ROMK1 is an inwardly rectifying K+ channel cloned from the outer medulla of rat kidney. We have determined the permeation and blocking characteristics for several monovalent cations in ROMK1 when expressed in Xenopus oocytes, using the two-electrode voltage-clamp technique. The selectivity sequence for monovalent cations as determined by reversal potential changes under bi-ionic conditions was K+ > Rb+ > Cs+ = NH4+ >> Na+ = Li+. The conductivity for the two permeant ions K+ and Rb+ was a saturable function of the external concentration, with Km values of 11.5 +/- 1.3 mmol l-1 (n = 19) and 47.3 +/- 4.8 mmol l-1 (n = 19), respectively. With mixtures of K+ and Rb+, the conductance ratio [Rb]/[K+Rb] was varied between 0 and 1. ROMK1 was blocked by both Cs+ and Ba2+ in a concentration- and voltage-dependent manner. The electrical distance (delta) at which Ba2+ and Cs+ blocked the channel was 0.41 and 0.69, respectively, suggesting that these two ions block at different sites within the pore. Taken together with previous reports, the results indicate that ROMK1 has a multi-ion pore, and that the N-terminus contributes to the pore structure.

The established opossum kidney (OK) cell line serves as a model system for ion and substrate transport in the renal proximal tubule. Previous experiments on OK cells revealed a channel-mediated Na⁺ conductance which is regulated by intracellular pH (pH_i). In this study we report on patch clamp experiments determining the properties and pHi dependence of a cation channel located in the apical membrane. This channel is selective for sodium over chloride but discriminates poorly between the monovalent cations Na⁺, K⁺, Li⁺ and Cs⁺. Its open probability (P_o) rises at depolarising membrane potentials. Under normal conditions the channel is inactive in the cell-attached patch mode and is activated upon excision. However, after excision the channel usually runs down within 30-90 s which cannot be overcome by either altering the Ca²⁺-concentration (10^-3mol/l, 10^-6mol/l, Ca²⁺-free) or adding 1 mmol/l Mg-ATP to the bath solution. In the cell-attached patch mode the channel could be activated by decreasing pH_i from pH 7.4 to pH 6.5, by either the ammonium prepulse technique or the nigericin K⁺ method, in more than 50% of the experiments performed. In the renal proximal tubule such a non-selective cation channel would constitute a functional Na⁺ channel and might therefore support Na⁺ reabsorption especially during the intracellular acidification due to hormonal inhibition of the Na⁺/H⁺ exchanger.