COMPARTMENTATION OF CEREBRAL GLUTAMATE IN-SITU AS DETECTED BY H-1 C-13 NMR/

Incorporation of C-13 label from either [1-C-13]glucose to glutamate C -4 and lactate C-3 or from [2-C-13]acetate to glutamate C-4 was monito red in situ in a superfused brain slice preparation by using H-1-detec ted/C-13-edited (H-1/C-13) n.m.r. spectroscopy. The fractional enrichm ents of both metabolites were determined by this means in both brain s lices and acid extracts of the preparations in order to assess their H -1-n.m.r. detectabilities. The H-1/C-13 Satellite resonances from glut amate C-4 and lactate C-3 in brain tissue were followed from 4 min onw ards in the presence of 5 mM [1-C-13]glucose. Fractional enrichment of glutamate C-4 in the slice preparations was higher than in their acid extracts throughout the incubation of 100 min; at 30 min the enrichme nt was 15.9+/-0.6% in the slice preparations and 10.6+/-0.9% in extrac ts and at 100 min 24.5+/-1.7% compared with 19.7+/-0.4% respectively. In contrast, lactate C-3 reached a steady-state fractional enrichment of approx. 43% by 15 min and there was no difference between the value s determined in the slice preparations and the acid extracts. There wa s a significant difference between the glutamate C-4 fractional enrich ments in the brain slices (7.4+/-0.6%) and extracts (5.1+/-0.3%) after 60 min of incubation with [2-C-13]acetate. Thus C-13 label from both glucose and exogenous acetate enters a pool of glutamate that is more amenable to H-1 n.m.r. detection than total acid-extracted brain bioch emical glutamate, whereas lactate is labelled with full H-1 n.m.r. vis ibility, The results are discussed in the light of the biochemical fac tors that affect glutamate H-1-n.m.r. susceptibility and thus its n.m. r. visibility.