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

Abstract Acute hyperammonemia was induced by 15 NH 4 + infusion in portacaval-shunted (PCS) and control rats to investigate its effects on cerebral metabolism of glutamine, glutamate and γ-aminobutyrate. Cerebral 15 N-metabolites were observed by 15 N-NMR spectroscopy in the ex vivo brain, removed in toto at the end of infusion. Key 15 N-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, [ γ - 15 N]glutamine, present at tissue concentrations of 3–5 μmol/g with 15 N enrichment of 36–48%, was observable within 6–13 min of data acquisition. [ α - 15 N]glutamine/glutamate, each present at 0.5–1 μmol/g (approx. 10% enrichment), were observed in 27 min. The results demonstrate the feasibility of observing these cerebral metabolites by 15 N-NMR within a physiological time scale. In a rat pretreate with glutamine synthetase inhibitor, l -methionine dl -sulfoximine, cerebral [ 15 N ]γ- aminobutyrate was observed after 910 min. In PCS rats, decreased 15 NH 4 + removal in the liver was accompanied by formation of approx. 2-fold higher concentration of cerebral [ γ - 15 N]glutamine relative to that in weight-matched controls. The result suggests that increased diffusion of blood-borne 15 NH 3 into the brain led to increased [ γ - 15 N]glutamine synthesis in astrocytes as well as ammonia-mediated inhibition of glutaminase.