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.