Specificity and modes of the anion exchanger in dog renal microvillus membranes

The transport of various organic anions via the pathway that mediates the exchange of urate or p-aminohippurate (PAH) for OH^- or Cl^- in dog renal microvillus membrane vesicles was investigated. The pH gradient-stimulated uptakes of tracer urate and PAH were significantly inhibited by 5 mM PAH, n-valerate, lactate, β-hydroxybutyrate, oxaloacetate, and cis-aconitate but not by 5 mM acetate, malate, oxalate, or citrate. The pH dependence of inhibition suggested that it was in their monovalent forms that these acid anions interacted with the urate exchange pathway. Outwardly directed gradients of succinate, lactate, and PAH stimulated uphill urate accumulation. Imposition of an inside-alkaline pH gradient stimulated the uphill accumulation of lactate and succinate. Na⁺ cotransport pathways for lactate and succinate were also present. In the presence of an inwardly directed Na⁺ gradient, lactate stimulated the uphill accumulation of urate, indicating that the pathways mediating Na⁺-lactate cotransport and lactate-urate exchange coexisted in at least some membrane vesicles. We conclude that the anion exchange pathway for urate in dog renal microvillus membrane vesicles has affinity for additional organic anions and can function in multiple exchange modes. Exchange of luminal urate or Cl^- for intracellular organic anions or OH^- is a possible mechanism for effecting uphill anion reabsorption in the proximal tubule.