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Parallel intermediate conductance K + (IK) and Cl - (CLC2) channel activity mediates electroneutral K + exit across basolateral membranes in rat distal colon.

Transepithelial K+ absorption requires apical K+ uptake and basolateral K+ exit. In colon, apical H+ ,K+ -ATPase mediates cellular K+ uptake, and it has been suggested that electroneutral basolateral K+ exit reflects K+ -Cl- cotransporter-1 (KCC1) operating in parallel with K+ and Cl- channels. Present study was designed to identify basolateral transporter(s) responsible for K+ exit in rat distal colon. Active K+ absorption was determined by measuring 86 Rb+ (K+ surrogate) fluxes in colonic epithelia mounted under voltage clamp conditions. With zero Cl- in the mucosal solution, net K+ absorption was reduced by 38%, indicating that K+ absorption was, at least in part, Cl- -dependent. Addition to the serosal solution of DIOA (KCC1 inhibitor) and Ba2+ (non-specific K+ channel blocker) inhibited net K+ absorption by 21% and 61% respectively, suggesting that both KCC1 and K+ channels contribute to basolateral K+ exit. Serosal addition of clotrimazole and TRAM34 (IK channel blockers) inhibited net K+ absorption, pointing to the involvement of IK channels in basolateral K+ exit. Serosal addition of GaTx2 (CLC2 blocker) also inhibited net K+ absorption, suggesting that CLC2-mediated Cl- exit accompanies IK channel-mediated K+ exit across the basolateral membrane. Net K+ absorption was not inhibited by serosal addition of either IbTX (BK channel blocker), apamin (SK channel blocker), chromanol 293B (KV 7 channel blocker) or CFTRinh172 (CFTR blocker). Immunofluoresence studies confirmed basolateral membrane co-localization of CLC2-like proteins and Na+ ,K+ -ATPase a-subunits. We conclude that active K+ absorption in rat distal colon involves electroneutral basolateral K+ exit, which may reflect IK and CLC2 channels operating in parallel.

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