An oscillatory current in sheep cardiac Purkinje fibers

The inward current ("oscillatory current") which may be present after the end of a depolarizing clamp was studied in sheep cardiac Purkinje fibers by means of a voltage-clamp method. The following results were obtained. In order to appear, the oscillatory current (Ios) requires a previous depolarization to approximately or equal to -20 mV or beyond and a repolarization to approximately or equal to -40 mV or to more negative potentials. The Ios requires a minimum duration of the depolarizing clamp and becomes larger with longer clamps. With repolarization to more negative potentials (approximately or equal to 90 mV), Ios becomes smaller and may disappear. Also Ios can be triggered twice if the potential is clamped to two different levels in succession. By several procedures which modify the other known currents (fast Na+ current, slow inward current, early outward current, plateau current Ix1 and pacemaker current), it can be demonstrated that Ios is not due to their oscillatory behavior and can occur in the absence of any one of them. Interventions which increase the contractile force presumably by increasing intracellular calcium stores enhance the Ios or may make it appear. In fact, these interventions may extend the voltage range over which Ios appears. These interventions include lowering potassium, increasing calcium, trains of depolarizing clamps, and administration of norepinephrine and of strophanthidin. It is concluded that Ios is a physiological event which is enhanced by certain procedures, and it appears to be of much importance in drive-induced arrhythmias under different conditions.