| Summary: | This study considered a number of dietary factors that may alter the balance
and constancy of amino acid concentrations in the plasma of rainbow trout. Fish
given five equal feedings per day had relatively constant plasma amino acid
concentrations and deposited more protein and less lipid than did fish fed the same
ration at one feeding per day. The rate at which amino acids from the single meal
feeding entered the circulation was evidently in excess of the rate at which those
amino acids could be utilized for immediate protein synthesis with the result that a
higher proportion of them was catabolized and the carbon skeletons employed in
lipid synthesis. Supplementation of dietary protein with lysine and methiorune in
the free form resulted in more rapid appearance of these amino acids in the plasma
than occurred with the intact dietary protein alone. A surge of plasma arginine,
alanine, histidine, and lysine concentrations observed at 36 li after the fish were fed
diets containing a mixture of protein sources suggested delayed digestion of
particular proteins. Isonitrogenous substitution of free glycine for that supplied by
gelatin delayed the time at which plasma glycine peaked postprandially compared
with the response to the gelatin-containing diet. When a diet was supplemented
with a mixture of free essential amino acids, elevated concentrations remained in
the plasma and muscle pools as long as 26 h after feeding, indicating that dietary
supplements of free amino acids may remain available for protein synthesis even
when the fish are fed once daily. Plasma concentrations of free amino acids in fish
fed different concentrations of dietary lipid indicated that dietary lipid at 24% of the
diet had no effect of the lipid on digestion of protein or absorption of amino acids.
Postprandial concentrations of plasma amino acids in fish fed fish meal that had been subjected to heat treatment showed that the predominant nutritional effect of
protein denaturation was a reduction in availability of threonine and histidine. In
conclusion, the responses of plasma amino acid concentrations to different dietary
conditions observed in this study indicate that they provide a useful tool for
investigating the effects of various nutritional factors on protein metabolism in fish.
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