The role of insulin and thyroid hormones in the regulation of muscle growth and protein turnover in response to dietary protein in the rat

Abstract

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.

The anabolic actions of growth hormone and thyroxine on protein metabolism in Snell dwarf and normal mice

“…These responses were similar to those following hypophysectomy, which caused cessa¬ tion of growth and decreased muscle Ks (Flaim, Li & Jefferson, 1978;Brown, Bates, Holliday & Millward, 1981). However, hypophysectomy is a rather drastic insult causing a lack of other hormones including insulin which is known to affect skeletal muscle KRNA directly (Jefferson, 1980;Jepson, Bates & Millward, 1988). The unchanged Ks in dwarf mouse heart is con¬ sistent with the finding that, after an initial period of atrophy, cardiac Ks is the same in longer term (greater than 24 days) hypophysectomized rats as in normal rats (Hjalmarson, Runnels, Kao & Morgan, 1975) and that heart RNA levels were then unaltered.…”

Effect of average daily gain on body size, metabolism, and milk production of Italian Holstein heifers raised on two different planes of nutrition and calving at two different ages

“…Thyroid hormone administration increased serum copper levels in humans as reviewed by Cousins (1985). Thyroid hormones are influenced by energy status (Bertoni et al, 1983), dietary protein (Jepson et al, 1988), and regulate metabolic activity. These findings lead to the hypothesis that the metabolic activity to support the growth metabolism influenced the plasma Cp.…”

Translational Control of Protein Synthesis in Muscle and Liver of Growth Hormone-Treated Pigs

“…Chronic alterations in protein synthesis are thought to be a result of an increase in the number of ribosomes (14,15), whereas acute changes in protein synthesis are thought to be due to changes in translation initiation (12,13). In the current study, the feeding-induced stimulation of protein synthesis in both skeletal muscle and liver was driven by an increase in translational efficiency, i.e.…”

“…Acute alterations in protein synthesis are driven primarily by changes in translation initiation (12, 13), whereas long-term alterations result from increases in ribosome number (14,15). Two key processes can regulate acute changes in translation initiation.…”