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.