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Hormonal and Nutritional Interrelationships and their Effects on Skeletal Muscle^{1, 2,}

Iowa State University³, Ames 50010

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.

¹ Journal Paper No. J-7640 of the Iowa Agriculture and Home Economics Experiment Station, Ames. Project No. 1790. The studies conducted in this report were supported in part by the Iowa State University Research Foundation.

² Invitational paper presented at the Symposium on Protein Synthesis and Muscle Growth held during the 65th Annual Meeting of the American Society of Animal Science, Lincoln, Nebraska, July 28 to August 1, 1973.

³ Department of Animal Science.