Base secretion in rat distal colon: ionic requirements.

To evaluate the ionic requirements of colonic base secretion, segments of rat distal colon were studied under short-circuited conditions. Net base flux was composed of an active secretory component and a diffusive component. Studied in the absence of a transepithelial HCO3- concentration gradient, active base secretion was dependent on the HCO3- concentration of the bathing solution but was not influenced by the CO2 tension or pH. Base secretion appeared to saturate with a Km of 33 +/- 9 mM and was inhibited by ouabain. The diffusive component was characterized by an apparent permeability coefficient to HCO3- of 8.9 +/- 0.9 x 10(-6) cm/s. In addition to requiring HCO3- on the serosal surface, net base secretion was inhibited by reducing the Na+ concentration in the serosal medium and the Cl- concentration in the mucosal medium. These data suggest that colonic base secretion involves HCO3- entry across the basolateral surface, energized by the Na+ gradient, and HCO3- exit across the apical surface in exchange for Cl-.