1. We have investigated the relations between changes in plasma insulin and 3,5,3'-triiodothyronine (T,), and muscle growth and protein turnover in the rat in response to diets of varying protein concentrations.2. Young rats were fed ad lib. on a control (180 g casein/kg) diet or low-protein diets containing 80, 45 and 0 g casein/kg in four separate experiments. Measurements were made of food intakes, muscle and body-weight growth rates, muscle protein turnover in vivo, plasma insulin, and plasma free and total T,.3. The food intakes of the 80 and 45 g casein/kg diet groups were variable, with the 80 g casein/kg diet group consuming either the same or more than the controls, and the 45 g casein/kg diet group consuming less or more than the controls. Body-weight and skeletal-muscle growth rates varied with the protein but not energy intakes, which in turn reflected both dietary composition and the food intake, with the hyperphagic 80 g casein/kg diet group of rats growing almost normally and the 0 g caseinlkg diet group losing body-weight and muscle mass. 4. Changes in rates of muscle growth were accompanied by parallel changes in rates of protein synthesis and degradation, as well as parallel changes in concentrations of plasma insulin and free T,, to the extent that all these variables were highly correlated with each other.5. Partial correlation analysis was used to separate interactions between variables. This indicated that dietary energy had no identifiable influence on muscle growth. In contrast dietary protein appeared to stimulate muscle growth directly by increasing muscle RNA content and inhibiting proteolysis, as well as increasing insulin and free T, levels. Insulin and free T, stimulated each other as well as muscle protein turnover; insulin stimulating the RNA activity particularly at low insulin levels, free T, stimulating the RNA content and both hormones stimulating proteolysis.6 . These apparent relations are shown to be consistent in the main part with previous studies of the mechanism of action of insulin and T,, but the possibility cannot be discounted that other anabolic hormones not measured in these studies are involved, particularly in the apparent direct influence of dietary protein on muscle.

Two pigs of 35 kg live weight were fitted with re-entrant duodenal cannulas anterior to the pancreatic duct and fed barley-soya or casein-wheat starch diets prior to the experiment. After 24 hours fasting they were given a single meal of 15N-labelled free amino acids-wheat starch (A) or 15N-labelled wheat (B) diets and digesta leaving the stomach was collected during 12 hours. The proportion of TCA soluble N in total N of digesta increased with time from 10 to 40% with diet B and decreased from 90 to 47% with diet A. Total N leaving the stomach within 12 hours accounted for 96% and 106% of N ingested on diet A and B, while the amount of 15N accounted for only 66 and 86% of 15N given in diets A and B, respectively. The content of endogenous N in the digesta was 1.22 g/12 h after feeding diet A and 1.67 g/12 h after feeding diet B. It was concluded that considerable amounts of N are secreted and absorbed in the part of the digestive tract proximal to the opening of the pancreatic duct.