Na+ sensitivity of ROMK1 K+ channel: Role of the Na+/H+ antiporter

To examine the extracellular Na+ sensitivity of a renal inwardly rectifying KC channel, we performed electrophysiological experiments on Xenopus oocyt es or a human kidney cell line, HEK293, in which we had expressed the clone d renal K+ channel, ROMK1 (Kir1.1). When extracellular Na+ was removed, the whole-cell ROMK1 currents were markedly suppressed in both the oocytes and HEK293 cells. Single-channel ROMK1 activities recorded in the cell-attache d patch on the oocyte were not affected by removal of Na+ from the pipette solution. However, macro-patch ROMK1 currents recorded on the oocyte were s ignificantly suppressed by Na+ removal from the bath solution. A blocker of Na+/H+ antiporters, amiloride, largely inhibited the Na+ removal-induced s uppression of whole-cell ROMK1 currents in the oocytes. The pH-insensitive K80M mutant of ROMK1 was much less sensitive to Na+ removal. Na+ removal wa s found to induce a significant decrease in intracellular pH in the oocytes using H+-selective microelectrodes. Coexpression of ROMK1 with NHE3, which is a Na+/H+ antiporter isoform of the kidney apical membrane, conferred in creased sensitivity of ROMK1 channels to extracellular Na+ in both the oocy tes and HEK293 cells. Thus, it is concluded that the ROMK1 channel is regul ated indirectly by extracellular Na+, and that the interaction between NHE transporter and ROMK1 channel appears to be involved in the mechanism of Na + sensitivity of ROMK1 channel via regulating intracellular pH.