Verapamil induced increases in Purkinje fiber automaticity

Verapamil slows the spontaneous rate of S A mode pacemaker cells and Purkinje fibers exposed to Na free, Ca rich solutions. In both cases the effect appears to depend on the action of verapamil to block Ca current. In the study the action of verapamil on the automaticity of Purkinje fibers maintained in normal Tyrode solution was investigated. Action potentials were recorded continuously from spontaneously beating fibers isolated from canine hearts and superfused with Tyrode solution containing verapamil (1 to 4 mg/l). Verapamil increased automaticity in all 7 preparations tested, as reflected by an increase in steady state spontaneous rate and a decrease in the degree of suppression of spontaneous rate following 2 min intervals of rapid stimulation, i.e., blunting of overdrive suppression. Accompanying this increase in automaticity were changes in the time course of repolarization. The rate of repolarization was increased during the plateau phase and decreased during phase 3. Maximum diastolic potential was not affected. Concentrations of manganese which had similar effects on action potential configuration slowed or blocked spontaneous firing. These data suggest that the ionic mechanisms responsible for latent automaticity in normal Purkinje fibers may be different from those of sinus node pacemaker cells.