Constitutive inactivation of the hKv1.5 mutant channel, H463G, in K+-free solutions at physiological pH

Extracellular acidification and reduction of extracellular K+ are known to decrease the currents of some voltage-gated potassium channels. Although the macroscopic conductance of WT hKv1.5 channels is not very sensitive to [K+](o) at pH 7.4, it is very sensitive to [K+](o) at pH 6.4, and in the mutant, H463G, the removal of K-o(+) virtually eliminates the current at pH 7.4. We investigated the mechanism of current regulation by K-o(+) in the Kv1.5 H463G mutant channel at pH 7.4 and the wild-type channel at pH 6.4 by taking advantage of Na+ permeation through inactivated channels. Although the H463G currents were abolished in zero [K+](o), robust Na+ tail currents through inactivated channels were observed. The appearance of H463G Na+ currents with a slow rising phase on repolarization after a very brief depolarization (2 ms) suggests that channels could activate directly from closed-inactivated states. In wild-type channels, when intracellular K+ was replaced by NMG(+) and the inward Na+ current was recorded, addition of 1 MM K+ prevented inactivation, but changing pH from 7.4 to 6.4 reversed this action. The data support the idea that C-type inactivation mediated at R487 in Kv1.5 channels is influenced by H463 in the outer pore. We conclude that both acidification and reduction of [K+](o) inhibit Kv1.5 channels through a common mechanism (i.e., by increasing channel inactivation, which occurs in the resting state or develops very rapidly after activation).