The role of apical and basolateral membranes in aldosteroneinduced active potassium (K) secretion in rat distal colon was investigated by measuring mucosal-to-serosal (Jm) and serosal-to-mucosal (s.m) 42K fluxes (jueq * h-.* cm-2) across isolated stripped mucosa under short-circuit conditions in normal and secondary-hyperaldosterone animals. In normal colons mucosal tetraethylammonium (TEA; 30 mM) or barium (Ba; 5 mM), but not cesium (Cs; 15 mM), reduced J,. without affecting J.s. In aldosterone animals (a) net K secretion (-0.54±0.11) was converted to net K absorption (0.63±0.15) by mucosal TEA, which produced a marked reduction in J,.

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