A 15N NMR study of in vivo cerebral glutamine synthesis in hyperammonemic rats

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.