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Elevated ammonium levels: differential acute effects on three glutamate transporter isoforms

Increased ammonium (NH 4 + /NH 3 ) in the brain is a significant factor in the pathophysiology of hepatic encephalopathy, which involves altered glutamatergic neurotransmission. In glial cell cultures and brain slices, glutamate uptake either decreases or increases following acute ammonium exposure but the factors responsible for the opposing effects are unknown. Excitatory amino acid transporter isoforms EAAT1, EAAT2, and EAAT3 were expressed in Xenopus oocytes to study effects of ammonium exposure on their individual function. Ammonium increased EAAT1- and EAAT3-mediated ( 3 H)glutamate uptake and glutamate transport currents but had no effect on EAAT2. The maximal EAAT3-mediated glutamate transport current was increased but the apparent affinities for glutamate and Na + were unaltered. Ammonium did not affect EAAT3-mediated transient currents, indicating that EAAT3 surface expression was not enhanced. The ammonium-induced stimulation of EAAT3 increased with increasing extracellular pH, suggesting that the gaseous form NH 3 mediates the effect. An ammonium-induced intracellular alkalinization was excluded as the cause of the enhanced EAAT3 activity because 1 ) ammonium acidified the oocyte cytoplasm, 2 ) intracellular pH buffering with MOPS did not reduce the stimulation, and 3 ) ammonium enhanced pH-independent cysteine transport. Our data suggest that the ammonium-elicited uptake stimulation is not caused by intracellular alkalinization or changes in the concentrations of cotransported ions but may be due to a direct effect on EAAT1/EAAT3. We predict that EAAT isoform-specific effects of ammonium combined with cell-specific differences in EAAT isoform expression may explain the conflicting reports on ammonium-induced changes in glial glutamate uptake. hyperammonemia ammonia Xenopus laevis oocytes excitatory amino acid transporter isoforms Copyright © 2012 the American Physiological Society « Previous | Next Article » Table of Contents This Article Published online before print December 2011 , doi: 10.​1152/​ajpcell.​00238.​2011 Am J Physiol Cell Physiol March 2012 vol. 302 no. 6 C880-C891 » Abstract Free Full Text Full Text (PDF) All Versions of this Article: ajpcell.00238.2011v1 302/6/C880 most recent Classifications Article Services Email this article to a friend Alert me when this article is cited Alert me if a correction is posted Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Download to citation manager Citing Articles Load citing article information Citing articles via Web of Science Google Scholar Articles by Søgaard, R. Articles by MacAulay, N. PubMed PubMed citation Articles by Søgaard, R. Articles by MacAulay, N. Related Content Load related web page information Current Issue March 2012, 302 (6) Alert me to new issues of Am J Physiol Cell Physiol About the Journal Information for Authors Submit a Manuscript Ethical Policies AuthorChoice PubMed Central Policy Reprints and Permissions Advertising Press Copyright © 2012 the American Physiological Society Print ISSN: 0363-6143 Online ISSN: 1522-1563 var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www."); document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E")); try { var pageTracker = _gat._getTracker("UA-2924550-1"); pageTracker._trackPageview(); } catch(err) {} http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png AJP - Cell Physiology The American Physiological Society

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