Effects of verapamil on electrophysiologic properties of blood superfused cardiac Purkinje fibers.

Blood superfusion techniques were used to study the effects of verapamil on the body surface electrocardiogram and blood pressure of intact, anesthetized dogs as well as transmembrane action potential characteristics of isolated Purkinje fibers. Verapamil, 100 to 600 microgram/kg, administered in sequential doses intravenously to the intact (donor) animal caused a dose-dependent increase in the P-R interval and a decrease in systemic arterial pressure while exerting no significant effect on the normal, fast response Purkinje fiber action potential. To study effects of verapamil in depolarized fibers, maintained stretch was applied to Purkinje fibers until resting potentials had decreased from approximately -91 mV to a stable level of approximately -55 mV. For these fibers, verapamil, 400 microgram/kg, given as a single injection almost totally suppressed the slow response action potential while prolonging the P-R interval by less than 30% above control. These experiments suggest that in the presence of normal sinus and atrioventricular nodal function, plasma concentrations of verapamil which abolish the slow response action potential should have no major effect on sinus rate and atrioventricular conduction.