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The snakehead Channa asiatica accumulates alanine during aerial exposure, but is incapable of sustaining locomotory activities on land through partial amino acid catabolism
1 Natural Sciences, National Institute of Education, Nanyang Technological
University, 1 Nanyang Walk, Singapore 637616, Republic of Singapore
2 Department of Biological Sciences, National University of Singapore, 10
Kent Ridge Road, Singapore 117543, Republic of Singapore
* Author for correspondence (e-mail: sfchew{at}nie.edu.sg)
Accepted 11 November 2002
The freshwater snakehead Channa asiatica is an obligatory
air-breather that resides in slow-flowing streams and in crevices near
riverbanks in Southern China. In its natural habitat, it may encounter bouts
of aerial exposure during the dry seasons. In the laboratory, the ammonia
excretion rate of C. asiatica exposed to terrestrial conditions in a
12h:12h dark:light regime was one quarter that of the submerged control.
Consequently, the ammonia contents in the muscle, liver and plasma increased
significantly, and C. asiatica was able to tolerate quite high levels
of ammonia in its tissues. Urea was not the major product of ammonia
detoxification in C. asiatica, which apparently did not possess a
functioning ornithine urea cycle. Rather, alanine increased fourfold to 12.6
µmolg-1 in the muscle after 48h of aerial exposure. This is the
highest level known in adult teleosts exposed to air or an ammonia-loading
situation. The accumulated alanine could account for 70% of the deficit in
ammonia excretion during this period, indicating that partial amino acid
catabolism had occurred. This would allow the utilization of certain amino
acids as energy sources and, at the same time, maintain the new steady state
levels of ammonia in various tissues, preventing them from rising further.
There was a reduction in the aminating activity of glutamate dehydrogenase
from the muscle and liver of specimens exposed to terrestrial conditions. Such
a phenomenon has not been reported before and could, presumably, facilitate
the entry of 
-ketoglutarate into the Krebs cycle instead of its
amination to glutamate, as has been suggested elsewhere. However, in contrast
to mudskippers, C. asiatica was apparently unable to reduce the rates
of proteolysis and amino acid catabolism, because the reduction in nitrogenous
excretion during 48 h of aerial exposure was completely balanced by
nitrogenous accumulation in the body. Alanine accumulation also occurred in
specimens exposed to terrestrial conditions in total darkness, with no change
in the total free amino acid content in the muscle. Exercise on land led to a
decrease in glycogen content, and an increase in lactate levels, with no
significant effect on ammonia and alanine contents in the muscle of C.
asiatica. Hence, unlike the mudskipper Periophthalmodon schlosseri,
C. asiatica was incapable of increasing the rate of partial amino acid
catabolism to sustain locomotory activities on land. Alanine formation
therefore appears to be a common strategy adopted by obligatory air-breathing
fishes to avoid ammonia toxicity (not a strategy to detoxify ammonia) on land,
but not all of them can utilize it to fuel muscular activities.
Key words: aerial exposure, alanine, amino acid, ammonia, Channa asiatica, glutamate dehydrogenase, proteolysis, snakehead
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