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First published online May 13, 2004
Journal of Experimental Biology 207, 1993-2002 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00901
Dogmas and controversies in the handling of nitrogenous wastes: The effect of feeding and fasting on the excretion of ammonia, urea and other nitrogenous waste products in rainbow trout
Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4K1
* Author for correspondence (e-mail: kajimur{at}mcmaster.ca)
Accepted 23 January 2004
Ammonia and urea are the primary forms of nitrogen excretion in teleost
fish. There exists, however, a discrepancy between the sum of ammonia plus
urea nitrogen and total nitrogen, indicating that `unknown' nitrogen end
products may play an important role in nitrogen metabolism. The current study
analysed a wide range of nitrogen end products in both fed and fasted juvenile
rainbow trout. Ammonia-N (53–68%) and urea-N (6–10%) were
confirmed as the most important forms of nitrogenous waste, but an interesting
finding was the considerable excretion of nitrogen as amino acids
(4–10%), via the gills, and as protein (3–11%), probably
via the body mucus. Use of anal sutures delineated an important role
for the gastrointestinal tract in the production of ammonia-N and urea-N in
fed fish, but amino acid-N and protein-N output by this route were both
negligible. Alternative nitrogen products – trimethylamine,
trimethylamine oxide, uric acid, and nitrite + nitrate – were not
excreted in detectable quantities. Creatine-N and creatinine-N outputs were
detected but contributed only a small fraction to total nitrogen excretion
(<1.4%). Despite the wide scope of nitrogenous end products investigated, a
considerable proportion (12–20%) of nitrogen excretion remains unknown.
Possible alternative end products and methodological considerations are
proposed to explain this phenomenon. The findings described above were used to
recalculate the nitrogen quotient
(NQ=
N/O2)
on trout that had been either fasted or fed various daily rations (1%, 3% or
5% dry food per unit wet body mass per day). Feeding increased oxygen
consumption (O2)
and total-N excretion (N).
The NQ is often used as a measure of protein utilisation in aerobic metabolism
and assumes that all protein (and amino acid) fuels are converted by oxidation
to nitrogenous waste products that are excreted. However, the results showed
that calculation of the NQ based on total nitrogen excretion may overestimate
protein utilisation in aerobic metabolism because of significant excretion of
N in the form of proteins and amino acids, whereas the use of summed ammonia-N
and urea-N excretion probably underestimates the contribution of protein
towards aerobic metabolism. These errors increase as ration increases, because
the discrepancy between total-N excretion and ammonia-N + urea-N excretion
increases.
Key words: amino acid, protein, nitrogen excretion, fish, nitrogen quotient
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