15N-NMR spectroscopy studies of ammonia transport and glutamine synthesis in the hyperammonemic rat brain

Ammonia transport and glutamine synthesis were studied in the hyperammonaemic rat brain in vivo using N-15-NMR spectroscopy at a plasma ammonia level of approximately 0.39 mM raised via an intravenous [N-15]-ammonium acetate infusion. The initial slope of the time course of the summed cerebral N-15-labelled metabolites was used to determine the rate of ammonia net transport during hyperammonemia as 0.13 +/- 0.02 mu mol/min/g (mean +/- SD; n = 5). Based on the total accumulation of glutamine and the 1:2 stoichiometric relationship between fluxes of four-carbon skeletons and nitrogen atoms, the rate of de novo glutamine synthesis through anaplerosis and subsequent glutamate dehydrogenase action 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 described model of glutamate-glutamine cycling between astrocytes and neurones. A large dilution was also observed in [2-N-15]glutamine, which supports the glutamate-glutamine cycle as being an important pathway for neuronal glutamate repletion in vivo.