COMPARATIVE-ANALYSIS OF C-13-ENRICHED METABOLITES RELEASED IN THE MEDIUM OF CEREBELLAR AND CORTICAL ASTROCYTES INCUBATED WITH [1-C-13]GLUCOSE

Rat cerebellar and cortical astrocytes cultured for 15 or 35 days were incubated with [1-C-13]glucose in the presence or absence of 4 mM exo genous glutamine and the release of C-13-enriched metabolites into cel l media was studied by C-13-NMR spectroscopy. In the presence of exoge nous glutamine, both cerebellar and cortical astrocytes consumed the a mino acid. In contrast, a net production of glutamine occurred in the absence of the amino acid. Simultaneously, a release of C-13-enriched glutamine into cell media was observed and was higher in the presence than in the absence of exogenous glutamine. This demonstrated the occu rrence of an isotopic-exchange process which may involve a futile cycl e at the level of glutamine synthetase and glutaminase activities. The C-13-enrichment ratio between glutamine carbons C2 and C3 was close t o 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 sy nthesized and exported into the medium of both cerebellar and cortical astrocytes. In contrast, citrate was specifically produced by cortica l astrocytes. Slight increases in alanine and glutamine productions we re observed for cortical astrocyte cultures between 15 and 35 days, wh ereas the amino acid production by cerebellar astrocytes increased sev eral-fold after 35 days compared with that at 15 days of culture.