QUANTITATIVE-DETERMINATION OF GLUTAMATE TURNOVER BY H-1-OBSERVED, C-13-EDITED NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY IN THE CEREBRAL-CORTEX EX-VIVO - INTERRELATIONSHIPS WITH OXYGEN-CONSUMPTION

The kinetics of glutamate C-13-4 label appearance from D-[1-C-13]-gluc ose and C-13-4 label disappearance from steady state following D-C-12- glucose incubation were quantified with LH-observed, C-13-edited nucle ar magnetic resonance (NMR) spectroscopy in the superfused brain slice s under largely varying oxygen consumption. Label incorporation to and from glutamate C-4 were fitted into mono- or bicompartmental models i n order to determine the respective rate constants and to assess the p resence of plausible multiple pools. At a steady-stale oxygen consumpt ion of similar to 4 mu mol/min/g dry weight, glutamate labelling could be fitted into a biexponential equation, suggesting that there were t wo compartments with a large difference in their rates (respective rat e constants of 0.022 and 0.149) and pool sizes (relative contributions of 91.2 and 8.8%, respectively). Stimulation of oxygen consumption in the brain slice preparations with either 40 mM KCl by 59.5+/-10.3% or 5 mu M carbonyl cyanide m-fluorophenyl hydrazone by 61.4+/-8.4% incre ased glutamate C-4 labelling rate constants to 0.058+/-0.009 and 0.054 +/-0.006, respectively. In the stimulated slice preparation, glutamate labelling could only be fitted into a monoexponential equation. C-13- 4 label disappearance, independent of oxygen uptake, could also only b e fitted into a monoexponential equation. There was a close match betw een the rate constants of label disappearance and appearance in non-st imulated and carbonyl cyanide m-fluorophenyl hydrazone-stimulated slic es. In the presence of 40 mM KCl label disappearance did not, however, increase. These data show that glutamate C-4 turnover from exogenous D-[1-C-13]-glucose can be used as an index of oxidative metabolism in situ under steady-state conditions as well as when oxygen metabolism i s strongly stimulated. The results are discussed with respect to the u se of NMR spectroscopy as a means of mapping brain oxidative metabolis m. (C) 1997 Elsevier Science Ltd.