Extracellular Histidine Residues Crucial for Na+ Self-inhibition of Epithelial Na+ Channels

Abstract Epithelial Na+ channels (ENaC) participate in the regulation of extracellular fluid volume homeostasis and blood pressure. Channel activity is regulated by both extracellular and intracellular Na+. The down-regulation of ENaC activity by external Na+ is referred to as Na+ self-inhibition. We investigated the structural determinants of Na+ self-inhibition by expressing wild-type or mutant ENaCs in Xenopus oocytes and analyzing changes in whole-cell Na+ currents following a rapid increase of bath Na+ concentration. Our results indicated that wild-type mouse αβγENaC has intrinsic Na+ self-inhibition similar to that reported for human, rat, and Xenopus ENaCs. Mutations at His239 (γH239R, γH239D, and γH239C) in the extracellular loop of the γENaC subunit prevented Na+ self-inhibition whereas mutations of the corresponding His282 in αENaC (αH282D, αH282R, αH282W, and αH282C) significantly enhanced Na+ self-inhibition. These results suggest that these two histidine residues within the extracellular loops are crucial structural determinants for Na+ self-inhibition.