Glutamine efflux from astrocytes is mediated by multiple pathways

Authors

  • Joachim W. Deitmer,

    1. School of Biochemistry & Molecular Biology, Australian National University, Canberra, Australia
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    • 1

      This study was carried out during Joachim W. Deitmer's leave of absence at the Australian National University. The present address for J. W. Deitmer is located at Abteilung für Allgemeine Zoologie, FB Biologie, Universität Kaiserslautern, Postfach 3049, D-67653 Kaiserslautern, Germany.

  • Angelika Bröer,

    1. School of Biochemistry & Molecular Biology, Australian National University, Canberra, Australia
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  • Stefan Bröer

    1. School of Biochemistry & Molecular Biology, Australian National University, Canberra, Australia
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Address correspondence and reprint requests to Stefan Bröer, School of Biochemistry and Molecular Biology, Australian National University, Canberra, ACT 0200, Australia. Tel.: +61-2-6125-2540, Fax: +61-2-6125-0313, E-mail: stefan.broeer@anu.edu.au

Abstract

The neurotransmitter glutamate, once released into the synaptic cleft, is largely recycled by the glutamate–glutamine cycle, which involves uptake into astrocytes, conversion into glutamine and subsequent release of glutamine from astrocytes as a precursor for neuroneal glutamate synthesis. We analysed glutamine efflux from cultured astrocytes by pre-loading cells with labelled glutamine for 30 min and subsequently measured glutamine efflux for 30 min. Efflux of pre-loaded glutamine was rapid and almost complete after 30 min with a first order rate of 0.11 ± 0.01/min. Efflux was 50% reduced when cells were depleted of intracellular Na+. Increasing intracellular Na+ concentration had a small stimulatory effect on glutamine efflux, indicating the participation of a Na+-dependent transport mechanism. About 50% of the basal efflux could not be inhibited by depletion of the intracellular Na+, suggesting the presence of an additional Na+-independent transport mechanism. Glutamine efflux was stimulated two- to threefold by addition of extracellular neutral amino acids, such as alanine or leucine. The stimulatory effects of alanine and leucine had a Na+-dependent and a Na+-independent component, suggesting the presence of two antiport mechanisms one involving Na+. When compared to the expression of glutamine transporter mRNAs in cultured astrocytes it appeared likely that glutamine efflux was mediated by SN1, LAT2, ASCT2 and an additional, yet unidentified, transporter that mediates about 40% of the basal efflux.

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