Both Na+ and Cl- gradients energize NaCl/L-glutamate cotransport in lobster hepatopancreatic brush border membrane vesicles.
Previous work with L-[3H]glutamate transport by lobster (Homarus americanus) hepatopancreatic brush border membrane vesicles (BBMV) indicated that the transport of this amino acid was stimulated by the presence of both Na+ and Cl- ions in the external medium, however, the specific catalytic or energetic role of each monovalent ion in amino acid transfer was not established (Ahearn and Clay (1987) J. Exp. Biol. 130, 175-191). The present study employs a variety of experimental treatments with this membrane preparation to clarify the nature of the ion dependency in the cotransport process. A zero-trans time course experiment using inwardly-directed transmembrane Na+ or Cl- gradients led to similar transient accumulations of the amino acid above equilibrium values in the presence of equilibrated concentrations of the respective counterions. The uptake overshoots observed in the presence of single ion gradients were significantly increased when gradients of both Na+ and Cl- were used simultaneously. When vesicles were pre-equilibrated with L-[3H]glutamate and either of the monovalent ions, an inwardly-directed gradient of each counterion led to the transient accumulation of additional labelled amino acid above its equilibrium concentration, indicating that either ion gradient was capable of energizing the net flow of L-glutamate. A cotransport stoichiometry of 1 Na+/1 Cl-/1 L-glutamate was established using the Static Head analysis where a balance of ion and amino acid driving forces were attained with a 7:1 Na+ or Cl- gradient (o greater than i) against a 7:1 L-glutamate gradient (i greater than o).
Balon
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Department of Zoology, University of Hawaii at Manoa, Honolulu 96822.
Ahearn
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D000818:Animals; D001692:Biological Transport; D002352:Carrier Proteins / Q000378:metabolism*; D002712:Chlorides / Q000378:metabolism*; D005971:Glutamates / Q000378:metabolism*; D018698:Glutamic Acid; D008099:Liver / Q000378:metabolism; D008871:Microvilli / Q000378:metabolism*; D008121:Nephropidae; D010179:Pancreas / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D049992:Sodium Chloride Symporters; D027981:Symporters
D002352:Carrier Proteins; D002712:Chlorides; D005971:Glutamates; D049992:Sodium Chloride Symporters; D027981:Symporters; D018698:Glutamic Acid; D012964:Sodium