Effect of glucose on glutamine metabolism in rat brain slices: a cellular metabolomic study with Effect of glucose ¹³C NMR.

To examine the effect of glucose on the cerebral metabolism of glutamine, rat brain slices were incubated with 5mM [3-(13)C]glutamine without and with 5mM unlabeled glucose. Tissue plus medium extracts were analyzed by using enzymatic and (13)C NMR techniques and fluxes through the enzymatic steps involved were calculated with a mathematical model. We demonstrate that glucose increased alanine, pyruvate and glutamate accumulations and decreased ammonium ions accumulation, aspartate accumulation and labeling, and GABA labeling. In order to determine the participation of glutamine synthetase when glucose was added to the incubation medium, we incubated rat brain slices with 5mM [3-(13)C]glutamine plus 5mM unlabeled glucose without and with 2mM methionine sulfoximine (MSO). The results indicate that 77% of the newly appeared glutamine was formed via glutamine synthetase and 23% from endogenous sources; the stimulation of [3-(13)C]glutamine removal by MSO also strongly suggests the existence of a cycle between [3-(13)C]glutamine and [3-(13)C]glutamate. This work also demonstrates that glucose increased fluxes through hexokinase, pyruvate kinase, lactate dehydrogenase, alanine aminotransferase, pyruvate carboxylase, pyruvate dehydrogenase, citrate synthase, flux from α-ketoglutarate to glutamate and flux through glutamine synthetase whereas it inhibited fluxes through aspartate aminotransferase, glutamic acid decarboxylase and GABA aminotransferase.