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.
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