Excretory nitrogen metabolism in the Chinese fire-belly newt Cynops orientalis in water, on land, or in high concentrations of environmental ammonia

Abstract

The Chinese fire-belly newt Cynops orientalis reverts to an aquatic mode of living when sexually mature. Despite living in water, sexually mature C. orientalis maintained high capacity for hepatic urea synthesis. However, it had a lower rate of urea production than other terrestrial amphibians because endogenous ammonia could diffuse out to the external medium as NH3. This conserves cellular energy because urea synthesis is energetically expensive. Simultaneously, C. orientalis also reduced the rate of urea excretion, and excreted 33% of the total nitrogenous waste as ammonia. Upon exposure to land, C. orientalis increased the rate of urea synthesis from accumulating endogenous ammonia. The increased rate of urea synthesis was within the inherent capacity of the hepatic ornithine-urea cycle; there was no induction of hepatic carbamoyl phosphate synthetase or ornithine transcarbamoylase activities and there was no reduction in ammonia production. When exposed to water containing 75 mmol.l-1 NH4Cl, the rates of both urea synthesis and urea excretion increased. Under such experimental conditions, the ornithine-urea cycle may be operating close to its limit; glutamine began to accumulate in the body, and endogenous ammonia production via amino acid catabolism was reduced.