To study ammonia metabolism through activities of glutamine synthetase (GS) and phosphate-activated glutaminase in pigs with acute liver failure

Abstract

Background: Acute liver failure results in disturbed ammonia metabolism, urea synthesis capacity decreases and hyperammonemia develops. Glutamine synthetase (GS) detoxifies (temporarily) ammonia to glutamine. However, glutamine is split by phosphate-activated glutaminase (PAG) again to ammonia and glutamate. We aimed to investigate whether organ specific changes in both GS and PAG activities are involved in the development of hyperammonemia in ALF. We also studied these relations in the context of a novel ammonia lowering treatment concept, L-Ornithine + Phenylbutyrate (OP). Methods: 24 female pigs were randomised into three equal groups: ALF (induced by liver devascularisation), ALF + OP and sham-operated. At the end of the experiment at T=8 hrs, the animals were sacrificed, tissue samples from the kidneys, lungs, muscles, duodenum, brain and ileum were dissected and frozen at -80 °C. To measure GS activity we applied a modified version of the methods previously described by Minet et al and Seiler et al. PAG activity was determined by measuring ammonia production following incubation for one hour at 37 °C with O-phthaladelhyde. Results: GS activity in ALF group, in almost all tissues studied, was increased. PAG activity, in all tissues studied, was increased. In ALF-OP treated animals our data showed reduction in PAG activity. Conclusion: Increased GS activity in muscles, lungs and small intestines contributes to ammonia detoxification during ALF. Increased PAG activity in kidney, small intestines and lungs contribute to systemic hyperammonemia in ALF. OP treatments lead to decrease in PAG activity, hence reduction in arterial ammonia and prevention of intracranial hypertension in pigs with ALF.