Effects of verapamil on action potentials of Purkinje fibers.

The effects of verapamil (1 and 2 micro M) on automaticity, membrane responsiveness curve and action potential configuration were studied in canine Purkinje fibers using conventional microelectrode techniques. The drug effects were concentration dependent, and more statistically significant changes were obtained during superfusion with 2 micron M verapamil. In spontaneously active fibers, the rate of discharge was decreased due to a decrease in the phase 4 depolarization. Verapamil failed to produce a consistent shift in the membrane responsiveness curve (Vmax versus take-off membrane potentials), although the development of early, slow action potentials was blocked. This blocking resulted in an increased effective refractory period and a shortened membrane responsiveness curve at the lower end. Maximum diastolic potential and action potential amplitude were decreased, and action potential duration was increased by verapamil. The results suggest that verapamil may selectively depress or block the slower action potentials which are thought to be important in the genesis of ventricular arrhythmias.