Regulation of the epithelial Na+ channel by cytosolic ATP.

The epithelial Na+ channel (ENaC), composed of three subunits (alphabetagamma), is expressed in various Na(+)-absorbing epithelia and plays a critical role in salt and water balance and in the regulation of blood pressure. By using patch clamp techniques, we have examined the effect of cytosolic ATP on the activity of the rat alphabetagammaENaC (rENaC) stably expressed in NIH-3T3 cells and in Madin-Darby canine kidney epithelial cells. The inward whole-cell current attributable to rENaC activity ran down when these cells were dialyzed with an ATP-free pipette solution in the conventional whole-cell voltage-clamping technique. This run down was prevented by 2 mM ATP (but not by AMP or ADP) in the pipette solution or by the poorly or non-hydrolyzable analogues of ATP (adenosine 5'-O-(thiotriphosphate) and adenosine 5'-(beta,gamma-imino)triphosphate) in both cell lines, suggesting that protection from run down was mediated through non-hydrolytic nucleotide binding. Accordingly, we demonstrate binding of ATP (but not AMP) to alpharENaC expressed in Madin-Darby canine kidney cells, which was inhibited upon mutation of the two putative nucleotide-binding motifs of alpharENaC. Single channel analyses indicated that the run down of currents observed in the whole-cell recording was attributable to run down of channel activity, defined as NPo (the product of the number of channels and open probability). We propose that this novel ATP regulation of ENaC may be, at least in part, involved in the fine-tuning of ENaC activity under physiologic and pathophysiologic conditions.