Ammonia metabolism in dogs with portacaval shunts

Organ Distribution of 13N Following Intravenous Injection of [13N]Ammonia into Portacaval-Shunted Rats

Ammonia is neurotoxic, and chronic hyperammonemia is thought to be a major contributing factor to hepatic encephalopathy in patients with liver disease. Portacaval shunting of rats is used as an animal model to study the detrimental metabolic effects of elevated ammonia levels on body tissues, particularly brain and testes that are deleteriously targeted by high blood ammonia. In normal adult rats, the initial uptake of label (expressed as relative concentration) in these organs was relatively low following a bolus intravenous injection of [¹³N]ammonia compared with lungs, kidneys, liver, and some other organs. The objective of the present study was to determine the distribution of label following intravenous administration of [¹³N]ammonia among 14 organs in portacaval-shunted rats at 12 weeks after shunt construction. At an early time point (12 s) following administration of [¹³N]ammonia the relative concentration of label was highest in lung with lower, but still appreciable relative concentrations in kidney and heart. Clearance of ¹³N from blood and kidney tended to be slower in portacaval-shunted rats versus normal rats during the 2-10 min interval after the injection. At later times post injection, brain and testes tended to have higher-than-normal ¹³N levels, whereas many other tissues had similar levels in both groups. Thus, reduced removal of ammonia from circulating blood by the liver diverts more ammonia to extrahepatic tissues, including brain and testes, and alters the nitrogen homeostasis in these tissues. These results emphasize the importance of treatment paradigms designed to reduce blood ammonia levels in patients with liver disease.

Effect of ammonia on plasma and cerebrospinal fluid amino acids in dogs with and without portacaval anastomoses

To elucidate the possible connection between ammonia-induced changes of plasma and cerebrospinal fluid (CSF) amino acid levels and the development of hepatic encephalopathy in dogs, beagle dogs were given an ammonium acetate infusion both before and following portacaval shunt (PCS). During ammonia-induced coma and after recovery in the dogs prior to PCS the plasma and CSF concentrations of most amino acids were decreased. Following PCS the plasma and CSF concentrations of the aromatic amino acids (AAA), phenylalanine and tyrosine, increased and the levels of the branched chain amino acids (BCAA), valine, leucine, and isoleucine, decreased during ammonia-induced coma. The CSF/plasma molar ratio for the AAA exhibited a marked increase after recovery as compared to the value during coma in the Eck-fistula dogs. With respect to the AAA, no correlation was observed between signs of neurologic impairment in the animals and the following parameters: glutamine and methionine levels of CSF, and the plasma molar ratio (Formula: see text). The data obtained do not support the hypothesis that high concentrations of phenylalanine and tyrosine in the brain may be primarily responsible for altered neurotransmission leading to the development of hepatic encephalopathy.