Lidocaine induces a slow inactivated state in rat skeletal muscle sodium channels

1. 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. 2. 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 I(M)) that involves structural rearrangements in the outer pore. 3. 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. 4. 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 I(M). 5. 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).