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Journal of Experimental Biology 130,175-191 (1987)
Published by Company of Biologists 1987

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

GREGORY A. AHEARN ¹ and LAUREL P. CLAY ¹

¹ Department of Zoology, 2538 The Mall, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA

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 (K_t), 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.

Key words: glutamate transport, brush border membrane vesicles, cotransport, Na⁺-dependence, ion gradients, gastrointestinal physiology, Homarus americanus, hepatopancreas

Accepted on February 18, 1987

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