CATION PERMEATION AND BLOCKADE OF ROMK1, A CLONED RENAL POTASSIUM CHANNEL

ROMK1 is an inwardly rectifying K+ channel cloned from the outer medul la of rat kidney. We have determined the permeation and blocking chara cteristics for several monovalent cations in ROMK1 when expressed in X enopus oocytes, using the two-electrode voltage-clamp technique. The s electivity sequence for monovalent cations as determined by reversal p otential changes under bi-ionic conditions was K+ > Rb+ > Cs+ = NH4(+) much greater than Na+ = Li+. The conductivity for the two permeant io ns K+ and Rb+ was a saturable function of the external concentration, with K-m values of 11.5 +/- 1.3 mmol l(-1) (n = 19) and 47.3 +/- 4.8 m mol l(-1) (n = 19), respectively With mixtures of K+ and Rb+, the cond uctance went through a clear minimum as the concentration ratio [Rb]/[ K+Rb] was varied between 0 and 1. ROMK1 was blocked by both Cs+ and Ba 2+ in a concentration- and voltage-dependent manner. The electrical di stance (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 resul ts indicate that ROMK1 has a multiion pore, and that the N-terminus co ntributes to the pore structure.