Open probability of the epithelial sodium channel is regulated by intracellular sodium

The regulation of epithelial Na+ channel (ENaC) activity by Na+ was studied in Xenopus oocytes using two-electrode voltage clamp and patch-clamp recording techniques. Here we show that amiloride-sensitive Na+ current (I-Na) is downregulated when ENaC-expressing cells are exposed to high extracellular [Na+]. The reduction in macroscopic Na+ current is accompanied by an increase in the concentration of intracellular Na+ ([Na+](i)) and is only slowly reversible. At the single-channel level, incubating oocytes in high-Na+ solution reduces open probability (P-o) approximately twofold compared to when [Na+] is kept low, by increasing mean channel closed times. However, increasing P-o by introducing a mutation in the beta-subunit (S518C) which, in the presence of [2-(trimethylammonium) ethyl] methane thiosulfonate (MTSET), locks the channel in an open state, could not alone abolish the downregulation of macroscopic current measured with exposure to high external [Na+]. Inhibition of the insertion of new channels into the plasma membrane using Brefeldin A revealed that surface channel lifetime is also markedly reduced under these conditions. In channels harbouring a beta-subunit mutation, R564X, associated with Liddle's syndrome, open probability in both high- and low-Na+ conditions is significantly higher than in wild-type channels. Increasing the P-o of these channels with an activating mutation abrogated the difference in macroscopic current observed between groups of oocytes incubated in high- and low-Na+ conditions. These findings demonstrate that reduction of ENaC P-o is a physiological mechanism limiting Na+ entry when [Na+](i) is high.