COMPARTMENTATION OF [C-14] GLUTAMATE AND [C-14] GLUTAMINE OXIDATIVE-METABOLISM IN THE RAT HIPPOCAMPUS AS DETERMINED BY MICRODIALYSIS

Metabolic compartmentation of amino acid metabolism in brain is exempl ified by the differential synthesis of glutamate and glutamine from th e identical precursor and by the localization of the enzyme glutamine synthetase in glial cells. In the current study, we determined if the oxidative metabolism of glutamate and glutamine was also compartmental ized. The relative oxidation rates of glutamate and glutamine in the h ippocampus of free-moving rats was determined by using microdialysis b oth to infuse the radioactive substrate and to collect (CO2)-C-14 gene rated during their oxidation. At the end of the oxidation experiment, the radioactive substrate was replaced by artificial CSF, 2 min-fracti ons were collected, and the specific activities of glutamate and gluta mine were determined. Extrapolation of the specific activity back to t he time that artificial CSF replaced C-14-amino acids in the microdial ysis probe yielded an approximation of the interstitial specific activ ity during the oxidation. The extrapolated interstitial specific activ ities for [C-14]glutamate and [C-14] glutamine were 59 +/- 18 and 2.1 +/- 0.5 dpm/pmol, respectively. The initial infused specific activitie s for [U-C-14] glutamate and [U-C-14] glutamine were 408 +/- 8 and 387 +/- 1 dpm/pmol, respectively. The dilution of glutamine was greater t han that of glutamate, consistent with the difference in concentration s of these amino acids in the interstitial space. Based on the extrapo lated interstitial specific activities,the rate of glutamine oxidation exceeds that of glutamate oxidation by a factor of 5.3. These data in dicate compartmentation of either uptake and/or oxidative metabolism o f these two amino acids. The presence of [C-14]glutamine in the inters titial space when [C-14]glutamate was perfused into the brain provided further evidence for the glutamate/glutamine cycle in brain.