Please use this identifier to cite or link to this item: https://doi.org/10.1242/jeb.00612
Title: A comparison of the effects of environmental ammonia exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro
Authors: Ip, Y.K. 
Tam, W.L.
Wong, W.P.
Loong, A.M.
Hiong, K.C.
Ballantyne, J.S.
Chew, S.F.
Keywords: Amino acid
Ammonia
Ammonia detoxification
Ammonia excretion
Carbamoyl phosphate synthetase
Elasmobranch
Himantura signifer
Nitrogen metabolism
Ornithine-urea cycle
Osmoregulation
Stingray
Urea
Urea excretion
Issue Date: Oct-2003
Citation: Ip, Y.K., Tam, W.L., Wong, W.P., Loong, A.M., Hiong, K.C., Ballantyne, J.S., Chew, S.F. (2003-10). A comparison of the effects of environmental ammonia exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro. Journal of Experimental Biology 206 (20) : 3625-3633. ScholarBank@NUS Repository. https://doi.org/10.1242/jeb.00612
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 NH3 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‰ 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.
Source Title: Journal of Experimental Biology
URI: http://scholarbank.nus.edu.sg/handle/10635/99816
ISSN: 00220949
DOI: 10.1242/jeb.00612
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