A new calcium current underlying the plateau of the cardiac action potential.

A small and very slow inward calcium current has been identified in isolated single ventricular cells using TTX and Cd2+ to block the sodium and fast calcium currents. Activation requires about 300 ms at the threshold potential of -60 mV, decreasing to 80 ms at the peak current voltage of -30 mV. Inactivation is five to ten times longer. Half steady-state activation and inactivation are at -50 and -45 mV respectively. The current is distinctively different in both its kinetics and pharmacology from the conventional calcium current described in single heart cells. It is proposed that it contributes significant current to help maintain a major portion of the long ventricular action potential.