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

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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 (ISC, 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 ISC 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 ISC significantly from 234 to 141 μA·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 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.

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