Abstract
Growth of skeletal muscle is dependent upon a properly functioning endocrine system with adequate nutrition. With undernutrition, either energy or protein, muscle fails to develop. In the young animal, there is a loss of RNA, reduced nuclear division and a normal ratio of protein per nucleus. With severe malnutrition, RNA loss is extensive, nuclear division stops and muscle loses protein so the ratio of protein per nucleus is decreased. Ribosomes isolated from these muscles are reduced in number and are less active in synthesizing protein. A deficiency of insulin or growth hormone results in similar changes in growing muscle. The defect in the intracellular mechanisms involved in protein synthesis seems to be fewer active ribosomes during insulin deficiency and fewer as well as less active ribosomes during growth hormone deficiency. Both growth hormone and insulin stimulate the uptake of amino acids by muscle cells, so the increased level of insulin after meals of carbohydrates and protein would favor more efficient utilization of the absorbed amino acids in muscle by reducing plasma amino acid levels and their oxidation by the liver. Growth hormone also seems to be necessary for DNA synthesis in skeletal muscle. The predominant effect of growth hormone on muscle size may be its influence on the number of nuclei per muscle fiber. Since the minimum level of plasma growth hormone or insulin necessary for maintaining synthesis of macromolecules in muscle is not known, the physiological implications of the differences in plasma hormone levels during various nutritional states remain to be more completely resolved, but the findings do suggest that some of the effects of diet on muscle growth may be mediated by alteration in the activity of the pancreas or pituitary. Copyright © 1974. American Society of Animal Science. Copyright 1974 by American Society of Animal Science.