HCO3- secretion in the rat colonic crypt is closely linked to Cl- secretion

Background & Aims: The mechanism of colonic HCO3- secretion has not been established largely because of a lack of experimental methods for its detailed study, The present studies were designed to establish whether the isolated, perfused crypt of the rat distal colon is an excellent model to study HCO3- movement and the mechanism of colonic HCO3- secretion, Methods: HCO3- secretion was determined in isolated, microperfused crypts by measuring [HCO3-] by microcalorimetry on nanoliter samples, Results: Net HCO3- absorption was observed during lumen and bath perfusion with an HCO,3--Ringer solution, Vasoactive intestinal polypeptide (60 nmol/L), acetylcholine (100 nmol/L), or dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP, 0.5 mmol/L) induced active HCO3- secretion that required bath but not lumen HCO3-/CO2. DBcAMP-stimulated HCO3- secretion was not affected by acetazolamide, an inhibitor of carbonic anhydrase, Removal of lumen Cl- did not alter DBcAMP-stimulated HCO3- secretion but reduced fluid secretion. DBcAMP-stimulated HCO3- secretion was closely linked to active Cl- secretion because HCO3- secretion was substantially reduced by removal of bath Cl-, by addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl- secretion, and by addition of lumen NPPB, a Cl- channel inhibitor, Conclusions: These studies establish that colonic crypt HCO3- secretion (1) is not a result of an apical membrane Cl--HCO3- exchange, (2) is tightly associated with Cl- secretion, and (3) primarily occurs via an apical membrane Cl- channel.