Phosphate transport across the basolateral membrane from rat kidney cortex: sodium-dependence?

Basolateral membrane vesicles were isolated from rat renal proximal tubules by a Percoll-centrifugation method. Transport of phosphate could be stimulated by a sodium gradient but transport of D-glucose was not or only slightly affected by sodium. The Percoll-basolateral membrane fraction showed sodium-independent trans-stimulation of phosphate by itself. To test whether sodium stimulation of phosphate transport is influenced by the crosscontamination of basolateral with brush border membranes, the basolateral membrane fraction obtained by the Percoll-method was applied to free-flow electrophoresis for further purification. Thereby a separation of basolateral from brush border marker enzymes was obtained. It was possible to correlate net sodium-dependent phosphate transport with the brush border marker enzyme activity. ATP-dependent calcium uptake and cytochalasin B sensitive, sodium-independent D-glucose uptake followed basolateral, whereas phlorizin sensitive, sodium-dependent D-glucose followed brush border marker enzymes. Sodium dependent phosphate uptake was inhibited by D-glucose, this inhibition was released by phlorizin. It is concluded that the sodium-dependent phosphate uptake present in the Percoll-basolateral membrane fraction is due to crosscontamination with brush border membranes and that phosphate translocation across the basolateral membrane is carrier-mediated and sodium-independent.