C-13 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 C-13-labeled and unlabeled glucos e and alanine, C-13 isotopomer analysis indicates two cytosolic pyruvate co mpartments in astrocytes. One pyruvate pool is in an exchange equilibrium w ith exogenous alanine and preferentially synthesizes releasable lactate. Th e second pyruvate pool, which is of glycolytic origin, is more closely rela ted to mitochondrial pyruvate, which is oxidized via tri carbonic acid (TCA ) cycle activity. In order to provide 2-oxoglutarate as a substrate for cyt osolic alanine aminotransferase, glycolytic activity is increased in the pr esence of exogenous alanine. Furthermore, in the presence of alanine, gluta mate is accumulated in astrocytes without subsequent glutamine synthesis. W e suggest that the conversion of alanine to releasable lactate proceeds at the expense of flux of glycolytic pyruvate through lactate dehydrogenase, w hich is used for ammonia fixation by alanine synthesis in the cytosol and f or mitochondrial TCA cycle activity. In addition, an intracellular traffick ing occurs between cytosol and mitochondria, by which these two cytosolic p yruvate pools are partly connected. Thus, exogenous alanine modifies astroc ytic glucose metabolism for the synthesis of releasable lactate disconnecte d from glycolysis. The data are discussed in terms of astrocytic energy met abolism and the metabolic trafficking via a putative alanine-lactate shuttl e between astrocytes and neurons. GLIA 34:200-212, 2001. (C) 2001 Wiley-Lis s, Inc.