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.
Kanamori
K
K
Magnetic Resonance Spectroscopy Laboratory, Huntington Medical Research Institutes, Pasadena, CA 91105.
Ross
B D
BD
Farrow
N A
NA
Parivar
F
F
D000641:Ammonia / Q000008:administration & dosage / Q000378:metabolism*; D000818:Animals; D001921:Brain / Q000378:metabolism*; D005974:Glutamate-Ammonia Ligase / Q000037:antagonists & inhibitors / Q000378:metabolism; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D008717:Methionine Sulfoximine / Q000494:pharmacology; D010472:Perchlorates / Q000494:pharmacology; D011167:Portacaval Shunt, Surgical; D051381:Rats; D011919:Rats, Inbred Strains
D010472:Perchlorates; D008717:Methionine Sulfoximine; D000641:Ammonia; D005974:Glutamate-Ammonia Ligase