Exogenous Glutamate Is Metabolized to Glutamine and Exported by Rat Primary Astrocyte Cultures

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Abstract
Rat cortical astrocytes in primary culture were examined for their capacity to transport and metabolize exogenous L-[U-14C]glutamate. After incubation for time periods up to 120 min, cells and incubation media were analyzed for labelled and endogenous glutamate and its metabolic products by HPLC coupled with fluorescence detection and liquid scintillation counting. Glutamine was the major labelled metabolite after 120 min, accounted for 38% of the original glutamate label, and was found primarily in the incubation medium. A further 13.5% of the label was recovered in deaminated metabolites of glutamate, 1.2% was associated with aspartate, 23% remained in glutamate, and 10.2% was found in acid-precipitated cell fraction. More tha 84% of the label was recovered in these fractions, suggesting that the maximum possible formation and loss of 14CO2 was 16%. The rate of total glutamine synthesis was 1.1 nmol .times. mg protein-1 .times. min-1 when 9 .mu.M exogenous glutamate was present. The total amount of glutamine synthesized greatly exceeded the consumption of glutamate, indicating that a substantial proportion of glutamine was synthesized from other carbon sources. Almost all of the newly formed glutamine was exported into the medium. These results indicate that astrocytes in primary culture, by accumulating glutamate, producing glutamine, and exporting it, are capable of carrying out the glial component of the glutamine cycle.