Modulation of Na+ current inactivation by stimulation of protein kinase C in cardiac cells.

Modulation of the inward Na+ current (I(Na)) by protein kinase C (PKC) was investigated by intracellular perfusion of a peptide corresponding to the catalytic subunit of PKC (PKCP). The effects of PKC activation independent of membrane-receptor pathways were studied in neonatal rat ventricular myocytes using whole-cell patch-clamp techniques. Perfusion with 2 nmol/L PKCP caused a depolarizing shift in steady state half-inactivation relative to control (-83.2+/-1.3 versus -74.9+/-1.6 mV for control versus PKCP, respectively) without a change in current-voltage relationships or peak I(Na). The development of resting inactivation was slowed by PKCP (tau, 69.1+/-7.6 [control] versus 100.4+/-5.1 ms). Open-channel inactivation, estimated by measuring I(Na) decay from peak current at test voltages between -10 and +30 mV was significantly slowed by PKCP. Recovery from inactivation was more rapid during PKCP perfusion, with a shortening of both the fast (tau(f)) and slow (tau(s)) components of tau (tau(f), 38.5+/-7.0 [control] versus 14.2+/-4.7 ms; tau(s), 163.4+/-47.9 [control] versus 51.3+/-9.2 ms). All of the effects of PKCP on I(Na) were antagonized by the PKC inhibitors chelerythrine chloride or staurosporine or by down-regulation of PKC using phorbol ester preincubation. We conclude that the actions of PKC on the Na+ channel result in slowing the development of inactivation and accelerating reactivation, resulting in less resting inactivation.