title: Stoichiometry of Na+-succinate cotransport in renal brush-border membranes.
abstract: The coupled transport of Na+ with succinate and citrate into rabbit renal brush-border membrane vesicles was studied. Initial rates of transport (J) were estimated from 1-s uptakes under well defined transmembrane conditions (membrane potential, pH, ionic composition). A Na+ gradient increased J up to 700 times compared to Na+ replacement with sorbitol. J was a sigmoid function of increasing Na+ concentration; the kinetic interaction of Na+ with the succinate-citrate transport system was described by the equation: J = Jmax/(1 + (Ks/[Na]))3. Assuming that the Na+ interaction involves equivalent, noninteracting binding sites, these results indicate that 3 Na+ ions are translocated/molecule of organic substrate. Consistent with the latter observation was the direct demonstration of a stoichiometric coupling between simultaneously determined fluxes of 22Na+ and 14C-labeled substrates. In excess of 2 Na+ ions were transported/molecule of organic substrate. These results explain the electrogenicity of the transport of succinate and citrate as evidenced by the response of J to K+ diffusion potentials and the effects of transport on the fluorescence of a potential-sensitive cyanine dye.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism*; D002951:Citrates / Q000378:metabolism; D019343:Citric Acid; D007668:Kidney / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials; D008871:Microvilli / Q000187:drug effects / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D013386:Succinates / Q000378:metabolism*; D019802:Succinic Acid
chemical_list: D002951:Citrates; D013386:Succinates; D019343:Citric Acid; D012964:Sodium; D019802:Succinic Acid