IDENTIFICATION OF SODIUM-DEPENDENT AND SODIUM-INDEPENDENT DICARBOXYLATE TRANSPORT-SYSTEMS IN RAT-LIVER BASOLATERAL MEMBRANE-VESICLES

The mechanisms involved in the hepatocellular uptake of Krebs-cycle intermediates were investigated in isolated basolateral (sinusoidal and lateral) rat liver plasma membrane (blLPM) vesicles. An inwardly directed Na+ gradient markedly stimulated uptake of 2-oxoglutarate and succinate into voltage- and pH-clamped blLPM vesicles. This Na+-dependent portion of the dicarboxylate uptake was characterized by (a) saturability with increasing substrate concentrations (K(m) = 6.4 - 10 mM; V(max) almost-equal-to 0.2 nmol min-1 mg protein-1), (b) cisinhibition by lithium (10 mM), other Krebs-cycle dicarboxylates (1 mM) and DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid; 1 mM) but not by sulphate, monocarboxylates, oxalate, acidic amino acids, bile salts and probenecid, (c) stimulation by an intravesicular negative K+-diffusion potential indicating electrogenic [(Na+)n>2-succinate] cotransport, and (d) a pH optimum for transport between 7.0 and 7.5. In the absence of Na+, an inside alkaline pH gradient also markedly stimulated 2-oxoglutarate uptake, This pH-gradient-driven 2-oxoglutarate uptake was insensitive to lithium, but could also be inhibited by DIDS and succinate. Furthermore, saturation kinetics demonstrated K. (almost-equal-to 34 mM) and V(max) (almost-equal-to 0.8 nmol min-1 mg protein-1) values that were clearly different from those of the Na+-dependent uptake system. These results indicate the occurrence of two separate dicarboxylate transport systems along the sinusoidal border of hepatocytes, one being a Na+-dicarboxylate symporter and the other representing an anion-exchange system.