Single Channel Studies and Kinetic Modeling of an Inactivation Deficient Voltage-Gated Sodium Channel

Abstract

Rapid inactivation is a hallmark of voltage-gated sodium channels critical for regulating the rate of electrical signaling between excitable cells. However, activation and inactivation processes can overlap making it difficult to determine which process is altered during a given perturbation. Moreover, at the single channel level rapid entry into inactivated states occludes less frequent channel activity. Thus, removing inactivation should simplify the interpretation of macroscopic effects and reveal the intrinsic gating behavior associated with channel activation. Here, we studied the single channel behavior of mutant sodium channels lacking fast inactivation (rat Nav1.4 L435W/L437C/A438W; Wang et al., 2003). These channels open repeatedly in response to voltage steps from −60 to 0 mV. Most of the voltage dependence of activation could be ascribed to the first latencies and the length of closures separating bursts of channel activity. Unlike non-inactivating voltage gated potassium channels which have a single open state (Hoshi et al., 1994), we observed two subconductance levels in single channels at all voltages, including during deactivation to −80, −100 and −120 mV. The dwell times in each conducting state during activation were not very voltage dependent, as were their relative probabilities following channel opening. To quantitatively explain these data, we developed a kinetic framework for the gating of these sodium channels. This work was funded by NIH grant GM084140.