Slow Inactivation of Nav Channels Described by Composite Movements of Voltage Sensors in Domains I, II and III

Abstract

Slow inactivation (SI) of voltage-gated sodium (NaV) channels is a primary determinant of their ‘availability’ and therefore the excitability of muscles and nerves. This process has been difficult to describe because it is a multi-timescale phenomenon that can be perturbed by mutations at many channel locales including the S4 voltage sensors (VS). Here, we compare ionic and gating currents during SI with changes in fluorescence from probes conjugated to the VS in each of the four domains (DI-DIV) of the rat skeletal muscle isoform (NaV1.4). We observe that onset and recovery of SI can be approximated by three time constants of ∼1, 10 and 100 s and that these temporal components are reflected in domain-specific fluorescence immobilization. The kinetics of immobilization of DI and DII correlate with the 10 s component and are potently inhibited by TTX. Immobilization of DIII reflects both the 1 and 100 s components and enhancing the speed and magnitude of SI via a mutation that removes fast inactivation (IQM) has the same effects on DIII immobilization. Limited immobilization observed for DIV shows no correlation with SI. Rapid gating is a hallmark of NaV channels and results from movements of VSs over ms. This study links slow, differential movements of VSs in DI, DII and DIII with the time constants required to model SI kinetics over multiple time domains. The findings offer a basis for understanding the effects on SI of previously reported disease-associated and experimental mutations in the pore, inactivation lid and VSs.