Rats were given intravenous 15NH4+ infusion at a rate of 2.2 or 5.5 mmol/h/kg body wt to induce hyperammonemia, as animal models of hepatic encephalopathy. Its effect on cerebral amino acid metabolism was studied in vivo by 15N NMR spectroscopy at 20.27 MHz for 15N. Cerebral [gamma-15N]glutamine (present at a tissue concentration of 4-9 mumol/g) and [alpha-15N]glutamate/glutamine (6 mumol/g) were clearly observed in living rats within 9-18 min. In portacaval-shunted rats, final cerebral [gamma-15N]glutamine concentrations were higher than those in controls after the same infusion period, presumably because decreased 15NH4+ removal in the liver led to increased 15NH3 diffusion into the astrocytes. In control rats, cerebral [gamma-15N]glutamine pool increased at a rate of 1.7 mumol/h/g when blood ammonia concentration was 0.8 mM. 15N enrichment in gamma-15N was 71%. From these observations, in vivo activity of glutamine synthetase in rat brain was estimated to be 3.5 mumol/h/g. Comparison with reported optimum in vitro activity suggests that in situ concentrations of some substrates and cofactors limit the activity of glutamine synthetase in vivo.

Volume MR imaging with MR spectroscopic imaging provided a noninvasive assessment of the presence and location of residual cancer after unsuccessful therapy and helped identify successful cryosurgery in patients who still had an elevated prostate-specific antigen level.

1. Rats were infused with 15NH4+ or L-[15N]alanine to induce hyperammonaemia, a potential cause of hepatic encephalopathy. HClO4 extracts of freeze-clamped brain, liver and kidney were analysed by 15N-n.m.r. spectroscopy in combination with biochemical assays to investigate the effects of hyperammonaemia on tissue concentrations of ammonia, glutamine, glutamate and urea. 2. 15NH4+ infusion resulted in a 36-fold increase in the concentration of blood ammonia. Cerebral glutamine concentration increased, with 15NH4+ incorporated predominantly into the gamma-nitrogen atom of glutamine. Incorporation into glutamate was very low. Cerebral ammonia concentration increased 5-10-fold. The results suggest that the capacity of glutamine synthetase for ammonia detoxification was saturated. 3. Pretreatment with the glutamine synthetase inhibitor L-methionine DL-sulphoximine resulted in 84% inhibition of [gamma-15N]glutamine synthesis, but incorporation of 15N into other metabolites was not observed. The result suggests that no major alternative pathway for ammonia detoxification, other than glutamine synthetase, exists in rat brain. 4. In the liver 15NH4+ was incorporated into urea, glutamine, glutamate and alanine. The specific activity of 15N was higher in the gamma-nitrogen atom of glutamine than in urea. A similar pattern was observed when [15N]alanine was infused. The results are discussed in terms of the near-equilibrium states of the reactions involved in glutamate and alanine formation, heterogeneous distribution in the liver lobules of the enzymes involved in ammonia removal and their different affinities for ammonia. 5. Synthesis of glutamine, glutamate and hippurate de novo was observed in kidney. Hippurate, as well as 15NH4+, was contributed by co-extracted urine. 6. The potential utility and limitations of 15N n.m.r. for studies of mammalian metabolism in vivo are discussed.

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Appropriate dietary manipulation may be beneficial in the prevention of recurrent urolithiasis in only a select group of patients.