Metabolic Studies of an Amino Acid Imbalance in Cold-exposed Rats

Young rats fed an amino acid-imbalanced diet based on 6% fibrin, and supplemented with 0.4% DL-methionine and 0.6% DL-phenylalanine had increased activities of liver arginine synthetase and arginase, and increased plasma levels of a number of amino acids. However, the activities and levels of these metabolites were decreased in a comparable group of animals exposed to 7°C. The radioactivity of the expired CO2, originating from 2 µc each of intraperitoneally administered carbonlabeled methionine, phenylalanine or alanine, indicated that the rats kept at room temperature may not be able to oxidize effectively the 2 imbalancing amino acids to maintain a proper balance in the amino acid pool for the protein synthesis. In contrast, the cold-exposed animals were able to catabolize the excessive levels of methionine and phenylalanine and utilize them for energy purposes. The remaining balanced portion of the amino acid pool was then apparently used for the formation of new tissues, as also indicated by greater gains in body weight. In another experiment it was shown that protein-depleted rats exposed to 3°C for 27 hours readily consumed the imbalanced diet, whereas the food intake of a comparable group of rats kept at 25°C decreased sharply 18 hours after the animals had been given the diets.