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 μeq·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 μM DIDS (0.5 ± 0.03 μeq·h−1·cm−2) but not by 5-nitro-3-(3-phenylpropyl-amino)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.
Stimulation of Bicarbonate Secretion by Luminal Short-Chain Fatty Acid in the Rat and Human Colon In Vitro.