Three distinct mechanisms of HCO3- secretion in rat distal colon

HCO3- secretion has long been recognized in the mammalian colon, but it has not been well characterized. Although most studies of colonic HCO3- secretion have revealed evidence of lumen Cl- dependence, suggesting a role for apical membrane Cl-/HCO3- exchange, direct examination of HCO3- secretion in isolated crypt from rat distal colon did not identify Cl--dependent HCO3- secretion but did reveal cAMP-induced, Cl--independent HCO3- secretion. Studies were therefore initiated to determine the characteristics of HCO3(-) secretion in isolated colonic mucosa to identify HCO3- secretion in both surface and crypt cells. HCO3- secretion was measured in rat distal colonic mucosa stripped of muscular and serosal layers by using a pH stat technique. Basal HCO3- secretion (5.6+/-0.03 mueq.h(-1).cm(-2)) was abolished by removal of either lumen Cl- or bath HCO3-; this Cl--dependent HCO3- secretion was also inhibited by 100 muM DIDS (0.5+/-0.03 mueq.h(-1).cm(-2)) but not by 5-nitro-3-(3-phenylpropylamino) benzoic acid (NPPB), a Cl- channel blocker. 8-Bromo-cAMP induced Cl--independent HCO3- secretion (and also inhibited Cl--dependent HCO3- secretion), which was inhibited by NPPB and by glibenclamide, a CFTR blocker, but not by DIDS. Isobutyrate, a poorly metabolized short-chain fatty acid (SCFA), also induced a Cl--independent, DIDS-insensitive, saturable HCO3- secretion that was not inhibited by NPPB. Three distinct HCO3- secretory mechanisms were identified: 1) Cl--dependent secretion associated with apical membrane Cl-/HCO3- exchange, 2) cAMP-induced secretion that was a result of an apical membrane anion channel, and 3) SCFA-dependent secretion associated with an apical membrane SCFA/HCO3- exchange.