Mechanism(s) of Chloride Transport in Human Distal Colonic Apical Membrane Vesicles

Previous studies from our laboratory have demonstrated the presence of an electroneutral Cl^–/HCO₃ ^– exchange process across the human proximal colonic apical membrane vesicles (AMV). However, very little is known about the mechanism(s) of chloride transport in the apical membrane of the human distal colon. Utilizing AMV purified from organ donor distal colonic mucosa and a rapid Millipore filtration technique, the mechanisms of ³⁶Cl^– uptake into these vesicles were examined. Outwardly directed OH^– and HCO₃ ^– gradients markedly increased the uptake of ³⁶Cl^– into these vesicles, demonstrating a transient accumulation over the equilibrium uptake. Voltage clamping in the presence of K⁺/valinomycin reduced the OH^– and HCO₃ ^– gradient-stimulated ³⁶Cl^– uptake into these vesicles by 30% indicating that the conductive Cl^– uptake pathway was present in these vesicles along with the electroneutral exchange process. Under voltage-clamped conditions, the inhibitors the bicarbonate transporters, DIDS and SITS (1 mM), inhibited OH^– and HCO₃ ^– gradient-stimulated ³⁶Cl^– uptake by 50%. Acetazolamide showed small but significant inhibition of chloride uptake. Amiloride, bumetanide, and furosemide failed to inhibit ³⁶Cl^– uptake. Chloride uptake into these vesicles exhibited saturation kinetics with an apparent K _m for chloride of 16.7 mM and a V _max of 5.9 nmol/mg/15 sec. Chloride, acetate, nitrate, but not sulfate (50 mM each), inhibited 5 mM ³⁶Cl^– uptake. Inwardly directed gradients of Na⁺, K⁺ or both together did not stimulate chloride uptake into these vesicles indicating that the uptake of Cl^– and Na⁺ in human distal colonic AMV does not involve Na-Cl or Na-K-2Cl cotransport. In conclusion, these studies demonstrate that Cl^– transport across the apical membranes of human distal colon involves both conductive pathway and electroneutral Cl^–/HCO₃ ^– (OH^–) exchange processes. In view of our previous demonstration of a Na⁺/H⁺ exchange process in these AMV, we propose that the operation of dual ion exchange mechanisms of Na⁺/H⁺ and Cl^–/HCO₃ ^– is the primary mode of electroneutral NaCl absorption across the apical membranes of the enterocytes of the human distal colon.