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.
Yu
Ling
L
Dept. of Physiology, Emory University School of Medicine, Atlanta, GA 30322, USA. lyu@physio.emory.edu
Eaton
Douglas C
DC
Helms
My N
MN
D000465:Algorithms; D000818:Animals; D002413:Cations, Divalent / Q000494:pharmacology*; D002460:Cell Line; D053503:Epithelial Sodium Channels / Q000378:metabolism; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism; D045604:Extracellular Fluid / Q000378:metabolism; D006639:Histidine / Q000378:metabolism; D007668:Kidney / Q000187:drug effects* / Q000378:metabolism*; D019216:Metals, Heavy / Q000494:pharmacology*; D018408:Patch-Clamp Techniques; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects* / Q000378:metabolism*; D014982:Xenopus laevis
D002413:Cations, Divalent; D053503:Epithelial Sodium Channels; D019216:Metals, Heavy; D015222:Sodium Channels; D006639:Histidine; D012964:Sodium