Contributions of charged residues in a cytoplasmic linking region to Na channel gating

Na channels inactivate quickly after opening, and the very highly positivel y charged cytoplasmic linking region between homologous domains III and IV of the channel molecule acts as the inactivation gate. To test the hypothes is that the charged residues in the domain III to domain IV linker have a r ole in channel function, we measured currents through wild-type and two mut ant skeletal muscle Na channels expressed in Xenopus oocytes, each lacking two or three charged residues in the inactivation gate. Microscopic current measures showed that removing charges hastened activation and inactivation . Macroscopic current measures showed that removing charges altered the vol tage dependence of inactivation, suggesting less coupling of the inactivati on and activation processes. Reduced intracellular ionic strength shifted t he midpoint of equilibrium activation gating to a greater extent, and shift ed the midpoint of equilibrium inactivation gating to a lesser extent in th e mutant channels. The results allow the possibility that an electrostatic mechanism contributes to the role of charged residues in Na channel inactiv ation gating. (C) 2000 Elsevier Science B.V. All rights reserved.