Cerebral cortex ammonia and glutamine metabolism in two rat models of chronic liver insufficiency-induced hyperammonemia: influence of pair-feeding.

Enhanced cerebral cortex ammonia uptake, subsequent glutamine synthesis, and glutamine release into the bloodstream have been hypothesized to deplete cerebral cortex glutamate pools. We investigated this hypothesis in rats with chronic liver insufficiency-induced hyperammonemia and in pair-fed controls to rule out effects of differences in food intake. Cerebral cortex plasma flow and venous-arterial concentration differences of ammonia and amino acids, as well as cerebral cortex tissue concentrations, were studied 7 and 14 days after surgery in portacaval-shunted/bile duct-ligated, portacaval-shunted, and sham-operated rats, while the latter two were pair-fed to the first group, and in normal unoperated ad libitum-fed control rats. At both time points, arterial ammonia was elevated in the chronic liver insufficiency groups and arterial glutamine was elevated in portacaval shunt/biliary obstruction rats compared to the other groups. In the chronic liver insufficiency groups net cerebral cortex ammonia uptake was observed at both time points and was accompanied by net glutamine release. Also in these groups, cerebral cortex tissue glutamine, many other amino acid, and ammonia levels were elevated. Tissue glutamate levels were decreased to a similar level in all operated groups compared with normal unoperated rats, irrespective of plasma and tissue ammonia and glutamine levels. These results demonstrate that during chronic liver insufficiency-induced hyperammonemia, the rat cerebral cortex enhances net ammonia uptake and glutamine release. However, the decrease in tissue glutamate concentrations in these chronic liver insufficiency models seems to be related primarily to nutritional status and/or surgical trauma.