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First published online May 13, 2004
Journal of Experimental Biology 207, 1977-1983 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00905

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Review Article

Dogmas and controversies in the handling of nitrogenous wastes: Ammonia tolerance in the oriental weatherloach Misgurnus anguillicaudatus

T. K. N. Tsui¹, D. J. Randall^1,*, L. Hanson², A. P. Farrell², S. F. Chew³ and Y. K. Ip⁴

¹ Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Hong Kong, China
² Department of Biological Sciences, Simon Fraser University, Vancouver, Canada
³ Natural Sciences, National Institute of Education, Nanyang, Technological University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore
⁴ Department of Biological Sciences, National University of Singapore, 10 Kent Ridge Road, Singapore 117543, Republic of Singapore

^* Author for correspondence (e-mail: BHRAND{at}cityu.edu.hk)

Accepted 26 January 2004

The oriental weatherloach Misgurnus anguillicaudatus is an extremely ammonia-tolerant fish. Many ammonia-protection mechanisms have been reported in this fish. Six strategies used by this fish to deal with the problem of excess ammonia are described. The fish can (1) reduce ammonia production through reduction in protein and/or amino acid catabolism; (2) reduce ammonia production and obtain energy through partial amino acid catabolism leading to alanine formation; (3) detoxify ammonia to glutamine; (4) tolerate very high ammonia levels in its tissues; (5) get rid of ammonia as NH₃ gas and, probably, (6) possesses background K⁺ channels that are impermeable to NH₄⁺. The effects of extracellular ammonia on the contraction performance of the heart from this fish were found to be the same as in rainbow trout, an ammonia-sensitive fish. It suggests that the hearts of most, if not all, fish species are protected against ammonia. MK-801, an NMDA receptor blocker, was found to have a protective effect against ammonia intoxication in the oriental weatherloach, which suggests that the NMDA receptor, as in mammals, is involved in ammonia toxicity.

Key words: oriental weatherloach, Misgurnus anguillicaudatus, ammonia, glutamine, alanine, K⁺ channel, membrane potential, NMDA receptor

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