Comparative analysis of 13C-enriched metabolites released in the medium of cerebellar and cortical astrocytes incubated with [1-13C]glucose.
Rat cerebellar and cortical astrocytes cultured for 15 or 35 days were incubated with [1-13C]glucose in the presence or absence of 4 mM exogenous glutamine and the release of 13C-enriched metabolites into cell media was studied by 13C-NMR spectroscopy. In the presence of exogenous glutamine, both cerebellar and cortical astrocytes consumed the amino acid. In contrast, a net production of glutamine occurred in the absence of the amino acid. Simultaneously, a release of 13C-enriched glutamine into cell media was observed and was higher in the presence than in the absence of exogenous glutamine. This demonstrated the occurrence of an isotopic-exchange process which may involve a futile cycle at the level of glutamine synthetase and glutaminase activities. The 13C-enrichment ratio between glutamine carbons C2 and C3 was close to 1 in the presence of exogenous glutamine whereas it was higher than 1 in its absence, indicating that pyruvate carboxylase was more active in the absence of glutamine. In addition to glutamine, alanine was synthesized and exported into the medium of both cerebellar and cortical astrocytes. In contrast, citrate was specifically produced by cortical astrocytes. Slight increases in alanine and glutamine productions were observed for cortical astrocyte cultures between 15 and 35 days, whereas the amino acid production by cerebellar astrocytes increased several-fold after 35 days compared with that at 15 days of culture.