Mechanism of Cl- translocation across small intestinal brush-border membrane. I. Absence of Na+-Cl- cotransport

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Abstract
Kinetic predictions from a putative Na+-Cl- cotransport system were tested in vesicles of isolated rat intestinal brush-border membranes. For the conditions of isotope exchange at equilibrium, the model predicts activation of the Na+ and Cl- exchange rates by increasing concentrations of the counterion, at least for concentrations well below the Km of the counterion. When Cl- was the test ion (150 mM), K+ plus monactin replaced Na+. When Na+ was the test ion (150 mM), SO42- replaced Cl. Contrary to the predictions of the cotransport model, the velocities of Na+ and Cl- exchange were constant regardless of the concentration of the putative cosubstrates Cl- and Na+, respectively. Cl- transport in the isolated vesicles was carrier mediated as judged by the criterion of saturability of transport (Km = 255 mM) and the pathways involved in net NaCl movements accounted for minimally 70 and 40% of the Na+ and Cl- exchange rates, respectively. These findings exclude a significant contribution of a Na+-Cl- cotransport mechanism to NaCl uptake across the intestinal brush-border membranes in the concentration range tested, i.e., above 25 mM. The findings are consistent with a double exchange of Na+ for H+ and of Cl- for OH- (HCO3-). A Donnan potential of 10 mV (inside negative) can explain differences in the equilibrium uptake of Na+, anions and glucose by intestinal brush-border membranes.

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