The activity of ROMK channels in rat kidney tubule cells was assessed as tertiapin-Q (TPNQ)-sensitive current under whole cell clamp conditions. With an external K(+) concentration of 5 mM and an internal K(+) concentration of 140 mM and the membrane potential clamped to 0 mV, TPNQ blocked outward currents in principal cells of the cortical collecting duct (CCD) outer medullary collecting duct and connecting tubule (CNT). The apparent K(i) was 5.0 nM, consistent with its interaction with ROMK. The TPNQ-sensitive current reversed at voltages close to the equilibrium potential for K(+). The currents were reduced when the pipette solution contained ATP. In the CCD, the average TPNQ-sensitive outward current (I(SK)) was 476 +/- 48 pA/cell in control animals on a 1% KCl diet. I(SK) increased to 1,255 +/- 140 pA when animals were maintained on a high-K (10% KCl) diet for 7 days and decreased to 314 +/- 46 pA after 7 days on a low-K (0.1% KCl) diet. In the CNT, I(SK) was 360 +/- 30 pA on control, 1,160 +/- 110 on high-K, and 166 +/- 16 pA on low-K diets. The results indicate that ROMK channel activity is highly regulated by dietary K in both the CCD and the CNT.