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First published online July 17, 2009
Journal of Experimental Biology 212, 2394-2402 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.030411
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Effects of crowding on ornithine–urea cycle enzyme mRNA expression and activity in gulf toadfish (Opsanus beta)

Tammy Laberge1,*, Patrick J. Walsh2 and M. Danielle McDonald1

1 Division of Marine Biology and Fisheries, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA
2 Department of Biology, Centre for Advanced Research in Environmental Genomics, University of Ottawa, 30 Marie Curie, Ottawa, ON, Canada K1N 6N5

* Author for correspondence (e-mail: tlaberge{at}rsmas.miami.edu)

Accepted 27 April 2009

The gulf toadfish (Opsanus beta) is a facultatively ureotelic fish that excretes primarily urea under conditions of crowding or confinement. To examine the relationship between ammonia production, urea production and the ornithine–urea cycle (O–UC) enzyme activity and mRNA expression, we subjected toadfish to two-day and seven-day crowding regimes. Plasma cortisol levels were measured and liver tissue was assayed for ammonia and urea concentrations. Liver glutamine synthetase (GS), carbamoyl phosphate synthetase III (CPS), ornithine carbamoyl transferase (OCT) and arginase (ARG) activities were also measured. Quantitative PCR was utilized to determine liver GS, CPS, OCT, ARG, argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) mRNA expression. Hepatic ammonia concentrations decreased with increased duration of crowding whereas liver urea and circulating cortisol levels increased. An elevation in enzyme activity with increased duration of crowding was observed for all four O-UC enzymes examined. By contrast, mRNA expression was variable for the O–UC enzymes and only CPS and ASS had mRNA expression levels that were elevated in crowded fish. These results suggest that the activities of O–UC enzymes are better predictors for urea production than O–UC enzyme mRNA expression levels.

Key words: nitrogen metabolism, transcription, carbamoyl phosphate synthetase, glutamine synthetase, ornithine carbamoyl transferase, argininosuccinate synthetase, argininosuccinate lyase, arginase, urea, ammonia, liver, cortisol


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