Cerebral Cortex Ammonia and Glutamine Metabolism During Liver Insufficiency‐Induced Hyperammonemia in the Rat

Abstract

Hyperammonemia has been suggested to induce enhanced cerebral cortex ammonia uptake, subsequent glutamine synthesis and accumulation, and finally net glutamine release into the blood stream, but this has never been confirmed in liver insufficiency models. Therefore, cerebral cortex ammonia- and glutamine-related metabolism was studied during liver insufficiency-induced hyperammonemia by measuring plasma flow and venous-arterial concentration differences of ammonia and amino acids across the cerebral cortex (enabling estimation of net metabolite exchange), 1 day after portacaval shunting and 2, 4, and 6 h after hepatic artery ligation (or in controls). The intra-organ effects were investigated by measuring cerebral cortex tissue ammonia and amino acids 6 h after liver ischemia induction or in controls. Arterial ammonia and glutamine increased in portacaval-shunted rats versus controls, and further increased during liver ischemia. Cerebral cortex net ammonia uptake, observed in portacaval-shunted rats, increased progressively during liver ischemia, but net glutamine release was only observed after 6 h of liver ischemia. Cerebral cortex tissue glutamine, gamma-aminobutyric acid, most other amino acids, and ammonia levels were increased during liver ischemia. Glutamate was equally decreased in portacaval-shunted and liver-ischemia rats. The observed net cerebral cortex ammonia uptake, cerebral cortex tissue ammonia and glutamine accumulation, and finally glutamine release into the blood suggest that the rat cerebral cortex initially contributes to net ammonia removal from the blood during liver insufficiency-induced hyperammonemia by augmenting tissue glutamine and ammonia pools, and later by net glutamine release into the blood. The changes in cerebral cortex glutamate and gamma-aminobutyric acid could be related to altered ammonia metabolism.

Glial uptake of neurotransmitter glutamate from the extracellular fluid studiedin vivoby microdialysis and13C NMR

“…The rates were determined from the time-courses of [3-13 C] and [2-13 C]GLU/GLN formation when brain concentration of GLN was 21.7 ± 5.4 lmol/g. The significant contribution of the anaplerotic pathway through glial pyruvate carboxylase is understandable because, at brain GLN concentration > 17 lmol/g, net efflux of brain GLN into the blood is observed (Dejong et al 1992), leading to stimulation of the anaplerotic pathway to replenish GLN carbons. In our present study, the rate of GLN synthesis was determined from the linear increase of [5-13 C, 15 N]GLN during the first 0.6 h of 15 NH 4 Ac infusion, and brain GLN concentration was 10.8 ± 0.76 lmol/g (n ¼ 6) after 0.85 ± 0.04 h of NH 4 Ac infusion.…”

Oxidative Stress and Mitogen-Activated Protein Kinase Phosphorylation Mediate Ammonia-Induced Cell Swelling and Glutamate Uptake Inhibition in Cultured Astrocytes

“…Thus, whereas total brain glutamate levels are decreased in various models of HE (Hindfelt et al, 1977;Bosman et al, 1990;Dejong et al, 1992), extracellular glutamate levels are consistently increased in rat models of acute liver failure (Moroni et al, 1983;Tossman et al, 1987;de Knegt et al, 1994). Ammonia has been shown to suppress high affinity glutamate uptake (Bender and Norenberg, 1996).…”

Steady-state in Vivo Glutamate Dehydrogenase Activity in Rat Brain Measured by 15N NMR

“…This in turn strongly suggests that the effluxing metabolites consisted mainly of endogenous [2][3][4][5][6][7][8][9][10][11][12][13][14] N]glutamate/glutamine. At brain glutamine levels observed in these rats, 8 -10 mol/g (32), efflux of glutamine from the brain has been shown not to occur (23) as discussed in detail previously (19,32 (20), and hence about 10% of [2][3][4][5][6][7][8][9][10][11][12][13][14][15] N]Glx, as expected from the observation that, in rat brain, the concentration of aspartate is only about 20% of that of glutamate (31). Hence, underestimate of the GDH activity resulting from the efflux of 15 N into aspartate, is at most 10%.…”

“…With the anesthetized rat still breathing, the cranium was opened by scissor dissection and the brain was removed in toto and rapidly frozen in liquid nitrogen for preparation of a perchloric acid extract, as described previously (21). This procedure takes Ͻ30 s and is as fast as similar dissection-freezing methods (22,23) that have been shown to yield brain ammonia, glutamate, and glutamine concentrations that are in good agreement with those obtained by freeze-blowing (24). [ 15 N]glutamate, [2][3][4][5][6][7][8][9][10][11][12][13][14][15] N]glutamine, and [5][6][7][8][9][10][11][12][13][14][15] N]glutamine in the brain extract were identified by 15 N NMR and quantified from the observed peak intensity (measured as integrated peak area) by comparison with those of standards, as described previously (19,20 (20,25), with the following modification.…”