Accounting for the role of the gastro-intestinal tract in the ammonia and urea nitrogen dynamics of freshwater rainbow trout on long-term satiation feeding

The contribution of the gut to the ingestion, production, absorption and excretion of the extra ammonia and urea nitrogen (urea-N) associated with feeding ('exogenous' fraction) has received limited attention. Analysis of commercial pellet food revealed appreciable concentrations of ammonia and urea-N. Long-term satiation feeding increased whole-trout ammonia and urea-N excretion rates by 2.5-fold above fasting levels. Blood was sampled from the dorsal aorta, posterior, mid- and anterior sub-intestinal veins, as well as the hepatic portal vein in situ. Ammonia, urea-N and fluid flux rates were measured in vitro using novel gut sac preparations filled with native chyme. The sacs maintained the extreme physico-chemical conditions of the lumen seen in vivo. Overall, these results confirmed our hypothesis that the stomach, and anterior intestine and pyloric caecae regions play important roles in ammonia and urea-N production and/or absorption. There was a very high rate of urea-N production in the anterior intestine and pyloric caecae, whereas the posterior intestine dominated for ammonia synthesis. The stomach was the major site of ammonia absorption, and the anterior intestine and pyloric caecae region dominated for urea-N absorption. Model calculations indicated that over 50% of the exogenous ammonia and urea-N excretion associated with satiation feeding was produced in the anaerobic gut. This challenges standard metabolic theory used in fuel-use calculations. The novel gut sac preparations gained fluid during incubation, especially in the anterior intestine and pyloric caecae, owing to marked hyperosmolality in the chyme. Thus, satiation feeding with commercial pellets is beneficial to the water balance of freshwater trout.