Mechanistic link between lidocaine block and inactivation probed by outer pore mutations in the rat μ1 skeletal muscle sodium channel

1. Mutations that disrupt Na⁺ channel fast inactivation attenuate lidocaine (lignocaine)-induced use dependence; however, the pharmacological role of slower inactivation processes remains unclear. In Xenopus oocytes, tryptophan substitution in the outer pore of the rat skeletal muscle channel (μ1-W402) alters partitioning among fast- and slow-inactivated states. We therefore examined the effects of W402 mutations on lidocaine block. 2. Recovery from inactivation exhibited three kinetic components (I(F), fast; I(M), intermediate; I(S), slow). The effects of W402A and W402S on I(F) and I(S) differed, but both mutants (with or without β₁ subunit coexpression) decreased the amplitude of I(M). In wild-type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use-dependent block was attenuated in both W402A and W402S. 3. To examine the pharmacological role of I(S) relative to I(M), drug-exposed β₁-coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (I(Na)) after a 1 s hyperpolarization interval that allowed recovery from I(M) but not I(S), suggesting that lidocaine affinity for I(S) is low. 4. Both W402 mutations reduced occupancy of I(M) in drug-free conditions, and also induced resistance to use-dependent block. We propose that lidocaine-induced use dependence may involve an allosteric conformational change in the outer pore.