mechanisms for regulation of the slow inward current: Theoretical basis for drug action☆

Regulation of the slow inward current appears to be an important mechanism by which the autonomic nervous system modifies cardiac function. Beta-adrenergic stimulation augments the slow inward current by increasing the number of functional slow inward current channels. This effect is mediated by cyclic adenosine monophosphate (cyclic AMP) and presumably involves phosphorylation of membrane proteins associated with the slow channels. Beta antagonists (propranolol) act by inhibiting beta-adrenergic activation of adenylate cyclase and thereby prevent increases in cyclic AMP. The calcium channel antagonists (verapamil) act directly at the level of the slow channels to inhibit the slow inward current independent of changes in cyclic AMP. Cholinergic stimulation attenuates beta-adrenergic stimulation of the slow inward current by one or both of two potential mechanisms: reduction in cyclic AMP formation and antagonism of the distal effects of cyclic AMP.