Dicarboxylate transport in human placental brush-border membrane vesicles.
Pathways for transport of dicarboxylic acid metabolites by human placental epithelia were investigated using apical membrane vesicles isolated by divalent cation precipitation. The presence of Na+/dicarboxylate cotransport was assessed directly by [14C]succinate tracer flux measurements and indirectly by fluorescence determinations of voltage sensitive dye responses. The imposition of an inwardly directed Na+ gradient stimulated vesicle uptake of succinate achieving levels approximately 5-fold greater than those observed at equilibrium. The increased succinate uptake was specific for Na+ as no stimulation was observed in the presence of Li+, K+ or choline+ gradients. In addition to concentrative accumulation of succinate, a direct coupling of Na+/succinate cotransport was suggested by the absence of a sizeable conductive pathway for succinate uptake and decreased succinate uptake levels associated with a more rapid decay of an imposed Na+ gradient. Na+ gradient-driven succinate uptake was not the result of parallel Na+/H+ and succinate/OH- exchange activities but was reduced by the Na+-coupled inhibitor harmaline. The voltage sensitivity of Na+ gradient-driven succinate uptake suggests Na+/succinate cotransport is electrogenic occurring with net transfer of positive charge. Substrate-specificity studies suggest the tricarboxylic acid cycle intermediates as candidates for transport by the Na+-coupled pathway. Decreasing pH increased the citrate-induced inhibition of succinate uptake suggesting divalent citrate as the preferred substrate for transport. Initial rate determinations of succinate uptake indicate succinate interacts with a single saturable site (Km 33 microM) with a maximal transport rate of 0.5 nmol/mg per min.
Ogin
C
C
Department of Pharmacology, State University of New York, Syracuse 13210.
Grassl
S M
SM
D001692:Biological Transport / Q000187:drug effects; D002951:Citrates / Q000494:pharmacology; D019343:Citric Acid; D003998:Dicarboxylic Acids / Q000378:metabolism*; D004563:Electrochemistry; D004848:Epithelium / Q000378:metabolism; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008871:Microvilli / Q000378:metabolism*; D010920:Placenta / Q000378:metabolism*; D011247:Pregnancy; D012964:Sodium / Q000494:pharmacology; D013050:Spectrometry, Fluorescence; D013386:Succinates / Q000378:metabolism
D002951:Citrates; D003998:Dicarboxylic Acids; D013386:Succinates; D019343:Citric Acid; D012964:Sodium