Presence of luminal K+, a prerequisite for active NaCl transport in the cortical thick ascending limb of Henle's loop of rabbit kidney

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 (I_SC, μA · 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 I_SC significantly from 215 to 133 μA·cm⁻². Addition of Ba²⁺ (10⁻³ mol·l⁻¹) which blocks the K⁺ conductance of the luminal membrane, to the K⁺-containing lumen perfusate decreased I_SC significantly from 234 to 141 μA·cm⁻². Combination of both manoeuvres: perfusion with a K⁺-free and Ba²⁺-containing solution almost abolished I_SC from a control of 237 to 56 μA · 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.