Role of the slow inward current during the repolarisation process in frog atrial trabeculae.

Voltage clamp experiments were performed to analyse the influence of the slow inward current (isi) on the repolarisation process in frog atrium. MnCl2 was used as isi blocker. The action potential was prolonged by depolarising pulses applied during the plateau. This was an expected result considering that, at positive potentials the time constant for isi inactivation increases. This effect was abolished by Mn ions which in turn block isi. The results suggest that inactivation of isi is of primary importance in determining the repolarisation rate. The action of Mn was not selective, since it reduced the background K current. This effect seems to be related to the decrease in Ca influx.