Characterization of cerebral glutamine uptake from blood in the mouse brain: implications for metabolic modeling of 13C NMR data

C-13 Nuclear Magnetic Resonance (NMR) studies of rodent and human brain using [1-C-13]/[1,6-C-13(2)]glucose as labeled substrate have consistently found a lower enrichment (similar to 25% to 30%) of glutamine-C4 compared with glutamate-C4 at isotopic steady state. The source of this isotope dilution has not been established experimentally but may potentially arise either from blood/brain exchange of glutamine or from metabolism of unlabeled substrates in astrocytes, where glutamine synthesis occurs. In this study, the contribution of the former was evaluated ex vivo using H-1-[C-13]-NMR spectroscopy together with intravenous infusion of [U-C-13(5)]glutamine for 3, 15, 30, and 60 minutes in mice. C-13 labeling of brain glutamine was found to be saturated at plasma glutamine levels >1.0 mmol/L. Fitting a blood-astrocyte-neuron metabolic model to the C-13 enrichment time courses of glutamate and glutamine yielded the value of glutamine influx, V-GIn(in), 0.036 0.002 mu mol/g per minute for plasma glutamine of 1.8 mmol/L. For physiologic plasma glutamine level (similar to 0.6 mmol/L), V-GIn(in) would be similar to 0.010 mu mol/g per Minute, which corresponds to similar to 6%, of the glutamine synthesis. rate and rises to similar to 11% for saturating blood glutamine concentrations. Thus, glutamine influx from blood contributes at Most similar to 20% to the dilution of astroglial glutamine-C4 consistently seen in metabolic studies using. [1-C-13]glucose.