Mechanism of Cl- translocation across small intestinal brush-border membrane. II. Demonstration of Cl--OH- exchange and Cl- conductance.

The mechanisms of Cl- transport across the brush-border membrane from rat small intestine were investigated in vitro in isolated vesicles and in vivo in ileal and jejunal segments. A Cl--OH- exchange mechanism was demonstrated in isolated vesicles by observing concentrative Cl- uptake driven by a pH gradient (extravesicular pH less than intravesicular pH). A Cl- conductance pathway was demonstrated by concentrative Cl- uptake driven by a K+ diffusion potential. The K+ diffusion potential was generated by a K+ concentration gradient in the presence of valinomycin (extravesicular K+ concentration greater than intravesicular K+ concentration). Furosemide and stilbene 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonate (SITS) strongly inhibited the Cl--OH- exchange but did not affect the Cl- conductance pathway, Na+-dependent glucose transport, and Na+-Na+ exchange. Under isotope exchange conditions at equilibrium, SITS inhibited Cl- transport up to 63%, indicating that this portion of Cl- transport is mediated by the Cl--OH- exchange transporter, with the remainder presumably due to the Cl- conductance pathway. In perfused small intestinal segments, Na+, Cl-, HCO3(-), and water were absorbed from the lumen in the absence of SITS. The presence of 5 mM SITS inhibited NaCl absorption and decreased HCO3(-) and water absorption in both jejunum and ileum but did not affect glucose absorption. The inhibition of in vivo salt absorption by SITS suggests that the Cl--OH- exchange mechanism plays a major role in NaCl absorption in intact enterocytes.