Contribution of the transient outward current to the repolarization of rabbit ventricular cells

Action potentials and membrane currents were recorded from a single ventricular cell from rabbit hearts by means of the suction pipette method. Action potentials elicited after a rest period longer than 2 sec had a distinct phase 1 with a low level of the second depolarization. Those after a diastolic interval shorter than 1 sec exhibited a small or absent phase 1 and attained the higher level of the second depolarization with prolonged action potential duration. In the whole cell patch clamp experiments, the transient outward current was activated upon strong depolarizations with an apparent threshold voltage around +30 mV. This current had a slow reactivation time-course without reaching a full recovery even after a rest period of 2 sec, while that of the calcium current achieved a full recovery after about 300 msec. These results suggest that the transient outward current contributes to form the phase 1 of the rabbit ventricular action potential and its slow recovery from inactivation unmasking the calcium current reactivation causes a loss of the phase 1 and an increase in the action potential duration at a sudden shortening of the diastolic interval.