Extracellular Zn2+ activates epithelial Na+ channels by eliminating Na+ self-inhibition

Inhibition of epithelial Na+ channel ( ENaC) activity by high concentrations of extracellular Na+ is referred to as Na+ self-inhibition. We investigated the effects of external Zn2+ on whole cell Na+ currents and on the Na+ self-inhibition response in Xenopus oocytes expressing mouse alphabetagamma ENaC. Na+ self-inhibition was examined by analyzing inward current decay from a peak current to a steady-state current following a fast switching of a low Na+ (1 mM) bath solution to a high Na+ (110 mM) solution. Our results indicate that external Zn2+ rapidly and reversibly activates ENaC in a dose-dependent manner with an estimated EC50 of 2 muM. External Zn2+ in the high Na+ bath also prevents or reverses Na+ self-inhibition with similar affinity. Zn2+ activation is dependent on extracellular Na+ concentration and is absent in ENaCs containing gammaH239 mutations that eliminate Na+ self-inhibition and in alphaS580Cbetagamma following covalent modification by a sulfhydryl-reactive reagent that locks the channels in a fully open state. In contrast, external Ni2+ inhibition of ENaC currents appears to be additive to Na+ self-inhibition when Ni2+ is present in the high Na+ bath. Pretreatment of oocytes with Ni2+ in a low Na+ bath also prevents the current decay following a switch to a high Na+ bath but rendered the currents below the control steady-state level measured in the absence of Ni2+ pretreatment. Our results suggest that external Zn2+ activates ENaC by relieving the channel from Na+ self-inhibition, and that external Ni2+ mimics or masks Na+ self-inhibition.