title: The role of bicarbonate ions and of adenosine 3',5'-monophosphate (cAMP) in chloride transport by epithelial cells of bullfrog small intestine.
abstract: In an HCO3-free medium, isolated segments of bullfrog small intestine, stripped of their external muscle layers, displayed a small, serosal positive PD that did not, on the average, differ significantly from zero. Similarly, in this medium, the mean values of Isc and of net Na+ and Cl- absorption under short-circuit conditions did not differ significantly from zero. External HCO3- (25 mM) induced a highly significant serosal negative PD and Isc and a large net absorption of Cl-. Net Cl- absorption exceeded Isc, i.e., there was a significant net flux, JR, which was consistent with a net secretion of HCO3-. The ratio of the internal Cl-activity of the absorptive cells (alpha Cli) to its equilibrium value was larger in the presence than in the absence of HCO3-. In the presence of HCO3-, cAMP, added to the serosal medium, reversed the serosal negative PD and Isc, and inhibited, though it did not completely abolish, net Cl- absorption. JR was unchanged; tissue Cl- and alpha Cli were reduced, and tissue Na+ decreased and tissue K+ increased. When HCO3- and Cl- were removed from the bathing medium, the electrical response of the tissue to cAMP, though greatly attenuated, was not completely abolished. Under these conditions, cAMP induced a significant net Na+ absorption. A model for ion transport in the absorptive cells of the small intestine is proposed that is consistent with these findings.
mesh_terms: D000818:Animals; D001001:Anura; D001639:Bicarbonates / Q000502:physiology*; D001693:Biological Transport, Active; D002712:Chlorides / Q000378:metabolism*; D000242:Cyclic AMP / Q000502:physiology*; D004848:Epithelium / Q000378:metabolism; D066298:In Vitro Techniques; D007421:Intestine, Small / Q000378:metabolism*; D008564:Membrane Potentials; D008954:Models, Biological; D011188:Potassium / Q000378:metabolism; D011892:Rana catesbeiana; D012964:Sodium / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D002712:Chlorides; D012964:Sodium; D000242:Cyclic AMP; D011188:Potassium