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

Rats were given intravenous 15 NH 4 + 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 15 N NMR spectroscopy at 20.27 MHz for 15 N. Cerebral [γ‐ 15 N]glutamine (present at a tissue concentration of 4–9 μmol/g) and [α‐ 15 N]glutamate/glutamine (6 μmol/g) were clearly observed in living rats within 9‐18 min. In portacaval‐shunted rats, final cerebral [γ‐ 15 N]glutamine concentrations were higher than those in controls after the same infusion period, presumably because decreased 15 NH   4 +removal in the liver led to increased 15 NH 3 diffusion into the astrocytes. In control rats, cerebral [γ‐ 15 N]glutamine pool increased at a rate of 1.7 μmol/h/g when blood ammonia concentration was 0.8 mM. 15 N enrichment in γ‐ 15 N was 71%. From these observations, in vivo activity of glutamine synthetase in rat brain was estimated to be 3.5 μmol/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 .