Effect of divalent heavy metals on epithelial Na+ channels in A6 cells

To better understand how renal Na(+) reabsorption is altered by heavy metal poisoning, we examined the effects of several divalent heavy metal ions (Zn(2+), Ni(2+), Cu(2+), Pb(2+), Cd(2+), and Hg(2+)) on the activity of single epithelial Na(+) channels (ENaC) in a renal epithelial cell line (A6). None of the cations changed the single-channel conductance. However, ENaC activity [measured as the number of channels (N) x open probability (P(o))] was decreased by Cd(2+) and Hg(2+) and increased by Cu(2+), Zn(2+), and Ni(2+) but was not changed by Pb(2+). Of the cations that induced an increase in Na(+) channel function, Zn(2+) increased N, Ni(2+) increased P(o), and Cu(2+) increased both. The cysteine modification reagent [2-(trimethylammonium)ethyl]methanethiosulfonate bromide also increased N, whereas diethylpyrocarbonate, which covalently modifies histidine residues, affected neither P(o) nor N. Cu(2+) increased N and stimulated P(o) by reducing Na(+) self-inhibition. Furthermore, we observed that ENaC activity is slightly voltage dependent and that the voltage dependence of ENaC is insensitive to extracellular Na(+) concentration; however, apical application of Ni(2+) or diethylpyrocarbonate reduced the channel voltage dependence. Thus the voltage sensor of Xenopus ENaC is different from that of typical voltage-gated channels, since voltage appears to be sensed by histidine residues in the extracellular loops of ENaC, rather than by charged amino acids in a transmembrane domain.