Role of Na-dependent Cl/HCO3 exchange in basolateral Cl transport of rabbit proximal tubules.

To analyze the rate of basolateral Cl exit and the magnitude and relative contributions of KCl cotransport and Na-dependent and -independent Cl/HCO3 exchange to Cl exit across the basolateral membrane (BLM) during transcellular Cl absorption, rabbit proximal convoluted tubules (PCT) were perfused with high-Cl, low-HCO3 plus formate solutions and bathed with plasma ultrafiltrate-like plus formate solutions. The initial rates of intracellular Cl activity (AiCl) reduction following bath Cl removal were compared when bath Cl was 0, when bath Na and Cl were 0, and when bath HCO3 and Cl were 0. The initial rate of AiCl reduction following bath Cl removal was 4.4 +/- 0.4 mM/s. After bath Na and Cl removal, this rate was reduced to 25.1 +/- 5.0%. After bath HCO3 and Cl removal, it was reduced to 18.0 +/- 4.8%. The difference between bath Na and Cl removal and bath HCO3 and Cl removal was not significant. Cl efflux following bath HCO3 and Cl removal may be due to a KCl symporter. The contribution of a KCl symporter was examined by raising bath K to 20 mM, thus eliminating the chemical driving force for KCl exit. After bath K increase, the initial rate of AiCl increase was 0.057 +/- 0.005 mM/s. These data suggest that Cl efflux at BLM of rabbit PCT is 1) large enough to explain transcellular Cl transport, 2) predominately due to an Na-dependent Cl/HCO3 exchanger, and 3) negligibly due to an Na-independent Cl/HCO3 exchanger or a KCl symporter.