A 15N-NMR study of isolated brain in portacaval-shunted rats after acute hyperammonemia.

Acute hyperammonemia was induced by 15NH4+ infusion in portacaval-shunted (PCS) and control rats to investigate its effects on cerebral metabolism of glutamine, glutamate and gamma-aminobutyrate. Cerebral 15N-metabolites were observed by 15N-NMR spectroscopy in the ex vivo brain, removed in toto at the end of infusion. Key 15N-metabolites in the brain and liver were quantitated and their specific activities measured by NMR and biochemical assays in perchloric acid extracts of the freeze-clamped organs. In the ex vivo brain, gamma-15N]glutamine, present at tissue concentrations of 3-5 mumol/g with 15N enrichment of 36-48%, was observable within 6-13 min of data acquisition. alpha-15N]glutamine/glutamate, each present at 0.5-1 mumol/g (approx. 10% enrichment), were observed in 27 min. The results demonstrate the feasibility of observing these cerebral metabolites by 15N-NMR within a physiological time scale. In a rat pretreated with glutamine synthetase inhibitor, L-methionine DL-sulfoximine, cerebral 15N]gamma-aminobutyrate was observed after 910 min. In PCS rats, decreased 15NH4+ removal in the liver was accompanied by formation of approx. 2-fold higher concentration of cerebral gamma-15N]glutamine relative to that in weight-matched controls. The result suggests that increased diffusion of blood-borne 15NH3 into the brain led to increased gamma-15N]glutamine synthesis in astrocytes as well as ammonia-mediated inhibition of glutaminase.