Tissue Distribution of α-Aminoisobutyric Acid and Nitrogen Metabolism in the Rat

Exposure to cold (7°C) resulted in rats increasing their food intake, independently of the nature of their diets. In spite of this, the nitrogen retention of these animals was lower than that of their respective controls maintained at room temperature (24°). The addition of toxic levels of amino acids to the diet of animals maintained at 24° greatly decreased the ratio of weight gain per gram of retained nitrogen. On the other hand, animals fed similar diets but exposed to the low environmental temperature were able to gain more weight in terms of the retained nitrogen. Since this increase in weight cannot be attributed to an increase in net protein synthesis over that of their controls, accumulation of other products such as lipids or water may be responsible. Exposure to cold, as well as the supplementation of the diets with toxic amounts of tryptophan or an isoleucine-free amino acid mixture, stimulated the urinary excretion of injected α-aminoisobutyric acid, with more rapid depletion of this unmetabolizable amino acid from the tissues.