Early outward current in rat single ventricular cells

Voltage clamp experiments were conducted using single ventricular myocytes which had been dissociated enzymatically from adult rat hearts in order to examine further the membrane currents which contribute to the unusual plateau of the rat action potential. Membrane currents were recorded, using a single microelectrode (switching) voltage clamp circuit. From holding potentials near the resting potential (-80 to -90 mV), depolarizing clamp steps above -20 mV elicited an early outward current which overlapped in time with the slow inward current and displayed time-dependent inactivation. This is the first demonstration of a transient potassium current in an isolated ventricular myocyte. The early outward potential was voltage-inactivated at holding potentials of -50 to -40 mV and was blocked by 4-aminopyridine. The current was not dependent on Cao or ICa and was blocked by Bao. Double pulse experiments revealed that the time course for the recovery of the early outward current at -80 mV was rapid, and had a tau of 25 msec. The possible functional significance of this current is discussed.