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Suppression of glial glutamine release to the extracellular fluid studied in vivo by NMR and microdialysis in hyperammonemic rat brain

Release of glial glutamine (GLN) to the extracellular fluid (ECF), mainly mediated by the bidirectional system N transporter SN1, was studied in vivo in hyperammonemic rat brain, using 15N‐nuclear magnetic resonance (NMR) to monitor intracellular [5‐15N]GLN and microdialysis/gradient 1H‐15N heteronuclear single‐quantum correlation NMR to analyse extracellular [5‐15N]GLN. GLNECF was elevated to 2.4 ± 0.2 mm after 4.5 h of intravenous ammonium acetate infusion. The [GLNi]/[GLNECF] ratio (i = intracellular) was 9.6 ± 0.9, compared with 17–20 in normal brain. GLNECF then decreased substantially at t = 4.9 ± 0.1 h. Comparison of the time‐courses of intra‐ and extra‐cellular [5‐15N]GLN strongly suggested that the observed decrease reflects partial suppression of glial GLN release to ECF. Suppression also followed elevation of GLNECF to 1.9 mm, resulting in a [GLN]i/[GLNECF] ratio of 8.4, upon perfusion of α‐(methylamino)isobutyrate which inhibits neuronal uptake of GLNECF mediated by sodium‐coupled amino acid transporter (SAT). The results provide first evidence for bidirectional operation of SN1 in vivo, and clarify the effect of transmembrane GLN gradient on glial GLN release at physiological Na+ gradient. Implications of the results for SN1 as an additional regulatory site in the glutamine/glutamate cycle and utility of this approach for examining the role of GLN in an experimental model of fulminant hepatic failure are discussed.