Lactate metabolism in the adult rat brain was investigated in relation with the concept of lactate trafficking between astrocytes and neurons. Wistar rats were infused intravenously with a solution containing either [3-13 C]lactate (534 mM) or both glucose (750 mM) and [3-13 C]lactate (534 mM). The time courses of both the concentration and 13 C enrichment of blood glucose and lactate were determined. The data indicated the occurrence of [3-13 C]lactate recycling through liver gluconeogenesis. The yield of glucose labeling was, however, reduced when using the glucose-containing infusate. After a 20-min or 1-h infusion, perchloric acid extracts of the brain tissue were prepared and subsequently analyzed by 13 C-and 1 H-observed/ 13 C-edited NMR spectroscopy. The 13 C labeling of amino acids indicated that [3-13 C]lactate was metabolized in the brain. Based on the alanine C3 enrichment, lactate contribution to brain metabolism amounted to 35% under the most favorable conditions used. By contrast with what happens with [1-13 C]glucose metabolism, no difference in glutamine C2 and C3 labeling was evidenced, indicating that lactate was metabolized in a compartment deprived of pyruvate carboxylase activity. This result confirms, for the first time from an in vivo study, that lactate is more specifically a neuronal substrate. Key Words: 13 C-NMR-LactateBrain metabolism-Neurons-Astrocytes.

C6 cells were used to establish a glioma-bearing rat model by stereotaxic injection in the left caudate nucleus. The tumor status was evaluated by magnetic resonance imaging and conventional histology. The glioma-bearing rats were infused for 1 h with a [1-13 C]glucose solution. Perchloric acid extracts of the tumor and the ipsilateral and contralateral hemispheres were analyzed by 13 C-NMR spectroscopy. The 13 C-labeling patterns in compounds, mainly amino acids, indicated no drastic modification of carbon metabolism in both ipsilateral and contralateral hemispheres, as compared with control rats, whereas profound metabolic differences between brain tissue and tumor were observed. Glutamine C4 enrichment was lower in the glioma than in the brain [mean Ϯ SD values, 5.4 Ϯ 2.3 (n ϭ 5) and 15.0 Ϯ 0.8% (n ϭ 10), respectively] and also lower than the glutamate C4 enrichment in the glioma (mean Ϯ SD value, 22.6 Ϯ 4.2%; n ϭ 5), indicating that tumor glutamine was neither synthesized inside the glioma nor taken up from the surrounding brain. The glutamine C4 enrichment in the serum (6.7 Ϯ 0.5%; n ϭ 10) suggested that the glioma imported glutamine from the blood, a process probably connected with angiogenesis.

13C and 1 H NMR spectroscopy was used to investigate the metabolism of L-lactate and D-glucose in C6 glioma cells. The changing of lactate and glucose concentration in the extracellular medium of C6 glioma cells incubated with 5.5 mM glucose and 11 mM lactate indicated a net production of lactate as the consequence of an active aerobic glycolysis. The 13 C enrichments of various metabolites were determined after 4-h cell incubation in media containing both substrates, each of them being alternatively labeled in the form of either [3-13 C]L-lactate or [1-13 C]D-glucose. Using 11 mM [3-13 C]L-lactate, the enrichment of glutamate C4, 69%, was found higher than that of alanine C3, 32%, when that of acetyl-CoA C2 was 78%. These results indicated that exogenous lactate was the major substrate for the oxidative metabolism of the cells. Nevertheless, an active glycolysis occurred, leading to a net lactate production. This lactate was, however, metabolically different from the exogenous lactate as both lactate species did not mix into a unique compartment. The results were actually consistent with the concept of the existence of two pools of both lactate and pyruvate, wherein one pool was closely connected with exogenous lactate and was the main fuel for the oxidative metabolism, and the other pool was closely related to aerobic glycolysis.Neoplastic cells preferentially utilize aerobic glycolysis for their energy needs rather than oxidative phosphorylations, that leads to an important lactate production (1). The reason for this behavior is not yet clearly understood. It has been proposed recently that it would be a means to minimize oxidative stress, in particular during the cell phases linked to biosynthesis and cell division (2). With regard to a tumor, aerobic glycolysis may increase lactate concentration inside the tumor itself and in its close vicinity (3, 4). Consequently, local changes in metabolic activities could be generated as an increased local lactate concentration provides a possible source for cell energy requirements in competition with glucose or other substrates. There is no data concerning the competition between lactate and glucose as fuel for the oxidative metabolism in glioma cells. It has been only reported that, in vitro, glioma cells utilize glucose, but that once it is depleted, they will take up pyruvate and lactate generated by aerobic glycolysis for further metabolism (5).To study the possible competition between glucose and lactate metabolism in glioma cells, we investigated the fate of each of these substrates when both were present in cell medium by using either [3-13 C]L-lactate or [1-13 C]D-glucose. We used the C6 glioma cell clone for which the metabolism of [1-13 C]glucose has been investigated already (6, 7). The initial glucose concentration (5.5 mM) was chosen higher than the apparent K m for transport (1.7 mM) (8), whereas the different lactate concentrations used (11, 5.5, and 1.1 mM) were higher or in the same range than the apparent K m for transport (1 mM) (9). The 13 C enrich...