Zinc is a voltage-dependent blocker of native and heterologously expressed epithelial Na+ channels

Zn2+ (1–1,000 μM) applied to the apical side of polarized A6 epithelia inhibits Na+ transport, as reflected in short-circuit current and conductance measurements. The Menten equilibrium constant for Zn2+ inhibition was 45 μM. Varying the apical Na+ concentration, we determined the equilibrium constant of the short-circuit current saturation (34.9 mM) and showed that Zn2+ inhibition is non-competitive. A similar effect was observed in Xenopus oocytes expressing αβγrENaC (α-, β-, and γ-subunits of the rat epithelial Na+ channel) in the concentration range of 1–10 μM Zn2+, while at 100 μM Zn2+ exerted a stimulatory effect. The analysis of the voltage dependence of the steady-state conductance revealed that the inhibitory effect of Zn2+ was due mainly to a direct pore block and not to a change in surface potential. The equivalent gating charge of ENaC, emerging from these data, was 0.79 elementary charges, and was not influenced by Zn2+. The stimulatory effect of high Zn2+ concentrations could be reproduced by intra-oocyte injection of Zn2+ (~10 μM), which had no direct effect on the amiloride-sensitive conductance, but switched the effect of extracellular Zn2+ from inhibition to activation.