... environmentally realistic levels in seawater, using it to make urea, an essential osmolyte. Elasmobranch Urea Ornithine–urea cycle Rh proteins Methylamine Glutamine synthetase Ammonia is a ubiquitous toxicant in the aquatic environment, and is a major threat to fish (Randall and Tsui, 2002...

... . Front. Physiol.   4 , 59 . Kajimura   M. , Walsh   P. J. , Mommsen   T. P. , Wood   C. M. (2006). The dogfish shark (Squalus acanthias) increases both hepatic and extrahepatic ornithine urea cycle enzyme activities for nitrogen conservation after feeding . Physiol. Biochem. Zool...

... to significant decreases in the activity of some ornithine–urea cycle enzymes (argininosuccinate synthetase/argininosuccinate lyase and arginase) from the liver of P. sinensis . As a decrease in urea synthesis occurred without the accumulation of ammonia and total free amino acids, it can be deduced that ammonia...

... for correspondence (e-mail: [email protected]) 10 6 2004 © The Company of Biologists Limited 2004 2004 amino acids ammonia ammonia excretion feeding glutamine mudskipper nitrogen metabolism ornithine-urea cycle Periophthalmodon schlosseri urea When confronted with aerial...

.... 300 , 1005 -1016. Felskie, A. K., Anderson, P. M. and Wright, P. A. (1998). Expression and activity of carbamoyl phosphate synthetase III and ornithine urea cycle enzymes in various tissues of four fish species. Comp. Biochem. Physiol. B 119 , 355 -364. Foss, D. L., Baarsch, M. J...

... was excreted in pulsatile events that accounted for 62.0±5.9% of total urea-N excreted. Excretion rates of urea-N and ammonia-N were 1.018±0.084 μmol N h –1 g –1 and 0.235±0.095 μmol N h –1 g –1 ,respectively. Field-collected eggs, larvae and juveniles expressed significant levels of the ornithine–urea cycle...

...Yuen K. Ip; Wai L. Tam; Wai P. Wong; Ai M. Loong; Kum C. Hiong; James S. Ballantyne; Shit F. Chew SUMMARY The white-edge whip tail ray Himantura signifer inhabits a freshwater environment but has retained the capability to synthesize urea de novo through the arginine-ornithine-urea cycle (OUC...

... endogenous ammonia production. Simultaneously, there were significant increases in urea levels in the muscle (8-fold), liver (10.5-fold) and plasma (12.6-fold) of specimens exposed to air for 6 days. Furthermore, there was an increase in the hepatic ornithine-urea cycle (OUC) capacity, with significant...

... the South American freshwater stingray Potamotrygon motoro, H. signifer has a functional ornithine-urea cycle (OUC) in the liver, with hepatic carbamoylphosphate synthetase III (CPS III) and glutamine synthetase (GS) activities lower than those of the marine blue-spotted fan tail ray Taeniura lymma . More...

.... (2001). Role of glutamine in cerebral nitrogen metabolism and ammonia neurotoxicity. Ment. Retard. Dev. Disabil. Res. Rev. 7 , 280 -286. Felskie, A. K., Anderson, P. M. and Wright, P. A. (1998). Expression and activity of carbamoyl phosphate synthetase III and ornithine urea cycle enzymes...

... of this teleost; uricolysis, argininolysis and the ornithine–urea cycle (OUC). Uricase, allantoinase, allantoicase and ureidoglycollate lyase of the uricolytic pathway were expressed in all early life stages and in adult liver of C. gariepinus . Uricase activity increased in starved larvae compared with yolk-sac...

... 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...

... of Kentucky, Lexington, KY 40506, USA. 28 04 1995 © The Company of Biologists Limited 1995 urea ammonia Opsanus beta ureotelism gills skin urine rectal fluid regurgitation ornithine–urea cycle stress Most teleost fish are obligate ammonioteles excreting the bulk...

... acanthias ornithine–urea cycle gill permeability kidney function nitrogenous waste excretion acid–base balance Since the classic work of Homer Smith (1929 , 1936), it has been recognized that marine elasmobranchs retain large amounts of urea, thereby raising the osmolality of their body...

... in normal acid–base regulation. 01 12 1993 © 1994 by Company of Biologists 1994 Lake Magadi urea acid–base balance high pH liver ornithine–urea cycle ammonia glutamine Oreochromis alcalicus grahami The tilapia Oreochromis alcalicus grahami lives in the highly alkaline...

...-to-water ammonia partial pressure diffusion gradient and NH 4 + electrochemical gradient. Urea excretion (J urea) almost doubled at 24–48h of pH10 exposure. Activities of ornithine–urea cycle enzymes in the liver were very low and there was no induction at pH10. However, all three enzymes of the uricolytic...