On the mechanism underlying the oscillatory current in cardiac Purkinje fibers

The mechanism underlying the oscillatory current (Ios) was investigated in sheep cardiac Purkinje fibers with a voltage-clamp technique. The Ios is initiated not only by repolarizing but also by a depolarizing clamp, provided that the preparation is preloaded with calcium by means of a conditioning clamp and that the depolarizing test clamp initiates the slow inward current. The Ios initiated by a depolarizing test clamp is usually smaller and has a longer time to peak than that initiated by a repolarizing clamp to the same potential. Brief depolarizing clamps can be followed by an Ios in fibers preloaded with calcium. The amplitude of Ios diminishes as the interval from the conditioning clamp increases. No Ios is initiated by repolarizing or depolarizing clamps to potentials positive to approximately -30 to -40 mV even when [Na]0 is lowered. If a test clamp is applied at a peak of Ios to potentials less negative than approximately -30 mV, the current disappears. The membrane conductance during the Ios is lower. It is concluded that Ios is initiated indirectly by repolarization and is due to a calcium-triggered release of calcium which may initiate an electrogenic Na-Ca exchange.