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, muA.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 Ki from the lumen reduced I,, significantly from 215 to 133 muA.cm(-2). Addition of Ba2+ (10(-3) mol.l(-1)) which blocks the K+ conductance of the luminal membrane, to the Kf-containing lumen perfusate decreased I-SC significanlty from 234 to 141 muA.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 muA.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.
