Functional Ureogenesis in the Gobiid Fish, Mugilogobius Abei

To examine the transition to ureogenesis, the gobiid fish Mugilogobius abei was immersed in 2 mmol l⁻¹ NH₄HCO₃ or a ¹⁵N-labelled ammonia solution [1 mmol l⁻¹ (¹⁵NH₄)₂SO₄, pH 8.0] for 4–8 days. When exposed to 2 mmol l⁻¹ NH₄HCO₃ or ¹⁵N-labelled ammonia solution for 4 days, the rate of urea excretion increased to seven times that of the control (in 20 % synthetic sea water) and remained at this level for 4 days. The proportion of nitrogen excreted as urea reached 62 % of total nitrogen excretion (ammonia-N + urea-N). ¹⁵N-enrichment of the amide-N in glutamine in the tissues of fish exposed to ¹⁵N-labelled ammonia was virtually the same as that of ammonia-N: i.e. approximately twice that of urea-N in the excreta and the tissues. Glutamine contents and glutamine synthetase activities in the liver and muscle increased greatly following exposure to ammonia. Urea and citrulline contents in the muscle and whole body of the exposed fish increased significantly, whereas uric acid contents remained unchanged. Carbamoyl phosphate synthetase III (CPSase III) mRNA expression and CPSase III activity were detected in the muscle, skin and gill, but levels were negligible in the liver. Furthermore, all other ornithine–urea cycle (O-UC) enzymes were also detected in muscle, skin and gill. Thus, M. abei clearly shows the transition from ammoniotely to ureotely under ammonia-loading condition and is able to produce urea mainly via the O-UC operating in multiple non-hepatic tissues as a means for ammonia detoxification.