Citrate uptake by basolateral and luminal membrane vesicles from rabbit kidney cortex

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Abstract
The mechanisms of tubular transport of citrate in renal basolateral and luminal membrane vesicles were studied under various experimental conditions. Both membrane preparations take up citrate by a Na+-dependent transport system, although with different characteristics. The uptake of citrate by basolateral membrane vesicles was insensitive to changes in membrane potential, which is indicative of electroneutral transport of the anion. The Na+-dependent uptake of citrate by luminal membrane vesicles was influence by the presence of Na+ salt anions of different permeabilities in the order: Cl > sulfate > gluconate. Further, addition of citrate to membrane vesicle-potential-sensitive dye suspensions resulted in optical changes of the dye, indicative of electrogenic transfer of this compound. The apparent affinity of the citrate transport system located in luminal membrane vesicles, in contrast to basolateral membrane vesicles, was sensitive to changes in medium pH and was higher than that of basolateral membrane vesicles in the pH range studied. On the basis of these results a model for the transport of citrate by rabbit kidney proximal tubule is proposed.