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First published online November 13, 2009
Journal of Experimental Biology 212, 3828-3836 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.034074

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The freshwater Amazonian stingray, Potamotrygon motoro, up-regulates glutamine synthetase activity and protein abundance, and accumulates glutamine when exposed to brackish (15) water

Y. K. Ip^1,*, A. M. Loong¹, B. Ching¹, G. H. Y. Tham¹, W. P. Wong¹ and S. F. Chew²

¹ Department of Biological Science, National University of Singapore, Kent Ridge, Singapore 117543, Republic of Singapore
² Natural Sciences & Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore

^* Author for correspondence (dbsipyk{at}nus.edu.sg)

Accepted 2 September 2009

This study aimed to examine whether the stenohaline freshwater stingray, Potamotrygon motoro, which lacks a functional ornithine—urea cycle, would up-regulate glutamine synthetase (GS) activity and protein abundance, and accumulate glutamine during a progressive transfer from freshwater to brackish (15) water with daily feeding. Our results revealed that, similar to other freshwater teleosts, P. motoro performed hyperosmotic regulation, with very low urea concentrations in plasma and tissues, in freshwater. In 15 water, it was non-ureotelic and non-ureoosmotic, acting mainly as an osmoconformer with its plasma osmolality, [Na⁺] and [Cl^–] comparable to those of the external medium. There were significant increases in the content of several free amino acids (FAAs), including glutamate, glutamine and glycine, in muscle and liver, but not in plasma, indicating that FAAs could contribute in part to cell volume regulation. Furthermore, exposure of P. motoro to 15 water led to up-regulation of GS activity and protein abundance in both liver and muscle. Thus, our results indicate for the first time that, despite the inability to synthesize urea and the lack of functional carbamoyl phosphate synthetase III (CPS III) which uses glutamine as a substrate, P. motoro retained the capacity to up-regulate the activity and protein expression of GS in response to salinity stress. Potamotrygon motoro was not nitrogen (N) limited when exposed to 15 water with feeding, and there were no significant changes in the amination and deamination activities of hepatic glutamate dehydrogenase. In contrast, P. motoro became N limited when exposed to 10 water with fasting and could not survive well in 15 water without food.

Key words: amino acids, freshwater stingray, glutamate, glutamine, glutamine synthetase, glutamate dehydrogenase, nitrogen metabolism, Potamotrygon motoro, urea

Abbreviations: ALT, alanine aminotransferase • AST, aspartate aminotransferase • CPS III, carbamoyl phosphate synthetase III • FAAs, free amino acids • GDH, glutamate dehydrogenase • GS, glutamine synthetase • OUC, ornithine-urea cycle • TEFAA, total essential free amino acid • TFAA, total free amino acid

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