Abstract
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.
Highlights
Oxalate shares the electroneutral pathway mediating Na؉-independent sulfate-carbonate exchange
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
Studies using microvillus membrane vesicles have indicated the presence of a Naϩ-sulfate cotransport system at the apical membrane of proximal tubule cells [9, 10]
Summary
Oxalate shares the electroneutral pathway mediating Na؉-independent sulfate-carbonate exchange. Studies using isolated renal microvillus membrane vesicles have identified a transport pathway mediating ClϪ-oxalate exchange across the apical membrane of proximal tubule cells [1]. Uptake of [35S]Sulfate and [14C]Oxalate—Freshly thawed microvillus membrane vesicles were suspended and washed twice in a medium consisting of 100 mM potassium gluconate and buffers to achieve experimental pH values of 7.5 (60 mM Hepes, 25 mM TMA-OH), 8.0 (60 mM Tris, 57 mM Hepes), or 6.0 (50 mM Mes, 10 mM Hepes, and 15 mM TMA-OH). The timed uptakes of [35S]sulfate and [14C]oxalate into the membrane vesicles were measured at 30 °C by a rapid filtration method described previously [8]. The uptake period was stopped by rapid addition of 3 ml of an ice-cold iso-osmotic solution (100 mM potassium gluconate, 89 mM mannitol, 40 mM Hepes, and 16 mM TMA-OH, pH 7.5). A metronome was used for the timing of uptake periods shorter than 15 s
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