title: A comparison of the effects of environmental ammonia exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro.
abstract: 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). The present study aimed to elucidate whether the capacity of urea synthesis in H. signifer could be upregulated in response to environmental ammonia exposure. When H. signifer was exposed to environmental ammonia, fairly high concentrations of ammonia were accumulated in the plasma and other tissues. This would subsequently reduce the net influx of exogenous ammonia by reducing the NH(3) partial pressure gradient across the branchial and body surfaces. There was also an increase in the OUC capacity in the liver. Since the ammonia produced endogenously could not be excreted effectively in the presence of environmental ammonia, it was detoxified into urea through the OUC. In comparison, the South American freshwater stingray Potamotrygon motoro, which has lost the capability to synthesize urea de novo, was unable to detoxify ammonia to urea during ammonia loading. No increase in glutamine was observed in the various tissues of H. signifer exposed to environmental ammonia despite a significant increase in the hepatic glutamine synthetase activity. These results indicate that the excess glutamine formed was channelled completely into urea formation through carbamoyl phosphate synthetase III. It has been reported elsewhere that both urea synthesis and urea retention were upregulated in H. signifer exposed to 20 per thousand water for osmoregulatory purposes. By contrast, for H. signifer exposed to environmental ammonia in freshwater, the excess urea formed was excreted to the external medium instead. This suggests that the effectiveness of urea synthesis de novo as a strategy to detoxify ammonia is determined not simply by an increase in the capacity of urea synthesis but, more importantly, by the ability of the animal to control the direction (i.e. absorption or excretion) and rate of urea transport. Our results suggest that such a strategy began to develop in those elasmobranchs, e.g. H. signifer, that migrate into a freshwater environment from the sea but not in those permanently adapted to a freshwater environment.
authors.affiliation: Department of Biological Science, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore. dbsipyk@nus.edu.sg
authors.forename: Yuen K
authors.initials: YK
authors.lastname: Ip
authors.forename: Wai L
authors.initials: WL
authors.lastname: Tam
authors.forename: Wai P
authors.initials: WP
authors.lastname: Wong
authors.forename: Ai M
authors.initials: AM
authors.lastname: Loong
authors.forename: Kum C
authors.initials: KC
authors.lastname: Hiong
authors.forename: James S
authors.initials: JS
authors.lastname: Ballantyne
authors.forename: Shit F
authors.initials: SF
authors.lastname: Chew
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000641:Ammonia / Q000493:pharmacokinetics / Q000633:toxicity*; D000704:Analysis of Variance; D000818:Animals; D001119:Arginase / Q000378:metabolism; D001124:Argininosuccinate Synthase / Q000378:metabolism; D019731:Carbon-Nitrogen Ligases / Q000378:metabolism; D002712:Chlorides / Q000097:blood; D018940:Elasmobranchii / Q000502:physiology*; D004781:Environmental Exposure; D005618:Fresh Water; D005974:Glutamate-Ammonia Ligase / Q000378:metabolism; D005973:Glutamine / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D009954:Ornithine Carbamoyltransferase / Q000378:metabolism; D009994:Osmolar Concentration; D012964:Sodium / Q000097:blood; D013045:Species Specificity; D015854:Up-Regulation / Q000187:drug effects* / Q000502:physiology; D014508:Urea / Q000378:metabolism*
chemical_list: D000596:Amino Acids; D002712:Chlorides; D005973:Glutamine; D000641:Ammonia; D014508:Urea; D012964:Sodium; D009954:Ornithine Carbamoyltransferase; D001119:Arginase; D019731:Carbon-Nitrogen Ligases; D005974:Glutamate-Ammonia Ligase; C024233:carbamoyl-phosphate synthetase (N-acetylglutamate); D001124:Argininosuccinate Synthase