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

Zn(2+) (1-1,000 microM) 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 Zn(2+) inhibition was 45 microM. Varying the apical Na(+) concentration, we determined the equilibrium constant of the short-circuit current saturation (34.9 mM) and showed that Zn(2+) inhibition is non-competitive. A similar effect was observed in Xenopus oocytes expressing alphabetagammarENaC (alpha-, beta-, and gamma-subunits of the rat epithelial Na(+) channel) in the concentration range of 1-10 microM Zn(2+), while at 100 microM Zn(2+) exerted a stimulatory effect. The analysis of the voltage dependence of the steady-state conductance revealed that the inhibitory effect of Zn(2+) 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 Zn(2+). The stimulatory effect of high Zn(2+) concentrations could be reproduced by intra-oocyte injection of Zn(2+) (approximately 10 microM), which had no direct effect on the amiloride-sensitive conductance, but switched the effect of extracellular Zn(2+) from inhibition to activation.