|
| |
|
|
|
|
||||
| Home Help Feedback Subscriptions Archive Search Table of Contents | ||||
First published online September 9, 2003
A comparison of the effects of environmental ammonia exposure on the Asian freshwater stingray Himantura signifer and the Amazonian freshwater stingray Potamotrygon motoro
1 Department of Biological Science, National University of Singapore, Kent
Ridge, Singapore 117543, Republic of Singapore
2 Department of Zoology, University of Guelph, Guelph, Ontario, Canada NIG
2W1
3 Natural Sciences, National Institute of Education, Nanyang Technological
University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore
* Author for correspondence (e-mail: dbsipyk{at}nus.edu.sg)
Accepted 15 July 2003
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.
Key words: ammonia, ammonia detoxification, ammonia excretion, amino acid, carbamoyl phosphate synthetase, elasmobranch, Himantura signifer, nitrogen metabolism, ornithine-urea cycle, osmoregulation, stingray, urea, urea excretion
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati 
Twitter What's this?
