Parallel Intermediate Conductance K and Cl 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 the 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. The present study was designed to identify basolateral transporter(s) responsible for K exit in rat distal colon. Active K absorption was determined by measuring Rb (K surrogate) fluxes across colonic epithelia under voltage-clamp conditions. With zero Cl in the mucosal solution, net K absorption was reduced by 38%, indicating that K absorption was partially Cl-dependent. Serosal addition of DIOA (KCC1 inhibitor) or Ba (nonspecific K channel blocker) inhibited net K absorption by 21% or 61%, respectively, suggesting that both KCC1 and K channels contribute to basolateral K exit. Clotrimazole and TRAM34 (IK channel blockers) added serosally inhibited net K absorption, pointing to the involvement of IK channels in basolateral K exit. GaTx2 (CLC2 blocker) added serosally 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 (K7 channel blocker), or CFTR (CFTR blocker). Immunofluorescence studies confirmed basolateral membrane colocalization of CLC2-like proteins and Na-K-ATPase α-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. This study demonstrates that during active electroneutral K absorption in rat distal colon, K exit across the basolateral membrane mainly reflects intermediate conductance K channels operating in conjunction with chloride channel 2, with a smaller, but significant, contribution from K-Cl cotransporter-1 (KCC1) activity.