Na+-Cr−-Glutamate Cotransport by Lobster Hepatopancreatic Brush Border Membrane Vesicles

Abstract

l-[3H]glutamate uptake by lobster hepatopancreatic brush border membrane vesicles, formed by a magnesium precipitation technique, was stimulated by a transmembrane NaCl gradient (o > i), but not by identical gradients of KCl, TMA-Cl, NaSCN or NaN03, suggesting that the amino acid transfer depends specifically upon both Na+ and Cl−. In the presence of a NaCl gradient (o > i), glutamate uptake was strongly dependent upon bilateral pH and increased markedly as pH was lowered from 7.0 to 4.0. NaCl-dependent l-[3H]glutamate uptake was not trans-stimulated by preloading vesicles with K+. At pH 5.0, NaCl-dependent glutamate uptake occurred by an electroneutral process, whereas Na+-dependent d-[3H]glucose uptake at this pH was electrogenic. l-[3H]glutamate influx exhibited both carrier-mediated and apparent diffusional transport components. Decreasing pH had no significant effect on glutamate influx (Kt), but tripled both maximal carrier-mediated entry rate (Jm) and the apparent diffusional permeability (P) of the membrane to this compound. l-[3H]glutamate influx was strongly trans-stimulated by vesicles preloaded with l- and d-glutamate, l- and d-aspartate, l-cysteate, l-tyrosine and l-asparagine, but not by vesicles preloaded with l-leucine, l-glutamine, l-proline, l-alanine or l-lysine. l-[3H]glutamate influx at pH 4.0 was a hyperbolic function of both external Na+ and Cl− concentrations at fixed concentrations of the respective counterions, suggesting cotransport between the amino acid and the two ions with a stoichiometry of 1 Na+: 1 Cl−: 1 glutamate.