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Abstract

Previous data from our laboratory have shown that active transport in the cortical thick ascending limb of Henle's loop (cTAL), as measured by the short circuit current (Lsc, microA X cm⁻²), requires the presence of Na⁺ and Cl⁻. The data were compatible with the model of secondarily active Cl⁻ reabsorption involving the cotransport of Na⁺ and Cl⁻ across the luminal membrane. The data suggested, furthermore, that 1 Na⁺ and 2 Cl⁻ interact with the luminal carrier. In the present study it was tested whether this reabsorptive mechanism also requires the presence of luminal K⁺. Isolated cTAL segments (n = 40) were perfused at high flow rates with a modified Ringer's solution. Removal of K⁺ from the lumen reduced Isc significantly from 215 to 133 microA X cm⁻². Addition of Ba²⁺ (10⁻³ mol X 1⁻¹) which blocks the K⁺ conductance of the luminal, membrane, to the K⁺-containing lumen perfusate decreased Isc significantly from 234 to 141 microA X cm⁻². Combination of both manoeuvres: perfusion with a K⁺-free and Ba²⁺-containing solution almost abolished Isc from a control of 237 to 56 microA X cm⁻². The results are compatible with the view that in rabbit cTAL the luminal carrier interacts with all 3 ions, possibly 1 Na⁺, 2 Cl⁻, and 1 K⁺. K⁺ recycles across the luminal membrane through its conductive pathway.