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Effects of hypoxia on the energy status and nitrogen metabolism of African lungfish during aestivation in a mucus cocoon

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Abstract

We examined the energy status, nitrogen metabolism and hepatic glutamate dehydrogenase activity in the African lungfish Protopterus annectens during aestivation in normoxia (air) or hypoxia (2% O2 in N2), with tissues sampled on day 3 (aerial exposure with preparation for aestivation), day 6 (entering into aestivation) or day 12 (undergoing aestivation). There was no accumulation of ammonia in tissues of fish exposed to normoxia or hypoxia throughout the 12-day period. Ammonia toxicity was avoided by increased urea synthesis and/or decreased endogenous N production (as ammonia), but the dependency on these two mechanisms differed between the normoxic and the hypoxic fish. The rate of urea synthesis increased 2.4-fold, with only a 12% decrease in the rate of N production in the normoxic fish. By contrast, the rate of N production in the hypoxic fish decreased by 58%, with no increase in the rate of urea synthesis. Using in vivo 31P NMR spectroscopy, it was demonstrated that hypoxia led to significantly lower ATP concentration on day 12 and significantly lower creatine phosphate concentration on days 1, 6, 9 and 12 in the anterior region of the fish as compared with normoxia. Additionally, the hypoxic fish had lower creatine phosphate concentration in the middle region than the normoxic fish on day 9. Hence, lowering the dependency on increased urea synthesis to detoxify ammonia, which is energy intensive by reducing N production, would conserve cellular energy during aestivation in hypoxia. Indeed, there were significant increases in glutamate concentrations in tissues of fish aestivating in hypoxia, which indicates decreases in its degradation and/or transamination. Furthermore, there were significant increases in the hepatic glutamate dehydrogenase (GDH) amination activity, the amination/deamination ratio and the dependency of the amination activity on ADP activation in fish on days 6 and 12 in hypoxia, but similar changes occurred only in the normoxic fish on day 12. Therefore, our results indicate for the first time that P. annectens exhibited different adaptive responses during aestivation in normoxia and in hypoxia. They also indicate that reduction in nitrogen metabolism, and probably metabolic rate, did not occur simply in association with aestivation (in normoxia) but responded more effectively to a combined effect of aestivation and hypoxia.

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Abbreviations

ADP:

Adenosine diphosphate

ATP:

Adenosine triphosphate

EDTA:

Ethylenediaminetetraacetic acid

EGTA:

Ethylene glycol-tetraacetic acid

FAA:

Free amino acid

GDH:

Glutamate dehydrogenase

α-KG:

α-Ketoglutarate

N:

Nitrogen

NADH:

β-Nicotinamide adenine dinucleotide, reduced

NAD:

β-Nicotinamide adenine dinucleotide

NaF:

Sodium fluoride

31P NMR:

31Phosphorus nuclear magnetic resonance

PMSF:

Phenylmethyl sulfonyl fluoride

TFAA:

Total free amino acid

TEFAA:

Total essential free amino acid

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Authors and Affiliations

  1. Department of Biological Science, National University of Singapore, Kent Ridge, Singapore, 117543, Singapore

    A. M. Loong, S. F. Ang, W. P. Wong & Y. K. Ip

  2. Alfred-Wegener-Institute for Marine and Polar Research, Am Handelshafen 12, 27570, Bremerhaven, Germany

    H. O. Pörtner, C. Bock & R. Wittig

  3. Institut für Zoophysiologie, Heinrich-Heine Universität, 40225, Düsseldorf, Germany

    C. R. Bridges

  4. Natural Sciences, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, Singapore, 637616, Singapore

    S. F. Chew

Authors
  1. A. M. Loong
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  2. S. F. Ang
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  3. W. P. Wong
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  4. H. O. Pörtner
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  5. C. Bock
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  6. R. Wittig
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  7. C. R. Bridges
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  8. S. F. Chew
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  9. Y. K. Ip
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Correspondence to Y. K. Ip.

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Communicated by I.D. Hume.

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Loong, A.M., Ang, S.F., Wong, W.P. et al. Effects of hypoxia on the energy status and nitrogen metabolism of African lungfish during aestivation in a mucus cocoon. J Comp Physiol B 178, 853–865 (2008). https://doi.org/10.1007/s00360-008-0273-9

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  • DOI: https://doi.org/10.1007/s00360-008-0273-9

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