Kinetics of glial glutamine efflux and the mechanism of neuronal uptake studied in vivo in mildly hyperammonemic rat brain

Abstract

Kinetics of glial glutamine (GLN) transport to the extracellular fluid (ECF) and the mechanism of GLNECF transport into the neuron - crucial pathways in the glutamine-glutamate cycle - were studied in vivo in mildly hyperammonemic rat brain, by NMR and microdialysis to monitor intra- and extracellular GLN. The minimum rate of glial GLN efflux, determined from the rate of GLNECF increase during perfusion of α-(methylamino) isobutyrate (MeAIB), which inhibits neuronal GLNECF uptake by sodium-coupled amino-acid transporter (SAT), was 2.88 ± 0.22 μmol/g/h at steady-state brain [GLN] of 8.5 ± 0.8 μmol/g. Our previous study showed that the rate of glutamine synthesis under identical experimental conditions was 3.3 ± 0.3 μmol/g/h. At steady-state glial [GLN], this is equal to its efflux rate to the ECF. Comparison of the two rates suggests that SAT mediates at least 87 ± 8% (= 2.88/3.3 x 100%) of neuronal GLNECF uptake. While MeAIB induced > 2-fold elevation of GLN ECF, no sustained elevation was observed during perfusion of the selective inhibitor of LAT, 2-amino-bicyclo[1,1,2]heptane-2-carboxylic acid (BCH), or of d-threonine, a putative selective inhibitor of ASCT2-mediated GLN uptake. The results strongly suggest that SAT is the predominant mediator of neuronal GLNECF uptake in adult rat brain in vivo. © 2006 The Authors.