Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes

With the development of air‐breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air‐breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia ex...

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Bibliographic Details
Published in:Journal of fish biology 2014-03, Vol.84 (3), p.603-638
Main Authors: Chew, S. F., Ip, Y. K.
Format: Article
Language:English
Subjects:
Air
alanine
amino acid
Amino Acids - metabolism
Ammonia
Ammonia - metabolism
Ammonia - toxicity
Animals
Brain - physiology
emersion
Fish
Fishes - physiology
gills
glutamine
Glutamine - biosynthesis
Hydrogen-Ion Concentration
Membrane Transport Proteins - metabolism
Metabolism
Nitrogen - metabolism
Proteolysis
Respiration
Toxicity
urea
Urea - metabolism
Volatilization
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Summary:With the development of air‐breathing capabilities, some fishes can emerge from water, make excursions onto land or even burrow into mud during droughts. Air‐breathing fishes have modified gill morphology and morphometry and accessory breathing organs, which would tend to reduce branchial ammonia excretion. As ammonia is toxic, air‐breathing fishes, especially amphibious ones, are equipped with various strategies to ameliorate ammonia toxicity during emersion or ammonia exposure. These strategies can be categorized into (1) enhancement of ammonia excretion and reduction of ammonia entry, (2) conversion of ammonia to a less toxic product for accumulation and subsequent excretion, (3) reduction of ammonia production and avoidance of ammonia accumulation and (4) tolerance of ammonia at cellular and tissue levels. Active ammonia excretion, operating in conjunction with lowering of ambient pH and reduction in branchial and cutaneous NH3 permeability, is theoretically the most effective strategy to maintain low internal ammonia concentrations. NH3 volatilization involves the alkalization of certain epithelial surfaces and requires mechanisms to prevent NH3 back flux. Urea synthesis is an energy‐intensive process and hence uncommon among air‐breathing teleosts. Aestivating African lungfishes detoxify ammonia to urea and the accumulated urea is excreted following arousal. Reduction in ammonia production is achieved in some air‐breathing fishes through suppression of amino acid catabolism and proteolysis, or through partial amino acid catabolism leading to alanine formation. Others can slow down ammonia accumulation through increased glutamine synthesis in the liver and muscle. Yet, some others develop high tolerance of ammonia at cellular and tissue levels, including tissues in the brain. In summary, the responses of air‐breathing fishes to ameliorate ammonia toxicity are many and varied, determined by the behaviour of the species and the nature of the environment in which it lives.
ISSN:0022-1112
1095-8649
DOI:10.1111/jfb.12279