Existence of two fast inactivation states in cardiac Na channels confirmed by two-stage action of proteolytic enzymes.

Fast inactivation of Na channels in neonatal cardiac cells was removed by the action of proteolytic enzymes trypsin or papain. Two stages were apparent in the time course of this process. During the first one, both number of channel reopenings and the mean open time increased markedly even though fast inactivation remained complete. The second stage was manifested by the disappearance of all signs of fast inactivation without further noticeable changes in channel mean open time. At the same time the nonrandom clustering of blank response (response without channel openings) trials became prominent. The data obtained support the interpretation of two separate fast inactivation states in cardiac Na channels as suggested in our previous papers (Zilberter et al. (1989) in Neuromuscular Junction (Sellin, L.C., Libelius, R. and Thesleff, S., eds.), pp. 43-50, Elsevier, Amsterdam, and Zilberter et al. (1991) J. Mol. Cell. Cardiol. 23, (Suppl.) 61-72).