Aberrant Expression and Distribution of Enzymes of the Urea Cycle and Other Ammonia Metabolizing Pathways in Dogs with Congenital Portosystemic Shunts

Abstract

The detoxification of ammonia occurs mainly through conversion of ammonia to urea in the liver via the urea cycle and glutamine synthesis. Congenital portosystemic shunts (CPSS) in dogs cause hyperammonemia eventually leading to hepatic encephalopathy. In this study, the gene expression of urea cycle enzymes (carbamoylphosphate synthetase (CPS1), ornithine carbamoyltransferase (OTC), argininosuccinate synthetase (ASS1), argininosuccinate lyase (ASL), and arginase (ARG1)), N-acetylglutamate synthase (NAGS), Glutamate dehydrogenase (GLUD1), and glutamate-ammonia ligase (GLUL) was evaluated in dogs with CPSS before and after surgical closure of the shunt. Additionally, immunohistochemistry was performed on urea cycle enzymes and GLUL on liver samples of healthy dogs and dogs with CPSS to investigate a possible zonal distribution of these enzymes within the liver lobule and to investigate possible differences in distribution in dogs with CPSS compared to healthy dogs. Furthermore, the effect of increasing ammonia concentrations on the expression of the urea cycle enzymes was investigated in primary hepatocytes in vitro. Gene-expression of CPS1, OTC, ASL, GLUD1 and NAGS was down regulated in dogs with CPSS and did not normalize after surgical closure of the shunt. In all dogs GLUL distribution was localized pericentrally. CPS1, OTC and ASS1 were localized periportally in healthy dogs, whereas in CPSS dogs, these enzymes lacked a clear zonal distribution. In primary hepatocytes higher ammonia concentrations induced mRNA levels of CPS1. We hypothesize that the reduction in expression of urea cycle enzymes, NAGS and GLUD1 as well as the alterations in zonal distribution in dogs with CPSS may be caused by a developmental arrest of these enzymes during the embryonic or early postnatal phase.

Saline is as effective as nitrogen scavengers for treatment of hyperammonemia

“…To our knowledge, this is the first randomized, placebo controlled study for the safety and efficacy of SB and SPA treatments of hyperammonemia. Due to the similarities of ammonia metabolism, dogs are considered a valid model for hyperammonemia in man 29 , 31 . In our study, SB and SPA treatments were safe and well-tolerated in both healthy dogs (SB + SPA) and dogs with CPSS (SB).…”

“…In dogs, UCED are extremely rare 25 , 26 and CPSS is the most common cause of hyperammonemia and HE 27 , 28 . CPSS result in severe reduction of ureagenesis, reduced expressions of urea cycle enzymes and consequently hyperammonemia 29 , 30 . Clinical symptoms of hyperammonemia in dogs (e.g.…”

Influence of Liver Fibrosis on Lobular Zonation

“…A zonated metabolic pattern is also known for ammonia metabolism [14]. While urea-cycle enzymes detoxify ammonia by high capacity and low affinity mechanism in the periportal and midzonal regions, low remaining ammonia concentrations are removed from the sinusoidal blood by a pericentral ring of glutamine synthetase positive hepatocytes that act by a low capacity, high affinity mechanism [15][16][17]. Further zonated functions include glycolysis, gluconeogenesis, glycogenesis, the TCA-cycle, glutamine metabolism and lipogenesis [2,7].…”

Hyperammonemic hepatic encephalopathy management through L-ornithin-L-aspartate administration in dogs

“…The reduction of ammonia detoxification in the liver is the most important cause of hyperammonemia [ 5 14 ]. Ammonia is detoxified via conversion to urea and glutamine in hepatic cells.…”

“…Ammonia is detoxified via conversion to urea and glutamine in hepatic cells. When hepatic failure occurs, the liver can no longer detoxify ammonia [ 2 4 5 14 ]. These conditions are common in the veterinary field [ 9 ].…”