Succinate and citrate transport in renal basolateral and brush-border membranes

Properties of the Na-dependent transport of succinate and citrate were determined in brush-border and basolateral membrane vesicles (BBMV and BLMV, respectively) isolated from the rabbit renal cortex. As has been shown in previous studies, the initial rate of uptake of these substrates into BBMV was markedly stimulated (50- to 150-fold) by an inwardly directed Na gradient. In BLMV, uptake of 5 μM succinate was also stimulated (150-fold) by a Na gradient, and the profile of the time course of accumulation was qualitatively and quantitatively different from that in BBMV. In contrast, basolateral uptake of 2 μM citrate was stimulated only one- to fivefold by a Na gradient. The external pH had little effect on uptake of succinate into BBMV or BLMV, or on uptake of citrate into BLMV. Uptake of citrate into BBMV, however, was stimulated 10-fold by reducing external pH from 8 to 5.5. Valinomycin-induced K diffusion potentials stimulated succinate uptake into BBMV and BLMV, suggesting that Na-coupled succinate transport across both membranes is electrogenic. The J(max) and apparent K(t) for uptake into BBMV were much greater than those in BLMV (90 vs. 5 nmol·mg^-1·min^-1 and 0.61 vs. 0.01 mM, respectively). Inhibition of succinate uptake by several polycarboxylates suggested that the BLMV transporter had a higher apparent affinity for dicarboxylates than for tricarboxylates, compared with the BBMV transporter. We conclude that BLMV possess a low-capacity, high-affinity Na-dependent transport pathway showing a high degree of specificity for dicarboxylic, rather than tricarboxylic, acids.