Larry D. Davis

Effects of Propranolol on the Transmembrane Potentials of Ventricular Muscle and Purkinje Fibers of the Dog

Papillary muscle-false tendon tissue preparations isolated from dog hearts were perfused with Tyrodes solution containing propranolol in concentrations ranging from 0.1 to 20.0 mg/liter. Transmembrane action potentials of both ventricular muscle fibers and Purkinje fibers were recorded. With sufficient concentration of drug, the velocity of the upstroke and the overshoot of both fiber types decreased. The curve relating upstroke velocity to level of membrane potential for Purkinje fibers was displaced to the right and down. The ability of both ventricular muscle fibers and Purkinje fibers to respond to rapid frequencies of stimulation was decreased. Repolarization of Purkinje fibers was accelerated by propranolol, but repolarization of ventricular muscle fibers was unaffected. Duration of the effective refractory period of Purkinje fibers decreased; that of ventricular muscle fibers was unchanged. Graded responses and decremental impulse conduction in Purkinje fibers were abolished in the presence of propranolol. Low doses of propranolol which caused no change in the transmembrane potential completely blocked the increase in Purkinje diastolic depolarization normally induced by epinephrine. The possible mechanisms by which propranolol might exert its antiarrhythmic actions on ventricular arrhythmias were discussed.

Electrophysiological Actions of Lidocaine on Canine Ventricular Muscle and Purkinje Fibers

Isolated canine papillary muscles with attached false tendons were perfused with Tyrodes solution containing lidocaine. Transmembrane action potentials of ventricular muscle fibers and Purkinje fibers were recorded with glass microelectrodes. Repolarization of Purkinje fibers was accelerated by lidocaine but that of ventricular muscle fibers was unaffected. The maximum rate of rise of the action potential of Purkinje fibers decreased only at 50.0 mg/liter lidocaine; that of ventricular muscle fibers was unchanged by any concentration. The curve relating rate of rise of premature responses to level of membrane potential for Purkinje fibers was unchanged with 5.0 mg/liter but shifted down and to the right with 10.0 and 50.0 mg/liter. The effective refractory period of Purkinje fibers shortened at 5.0 and lengthened at 50.0 mg/liter. The level of membrane potential needed to elicit premature propagated responses in Purkinje fibers with a standardized test stimulus increased and the earliest responses obtained in the presence of lidocaine were relatively large. Duration of the effective refractory period of ventricular muscle fibers was unchanged with 5.0 mg/liter but lengthened progressively with 10.0 and 50.0 mg/liter. Lidocaine impaired the ability of both fibers to respond to rapid frequencies of stimulation, slowed the rate of inherent diastolic depolarization of driven Purkinje fibers, decreased the rate of discharge of spontaneously beating preparations, and minimized the increase in rate and magnitude of diastolic depolarization caused normally by epinephrine. The possible role of these effects of lidocaine in regard to its antiarrhythmic actions was discussed.

Effect of Changes in Cycle Length on Diastolic Depolarization Produced by Ouabain in Canine Purkinje Fibers

Isolated canine papillary muscle–false tendon tissue preparations stimulated at a cycle length of 630 msec were treated with ouabain (2.1 × 10−7M) until an increase in the slope of diastolic depolarization of Purkinje fibers was produced. Then the effects of changes in cycle length on the slope were tested. Sustained shortening of the cycle length increased the slope, and sustained lengthening had the opposite effect. An abrupt decrease in cycle length caused by a stimulus during phase 3 of a driven beat induced an increase in the slope of diastolic depolarization for one or more subsequent cycles. This procedure occasionally led to the spontaneous generation of an action potential which propagated into surrounding tissue. Suspension of stimulation often was followed by spontaneous beats. Shortening of the cycle length decreased the time of onset and increased the number and frequency of spontaneous beats. These effects were correlated with increases in the slope of diastolic depolarization. The diastolic depolarization of Purkinje fibers in false tendons was increased earlier and to a greater degree by ouabain than was the depolarization of Purkinje fibers on the muscle surface. These findings suggest that enhancement of diastolic depolarization of Purkinje fibers to the point of spontaneous discharge of action potentials may be a common means by which digitalis produces a variety of ventricular arrhythmias observed in intact animals.

Evidence for Specialized Fibers in the Canine Right Atrium

Right atria from excised beating dog hearts were immersed in Tyrode solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in different anatomical areas of this preparation. Fibers with action potentials similar in contour to those of ventricular Purkinje fibers were found along the caval border of the crista terminalis in the area of the posterior internodal tract. The action potential of these fibers (plateau fibers) had a resting potential of −85 to −95 mv, a sharp spiked overshoot, a long plateau phase, and inherent diastolic depolarization. The maximum velocity of the action potential upstroke of plateau fibers was consistently greater than that of simultaneously recorded fibers which lacked a plateau phase (regular fibers). Epinephrine or isoproterenol produced an increase in both rate and magnitude of diastolic depolarization of plateau fibers and on occasion converted them to true pacemakers. Acetylcholine accelerated repolarization of plateau fibers with disappearance of the plateau. Increases in extracellular concentration of potassium ions from 2.7 to 10.8 mM rendered regular atrial fibers inexcitable, but plateau fibers continued to show action potentials. Plateau atrial fibers possess several characteristics exhibited by specialized conducting and impulse generating fibers. The possibility that these fibers constitute the posterior internodal tract and function in preferential conduction of excitation to the A-V node was discussed.

Effect of Calcium Concentration on the Transmembrane Potentials of Purkinje Fibers

The effects of variation in calcium concentration on the action potential of Purkinje fibers isolated from the dog heart were studied. Action potentials recorded during perfusion with Tyrode solution containing 2.7 mM calcium chloride were compared with those recorded during subsequent perfusion with solutions containing 0.675 (1/4X), 1.35 (1/2X), 5.4 (2X), or 10.8 (4X) mM calcium chloride. In both 1/2X and 1/4X solutions, the time required to repolarize to minus 60 mv and the duration of the action potential were increased. There were significant decreases in the slopes of phases 2 and 3 and the terminal phase of repolarization, while the slope of phase 1 increased. In 2X or 4X solutions, repolarization was speeded mainly by an earlier onset of phase 3. As a result the time to repolarize to minus 60 mv and the duration of the action potential were decreased significantly. The slope of phase 1 decreased significantly in 4X solution. In both high calcium solutions the rate and magnitude of diastolic depolarization increased. It was shown that the rate of the calcium-enhanced diastolic depolarization was dependent on the stimulus rate. The possible role of these changes in transmembrane potential in causing the ventricular arrhythmias following CaCl2 infusion in intact animals is discussed.

Electrophysiological Characteristics of Canine Atrial Plateau Fibers

Right atria from dog hearts were immersed in Tyrodes solution with the endocardial surface of the anterior wall exposed. Glass microelectrodes were used to impale fibers in different anatomical areas of this preparation. Electrical characteristics of atrial plateau fibers located along the caval border of the crista terminalis were compared with those of regular atrial fibers from other sites. The former conducted at a rate (0.803±0.110 M/sec) two to three times more rapidly than pathways composed of regular atrial fibers. A period of supernormal excitability was demonstrated for plateau fibers but not for regular fibers. Graded premature responses could be elicited in plateau fibers by both electrical stimulation and propagated action potentials. Duration of the effective refractory period of plateau fibers (196.1±2.5 msec) was longer than that of regular fibers (147.8±5.4 msec). Plateau fibers required a lower level of membrane potential (−62.1 ± 0.8 mv) to generate a propagated action potential than did regular fibers (−65.5 ± 1.3 mv). Regular atrial fibers could respond at faster rates to high frequency stimulation than plateau fibers.

Production of slow responses in canine cardiac Purkinje fibers exposed to reduced pH

Abstract Isolated canine papillary muscle-false tendon tissue preparations were exposed to reduced pH at a level of 6.0 or 5.2. Effects on the transmembrane potential of Purkinje fibers and ventricular muscle fibers were recorded. Ventricular fibers were not significantly altered by these levels of pH relative to the changes observed in Purkinje fibers. In Purkinje fibers acid perfusion initially caused decreases in maximum diastolic potential, rising velocity and overshoot and an increase in duration of the action potential. Subsequently in many fibers these effects progressed and resulted in an action potential with all of the characteristics of what has been termed the “slow response.” The maximum diastolic potential was 50 ± .9 mV, the overshoot was 20 ± 1.1 mV and the action potential duration was 322 ± 11 ms. Phase 4 depolarization of variable degree usually became evident with the onset of slow responses. In a number of experiments the preparation escaped from the drivestimulus and discharged spontaneously at a more rapidrate. In other experiments suspension of drive stimulation resulted in spontaneous discharge of action potentials generated by the mechanism of phase 4 depolarization. Such excitation propagated and caused excitation of other Purkinje fibers as well as ventricular muscle fibers. At times disturbances of conduction were noted and ranged from simple slowing in conduction of excitation to partial or complete block of conduction between regions of the preparation. Agents known to block the slow response were tested for their ability to block pH-induced responses. MnC1 2 or verapamil depressed the upstroke and overshoot of the slow response and slowed or stopped the spontaneous discharge of action potentials in acid medium. The possible role of pH-induced slow responses in causing cardiac arrhythmias, especially those arising secondary to cardiac ischemia, was considered.

Effects of Lidocaine, Propranolol, and Sotalol on Ouabain-lnduced Changes in Transmembrane Potential of Canine Purkinje Fibers

Isolated canine papillary muscle-false tendon preparations stimulated at 95/min were perfused with Tyrodes solution containing ouabain, 2.1 × 10−7M. Action potentials of Purkinje fibers were recorded. Initially ouabain increased the slope of phase-4 depolarization. Subsequently it decreased the maximum diastolic potential and the rising velocity of phase 0, the amplitude, and the duration of the action potential. The slope of phase-4 depolarization increased progressively, and eventually the configuration of the action potential changed to resemble that of sinoatrial node fibers. Onset of enhanced phase-4 depolarization was delayed significantly by treatment before exposure to ouabain with lidocaine, 3.7 × 10−5M, or propranolol, 2.1 × 10−5M, but not with sotalol, 3.2 × 10−4M. Enhanced phase-4 depolarization produced by exposure to ouabain was reduced in slope by subsequent treatment with lidocaine or propranolol but not sotalol. Application of lidocaine or propranolol to fibers with action potentials in the configuration characteristic of sinoatrial node fibers returned the contour of the action potentials toward normal; sotalol was not effective in this regard. These results, coupled with the relative effectiveness of these drugs in intact animals with digitalis arrhythmias, support the hypothesis that enhanced phase-4 depolarization of Purkinje fibers is a factor in the production of digitalis-induced ventricular arrhythmias.

Effects of Cyclopropane and of Hypoxia on Transmembrane Potentials of Atrial, Ventricular and Purkinje Fibers

The effects of cyclopropane (6 to 8 vol% in Tyrode solution) and of hypoxia, on the transmembrane potentials of atrial, ventricular and Purkinje fibers of the dog heart, were studied. Action potentials were recorded during perfusion with solution equilibrated with 95% O2, and 5% CO2 (control-Tyrode); during cyclopropane-Tyrode perfusion and during perfusion with solution in which nitrogen replaced cyclopropane (nitrogen-Tyrode). Changes in transmembrane potential observed during cyclopropane-Tyrode treatment were considered to be due to an action of cyclopropane only if they recovered or failed to appear during nitrogen-Tyrode perfusion. By interpretation on this basis it could not be determined that cyclopropane exerted an effect on atrial or ventricular fibers. In Purkinje fibers cyclopropane caused a significant increase in rate of repolarization during the plateau (phase 2) while the rate during the period of rapid repolarization (phase 3) decreased. The time required to repolarize to minus 60 millivolts was shortened significantly while the durations of the terminal phase of repolarization and of the total action potential were lengthened. A relative hypoxia averaging 300 mm Hg accelerated repolarization and reduced resting potential of atrial cells. Ventricular cells were affected similarly but only after two or more periods of hypoxia. Purkinje fibers were shown to be most resistant to this level of hypoxia.

Effects of Calcium and Cylopropane on Purkinje Fibers

Purkinje fibers isolated from the dog heart were perfused with Tyrode solution containing cyclopropane (6 to 8 vol.%) and normal calcium concentration (2.7 mmoles/liter). Transmembrane action potentials recorded during perfusion were compared with those obtained during subsequent perfusion with cyclopropane in solution containing 0.675(¼×), 1.35(½×), 5.4(2×) or 10.8(4×) mmoles/liter of CaCl2. In either of the low calcium solutions, the acceleration of repolarization during the plateau of the action potential initiated by cyclopropane was reversed, with the result that the contour of the action potential returned to that present prior to treatment with cyclopropane. Continued treatment with ¼× solution caused a general deterioration of the action potential. In both 2× and 4× solutions, the action of cyclopropane on repolarization was enhanced, that is repolarization occurred more rapidly. In addition the latter procedure increased the rate and magnitude of diastolic depolarization. The possible effects of these changes in configuration of action potential on ventricular excitability and on the tendency for the development of arrhythmias during cyclopropane anesthesia are discussed.