13C isotopomer analysis of glucose and alanine metabolism reveals cytosolic pyruvate compartmentation as part of energy metabolism in astrocytes

After incubation of glial cells with both (13)C-labeled and unlabeled glucose and alanine, (13)C isotopomer analysis indicates two cytosolic pyruvate compartments in astrocytes. One pyruvate pool is in an exchange equilibrium with exogenous alanine and preferentially synthesizes releasable lactate. The second pyruvate pool, which is of glycolytic origin, is more closely related to mitochondrial pyruvate, which is oxidized via tri carbonic acid (TCA) cycle activity. In order to provide 2-oxoglutarate as a substrate for cytosolic alanine aminotransferase, glycolytic activity is increased in the presence of exogenous alanine. Furthermore, in the presence of alanine, glutamate is accumulated in astrocytes without subsequent glutamine synthesis. We suggest that the conversion of alanine to releasable lactate proceeds at the expense of flux of glycolytic pyruvate through lactate dehydrogenase, which is used for ammonia fixation by alanine synthesis in the cytosol and for mitochondrial TCA cycle activity. In addition, an intracellular trafficking occurs between cytosol and mitochondria, by which these two cytosolic pyruvate pools are partly connected. Thus, exogenous alanine modifies astrocytic glucose metabolism for the synthesis of releasable lactate disconnected from glycolysis. The data are discussed in terms of astrocytic energy metabolism and the metabolic trafficking via a putative alanine-lactate shuttle between astrocytes and neurons.