title: Mercuric chloride uncouples glutamate uptake from the countertransport of hydroxyl equivalents.
abstract: The cotransport of sodium and glutamate by system X(AG)- is believed to be coupled to the countertransport of potassium and hydroxyl ion equivalents. Accordingly, the uptake of glutamate or D-aspartate in astrocytes is accompanied by an intracellular acidification. Here, we report that HgCl2 blocks the glutamate-induced acidification with an approximate 50% inhibitor concentration (IC50) of 55 nM, an order of magnitude below its IC50 for inhibition of glutamate uptake. At 100 nM HgCl2, glutamate-induced acidification was abolished, whereas glutamate uptake was unaffected. D-Aspartate-induced acidification was equally sensitive to HgCl2, indicating that HgCl2 blocked a transporter-mediated, rather than a receptor-mediated, acidification. Unaltered responses to acute acid and alkaline loads showed that HgCl2 was not acting indirectly via a change in pH regulation. We conclude that HgCl2 acted directly on the glutamate transporter to uncouple the uptake of glutamate from the export of hydroxyl equivalents. In contrast, two other sulfhydryl reagents, p-chloromercuribenzensulfonate and N-ethylmaleimide, failed to discriminate between glutamate-induced acidification and glutamate uptake. An additional effect of > or = 100 nM HgCl2, in this case shared by p-chlormercuribenzenesulfonate, was transient intracellular acidification. There is evidence that glutamate transport is regulated by intracellular pH. Mercuric mercury may disrupt the regulation of glutamate transport at lower concentrations than those that block transport.
authors.affiliation: Department of Pharmacology and Experimental Therapeutics, University of Maryland School of Medicine, Baltimore 21201, USA.
authors.forename: T N
authors.initials: TN
authors.lastname: Nagaraja
authors.forename: N
authors.initials: N
authors.lastname: Brookes
mesh_terms: D002731:4-Chloromercuribenzenesulfonate / Q000494:pharmacology; D027322:Amino Acid Transport System X-AG; D000818:Animals; D000831:Animals, Newborn; D001224:Aspartic Acid / Q000378:metabolism*; D001253:Astrocytes / Q000187:drug effects / Q000378:metabolism*; D001692:Biological Transport / Q000187:drug effects; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D005033:Ethylmaleimide / Q000494:pharmacology; D050491:Glutamate Plasma Membrane Transport Proteins; D018698:Glutamic Acid / Q000378:metabolism*; D006706:Homeostasis; D006863:Hydrogen-Ion Concentration; D006878:Hydroxides / Q000378:metabolism*; D007700:Kinetics; D008627:Mercuric Chloride / Q000494:pharmacology*; D051379:Mice; D013439:Sulfhydryl Reagents / Q000494:pharmacology; D027981:Symporters
chemical_list: D027322:Amino Acid Transport System X-AG; D002352:Carrier Proteins; D050491:Glutamate Plasma Membrane Transport Proteins; D006878:Hydroxides; D013439:Sulfhydryl Reagents; D027981:Symporters; D001224:Aspartic Acid; D018698:Glutamic Acid; D008627:Mercuric Chloride; D002731:4-Chloromercuribenzenesulfonate; C031356:hydroxide ion; D005033:Ethylmaleimide