Flow-dependent K⁺ secretion in the cortical collecting duct is mediated by a maxi-K channel

K⁺ secretion by the cortical collecting duct (CCD) is stimulated at high flow rates. Patch-clamp analysis has identified a small-conductance secretory K⁺ (SK) and a high-conductance Ca²⁺-activated K⁺ (maxi-K) channel in the apical membrane of the CCD. The SK channel, encoded by ROMK, is believed to mediate baseline K⁺ secretion. The role of the stretch- and Ca²⁺-activated maxi-K channel is still uncertain. The purpose of this study was to identify the K⁺ channel mediating flow-dependent K⁺ secretion in the CCD. Segments isolated from New Zealand White rabbits were microperfused in the absence and presence of luminal tetraethylammonium (TEA) or charybdotoxin, both inhibitors of maxi-K but not SK channels, or apamin, an inhibitor of small-conductance maxi-K⁺ channels. Net K⁺ secretion and Na⁺ absorption were measured at varying flow rates. In the absence of TEA, net K⁺ secretion increased from 8.3 ± 1.0 to 23.4 ± 4.7 pmol · min^-1·mm^-1 (P < 0.03) as the tubular flow rate was increased from 0.5 to 6 nl·min^-1·mm^-1. Flow stimulation of net K⁺ secretion was blocked by luminal TEA (8.2 ± 1.2 vs. 9.9 ± 2.7 pmol·min^-1·mm^-1 at 0.6 and 6 nl·min^-1·mm^-1 flow rates, respectively) or charybdotoxin (6.8 ± 1.6 vs. 8.3 ± 1.6 pmol·min^-1·mm^-1 at 1 and 4 nl·min^-1·mm^-1 flow rates, respectively) but not by apamin. These results suggest that flow-dependent K⁺ secretion is mediated by a maxi-K channel, whereas baseline K⁺ secretion occurs through a TEA- and charybdotoxin-insensitive SK (ROMK) channel.