Transition metals inhibit epithelial sodium channels via distinct mechanisms

Abstract

The epithelial Na+ channel (ENaC) plays an essential role in body fluid volume homeostasis. We studied the effects of extracellular Mn2+, Co2+, Ni2+ and Hg2+ on human, mouse and rat ENaCs expressed in Xenopus oocytes. External Mn2+ inhibited amiloride‐sensitive currents of human and mouse ENaCs with maximal inhibition of 50% (hENaC) and 30% (mENaC) at 10 mM and did not affect the current of rat ENaC. Extracellular Co2+ inhibited all three ENaCs with estimated inhibitory constants (IC50) of 0.1 mM (hENaC), 2 mM (mENaC) and >10 mM (rENaC). Ni2+ inhibited hENaC, similar to the previously reported effect on mENaC and rENaC, but with 10‐fold higher affinity (IC50 of 50 μM). External Hg2+ blocked all three ENaCs with similar affinity (IC50 of ~10 μM). Mutations of the His residues implicated in Ni2+ block of mENaC greatly reduced the inhibitory effects of Co2+ on human and mouse ENaCs. However, for Ni2+ inhibition of hENaC, αH255A did not change the effect and γH233A significantly increased its maximal inhibition. Mouse ENaC inhibition by Hg2+ was not significantly altered by homologous mutations. Our results indicate that external Mn2+, Co2+ and Ni2+ differentially alter activities of human, mouse and rat ENaCs whereas Hg2+ has a similar inhibition on all three ENaCs. Mutational analyses suggest that these metals inhibit ENaC activity through distinct mechanisms. (Supported by NIH ES014701 and DK079307)