Actions of verapamil on Purkinje fibers from normal and infarcted heart tissues.

Dogs were anesthetized and a two-stage occlusion of the left anterior descending coronary artery was performed. They were sacrificed 24 hr later and the experimental preparations, which included both normal and infarcted tissues, were dissected from the left ventricles. Effects of 1 microM verapamil on action potentials of Purkinje fibers from normal and infarcted zones (NZ and IZ) were studied using conventional microelectrode techniques. Verapamil produced a significant decrease in action potential amplitude in the cells from both NZ and IZ, with the IZ cells showing a greater reduction. A measured decrease in action potential duration (at 50% repolarization) was significant only in the NZ. Both NZ and IZ cells showed a significant increase in refractory period after verapamil superfusion Vmax was decreased only in the IZ in which slow responses were further depressed by the drug. The rapidly repetitive responses induced in some preparations by early premature stimuli were reduced or abolished during verapamil superfusion. These results suggest that verapamil may preferentially alter the slow responses seen in infarcted tissues, thereby reducing a source of reentrant activity and associated arrhythmias.