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. Recovery from inactivation exhibited three kinetic components (IF, fast; IM, intermediate; IS, slow). The effects of W402A and W402S on IF and IS differed, but both mutants (with or without β1 subunit coexpression) decreased the amplitude of IM. In wild‐type channels, lidocaine imposed a delayed recovery component with intermediate kinetics, and use‐dependent block was attenuated in both W402A and W402S. To examine the pharmacological role of IS relative to IM, drug‐exposed β1‐coexpressed channels were subjected to 2 min depolarizations. Lidocaine had no effect on sodium current (INa) after a 1 s hyperpolarization interval that allowed recovery from IM but not IS, suggesting that lidocaine affinity for IS is low. Both W402 mutations reduced occupancy of IM 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.

The R1623Q mutation produces inactivation gating defects that differ mechanistically from those caused by LQT3 mutations. These findings provide a biophysical explanation for this severe long-QT phenotype and extend our understanding of the mechanistic role of the S4 segment in cardiac sodium channel inactivation gating and class I antiarrhythmic drug action.

Local anaesthetics such as lidocaine (lignocaine) interact with sodium channels in a manner that is exquisitely sensitive to the voltage‐dependent conformational state of the ion channel. When depolarized in the presence of lidocaine, sodium channels assume a long‐lived quiescent state. Although studies over the last decade have localized the lidocaine receptor to the inner aspect of the aqueous pore, the mechanistic basis of depolarization‐induced ‘use‐dependent’ lidocaine block remains uncertain. Recent studies have shown that lowering the extracellular Na+ concentration ([Na+]o) and mutations in the sodium channel outer P‐loop modulate occupancy of a quiescent ‘slow’ inactivated state with intermediate kinetics (termed IM) that involves structural rearrangements in the outer pore. Site‐directed mutagenesis and ion‐replacement experiments were performed using voltage‐clamped Xenopus oocytes and cultured (HEK‐293) cells expressing wild‐type and mutant rat skeletal muscle (μ1) sodium channels. Our results show that lowering [Na+]o potentiates use‐dependent lidocaine block. The effect of [Na+]o is maintained despite a III‐IV linker mutation that partially disrupts fast inactivation (F1304Q). In contrast, the effect of lowering [Na+]o on lidocaine block is reduced by a P‐loop mutation (W402A) that limits occupancy of IM. Our findings are consistent with a simple allosteric model where lidocaine binding induces channels to occupy a native slow inactivated state that is inhibited by [Na+]o.