N-15-NMR SPECTROSCOPY STUDIES OF AMMONIA TRANSPORT AND GLUTAMINE SYNTHESIS IN THE HYPERAMMONEMIC RAT-BRAIN

Ammonia transport and glutamine synthesis were studied in the hyperamm onaemic rat brain in vivo using N-15-NMR spectroscopy at a plasma ammo nia level of approximately 0.39 mM raised via an intravenous [N-15]-am monium acetate infusion. The initial slope of the time course of the s ummed cerebral N-15-labelled metabolites was used to determine the rat e of ammonia net transport during hyperammonemia as 0.13 +/- 0.02 mu m ol/min/g (mean +/- SD; n = 5). Based on the total accumulation of glut amine and the 1:2 stoichiometric relationship between fluxes of four-c arbon skeletons and nitrogen atoms, the rate of de novo glutamine synt hesis through anaplerosis and subsequent glutamate dehydrogenase actio n was calculated to be 0.065 +/- 0.01 mu mol/min/g. The rate of total glutamine synthesis was estimated to be 0.20 +/- 0.06 mu mol/min/g (n = 5) by fitting the [5-N-15]glutamine time course to a previously desc ribed model of glutamate-glutamine cycling between astrocytes and neur ones. A large dilution was also observed in [2-N-15]glutamine, which s upports the glutamate-glutamine cycle as being an important pathway fo r neuronal glutamate repletion in vivo.