Characterization of the External Sodium Inhibition of hERG Potassium Channels

Abstract

In the absence of external K+, external Na+ (Na+o) inhibited the human ether-a-go-go-related gene (hERG)-encoded channel K+ current (IhERG-K) with an IC50 of 4.4 ± 0.7 mM, and Na+ current (IhERG-Na) with an IC50 of 3.5 ± 0.4 mM (P > 0.05). Using the whole cell patch clamp and site-directed mutagenesis methods on recombinant hERG channels, we found that Na+o-induced inhibition of hERG current was intrinsically independent of either activation or inactivation of the channel. In the absence of K+, Na+o inhibited IhERG-Na in a time-dependent manner with a time constant of 15.0 ± 0.4 s. The recovery of IhERG-Na from inhibition after washout of Na+o was also time dependent, with a time constant of 27.1 ± 1.0 s. K+ competes with Na+ for binding to interfere with Na+o mediated inhibition of hERG channels. When IhERG-K was maximally inhibited by 135 mM Na+o, outflowing K+ during channel opening dynamically relived Na+o-induced inhibition. As well, addition of K+ to the 135 mM Na+-containing bath solutions relieved the Na+o-mediated inhibition of IhERG-K with an EC50 of 1.2 mM. Point mutations in the hERG pore region were identified to completely eliminate hERG Na+ sensitivity. We propose that K+o and Na+o compete for a binding site(s) close to the permeation pathway of the channel to determine the fate of hERG function. While K+-bound hERG channels gate normally, Na+-bounding fosters the hERG channel entering into a nonfunctional conformation.