title: H+ and HCO3- flux across apical surface of rat distal colon.
abstract: Colonic ion transport is postulated to occur via simultaneous operation of Na(+)-H+ exchange and Cl(-)-HCO3- exchange. Accordingly H+ and HCO3- should be transported simultaneously by the colon. To assess simultaneous H+ and HCO3- transport, net acid-base flux was measured in isolated segments of rat distal colon. When both tissue surfaces were bathed in symmetrical solutions containing Cl-, net base was secreted (-1.0 +/- 0.1 mu eq.cm-2.h-1). Cl- substitution with gluconate in the mucosal medium caused net base flux to switch from secretion to absorption (2.0 +/- 0.2 mu eq.cm-2.h-1). To evaluate whether base absorption was dependent on H+ secretion via Na(+)-H+ exchange, mucosal Na+ was substituted with N-methylglucamine, and amiloride, an inhibitor of Na(+)-H+ exchange, was applied. Na+ substitution and 1 mM amiloride inhibited base absorption by 37 and 38%, respectively, suggesting operation of Na(+)-H+ exchange. Because base absorption persisted, an additional mechanism was considered, HCO3- absorption via Cl(-)-HCO3- exchange. This was evaluated with an inhibitor of Cl(-)-HCO3- exchange 4-acetamido-4'-isothiostilbene-2,2'-disulfonic acid (SITS). SITS (1 mM) inhibited HCO3- absorption by 40%. The effects of amiloride and SITS were additive, suggesting that the Na(+)-H+ and Cl(-)-HCO3- exchangers operate simultaneously. Amiloride also inhibited H+ secretion when net HCO3- was secreted, suggesting that the direction of HCO3- movement does not influence Na(+)-H+ exchange activity. These data suggest that the colon transports both H+ and HCO3- across the apical surface via Na(+)-H+ exchange and Cl(-)-HCO3- exchange; H+ is secreted via Na(+)-H+ exchange, whereas HCO3- can be secreted or absorbed via Cl(-)-HCO3- exchange.
authors.affiliation: Department of Medicine, Veterans Affairs Medical Center, Philadelphia, Pennsylvania.
authors.forename: G M
authors.initials: GM
authors.lastname: Feldman
authors.forename: R L
authors.initials: RL
authors.lastname: Stephenson
mesh_terms: D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D002712:Chlorides / Q000378:metabolism*; D003106:Colon / Q000378:metabolism*; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000502:physiology; D007700:Kinetics; D008297:Male; D009130:Muscle, Smooth / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D001639:Bicarbonates; D002712:Chlorides; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D000584:Amiloride