Pathways for oxalate transport in rabbit renal microvillus membrane vesicles

Recent evidence suggests that apical membrane Cl^--oxalate exchange plays a major role in mediating Cl^absorption in the renal proximal tubule. To sustain steady-state Cl^- absorption by a mechanism of exchange for intracellular oxalate requires the presence of one or more pathways for recycling oxalate from lumen to cell. Accordingly, we evaluated the mechanisms of oxalate transport in luminal membrane vesicles isolated from the rabbit renal cortex. We found that transport of oxalate by Na⁺ cotransport is negligible compared to the transport of sulfate. In contrast, we demonstrated that oxalate shares the electroneutral pathway mediating Na⁺-independent sulfate-carbonate exchange. We also demonstrated the presence of OH^--oxalate exchange (indistinguishable from H⁺-oxalate cotransport). The process of OH^--oxalate exchange was electrogenic and partially inhibited by Cl^-, indicating that it occurs, at least in part, as a mode of the Cl^--oxalate exchanger described previously. An additional component of OH^--oxalate exchange was insensitive to inhibition by either Cl^- or sulfate, suggesting that it takes place by neither the Cl^--oxalate exchanger nor the sulfate-carbonate exchanger. We conclude that multiple anion exchange mechanisms exist by which oxalate can recycle from lumen to cell to sustain Cl^- absorption occurring via apical membrane Cl^--oxalate exchange in the renal proximal tubule.