Mechanism of Cl- Translocation Across Small Intestinal Brush-border Membrane. I. Absence of Na+-Cl- Cotransport

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, however, 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.