Block of single cardiac Na+ channels by antiarrhythmic drugs: the effect of amiodarone, propafenone and diprafenone.

Cell-attached patch-clamp experiments were performed in cultured cardiocytes of neonatal rats at 19 degrees C to analyze elementary currents through single Na+ channels under control conditions and in the presence of the class 1 antiarrhythmic drugs amiodarone, propafenone, and diprafenone. As observed in a cell-attached patch with only one functioning Na+ channel, repetitive stepping of the membrane at 0.4 Hz triggered periodically channel openings except during a silent period of about 1.5 min. The latter began and ceased abruptly and did not fit the monoexponential distribution of the run length of sweeps without activity (blank sweeps). Treating the cardiocytes with amiodarone, propafenone or diprafenone (10 to 20 mumol/liter) led rapidly to a blockage and reduced the likelihood that membrane depolarization triggers the opening of Na+ channels. The number of blank sweeps increased at the expense of the number of sweeps with activity. The fraction of activity sweeps with superpositions, indicating the simultaneous activation of two or more Na+ channels, also declined. As tested with amiodarone, the run length of blank sweeps is voltage- and time-dependent, analogous to the intensity of the block of macroscopic Na+ currents. Open time, open-time distribution, unitary current size and the tendency to reopen did not differ in unblocked cardiac Na+ channels (i.e. that channel fraction capable of opening in the presence of amiodarone or propafenone) from the respective control values obtained before superfusing the cardiocytes with these drugs. Apart from its blocking action, the propafenone derivative diprafenone exerted additionally a modifying effect and reduced mean open time by up to 45%. In contrast to the block, this reduction in conducting state proved insensitive to changes in holding potential, at least between -130 and -150 mV, the range tested. This means that block was attenuated on hyperpolarization whereas the reduction in open time persisted. It is concluded that, in the presence of these drugs, unblocked cardiac Na+ channels share a number of properties with normal Na+ channels in the absence of these drugs. Shortening of channel lifetime by diprafenone may be indicative of a channel modification brought about possibly by a receptor-mediated facilitation of the transition from the open to the inactivated state.