Plasma concentrations of amino acids in rainbow trout (Oncorhynchus mykiss) in response to nutritional and feeding management

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.Land and Food Systems, Faculty ofGraduat