The role of inactivation in open-channel block of the sodium channel: studies with inactivation-deficient mutant channels.

Inactivation has been implicated as an important determinant of the block of Na+ channel by local anesthetic-class drugs. This proposition has been difficult to examine because agents used to modify inactivation change other channel properties and both inactivated and blocked channels do not conduct. We used site-directed mutagenesis of Phe1304 to glutamine in the linker between the third and fourth domains of the mu-1 Na+ channel to slow inactivation. Wild-type and mutant channels were expressed in frog oocytes. Macropatch and single-channel currents were recorded in cell-attached membrane patches. The F1304Q mutation increased mean open time (1.7 fold at -20 mV) and reduced the probability that the channel would fail to open. Closed times were best fit by a double-exponential function, suggesting that the inactivated state transitions were no longer absorbing. In wild-type channels, 100 microM disopyramide decreased mean open time from 1.64 +/- 0.08 to 0.34 +/- 0.04 msec. Total open time per trial was decreased 2-fold. There also was a marked increase in the fraction of null sweeps. In the inactivation-deficient mutant channel, mean and total open times were also reduced. These data indicate that even when inactivation is slowed by a localized specific mutation, open-channel block by disopyramide persists. Inactivation may not be a necessary requirement for open-channel block.