Coupling between sodium and succinate transport across renal brush border membrane vesicles

The coupling between Na+ and succinate transport through the renal brush border Na+-dicarboxylate cotransporter was examined in a vesicle preparation. Initial rates of Na+ and succinate uptake were measured simultaneously using radioactive isotopes under zero-trans (Na+ and succinate-free intravesicular solutions) and voltage-clamped conditions (p.d. = 0 mV). The coupling coefficient is defined as the ratio of the succinate-dependent Na+ flux to the Na+-dependent succinate flux. The cis Na+ and succinate concentrations were varied from 0.05-5 mM succinate and 25-150 mM Na+. The coupling coefficient was approximately 3 and was independent of the succinate and Na+ concentrations tested. These results suggest that only the free carrier and the fully loaded carrier (carrier + 3 Na+ + succinate2-) are able to cross the brush border membrane; i.e., there is little evidence of slippage in the coupling between Na+ and succinate fluxes through the membrane via this carrier. A coupling coefficient of 3 is consistent with the electrogenic nature of the Na-dicarboxylate cotransporter.