Increased urea synthesis and/or suppressed ammonia production in the African lungfish, Protopterus annectens, during aestivation in air or mud |
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Authors: | Ai M Loong Cheryl Y M Pang Kum C Hiong Wai P Wong Shit F Chew Yuen K Ip |
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Institution: | (1) Department of Biological Sciences, National University of Singapore, Kent Ridge, Singapore, 117543, Republic of Singapore;(2) Natural Sciences and Science Education, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore, 637616, Republic of Singapore |
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Abstract: | The objective of this study was to elucidate how the African lungfish, Protopterus annectens, ameliorated ammonia toxicity during 12 or 46 days of aestivation in air or in mud. Twelve days of aestivation in air led
to significant increases in contents of urea, but not ammonia, in tissues of P. annectens. The estimated rate of urea synthesis increased 2.7-fold despite the lack of changes in the activities of hepatic ornithine–urea
cycle enzymes, but there was only a minor change in the estimated rate of ammonia production. After 46 days of aestivation
in air, the ammonia content in the liver decreased significantly and contents of urea in all tissues studied increased significantly,
indicating that the fish shifted to a combination of increased urea synthesis (1.4-fold of the day 0 value) and decreased
ammonia production (56% of the day 0 value) to defend against ammonia toxicity. By contrast, 12 days of aestivation in mud
produced only minor increases in tissue urea contents, with ammonia contents remained unchanged. This was apparently achieved
through decreases in urea synthesis and ammonia production (40 and 15%, respectively, of the corresponding day 0 value). Surprisingly,
46 days of aestivation in mud resulted in no changes in tissue urea contents, indicating that profound suppressions of urea
synthesis and ammonia production (2.6 and 1.2%, respectively, of the corresponding day 0 value) had occurred. This is the
first report on such a phenomenon, and the reduction in ammonia production was so profound that it could be the greatest reduction
known among animals. Since fish aestivated in mud had relatively low blood pO2 and muscle ATP content, they could have been exposed to hypoxia, which induced reductions in metabolic rate and ammonia production.
Consequently, fish aestivating in mud had a lower dependency on increased urea synthesis to detoxify ammonia, which is energy
intensive, than fish aestivating in air. |
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Keywords: | Aestivation Hypoxia Lungfish Nitrogen metabolism Protopterus annectens Urea |
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