Nutritional and hormonal control of muscle and peripheral tissue metabolism in farm species

Alterations in the rate of protein turnover in muscle have marked effects on production efficiency, environmental pollution and perhaps also meat quality. Nutrient and hormonal responses produce differential changes in protein synthesis and degradation; these determine the quality of the end product obtained. Breed selection appears, from limited data, to "protect" the selected characteristic (e.g., muscle gain, wool growth) against nutritional deprivation through maintenance of protein synthesis rates. At intakes below maintenance, the anti-catabolic effects of nutrients appear to operate mainly through inhibition of protein breakdown. In all but the very young animals, this probably involves the anti-proteolytic action of insulin. In contrast, general increases in either protein or total nutrient supply stimulate both protein synthesis and degradation in muscle at intakes above maintenance. These responses are similar to the chronic mechanisms of action of growth hormone (GH) while the majority of acute events reported for insulin-like growth factor-1 (IGF-1) appear to involve primarily changes in protein synthesis. Current data favours ICF-1 action on muscle via an endocrine rather than autocrine (or paracrine) mechanism. Increased protein or energy supply alters the efficiency with which protein is synthesised per unit of cellular RNA, while the GH response involves increases in total RNA but no change in synthetic efficiency. A biphasic model involving insulin and the GH/IGF-1 axis in regulation of nutrient responses is proposed. Muscle sensitivity and responsiveness to insulin, GH and IGF-1 may be regulated by the availability of both total and specific amino acids. Insulin action is specifically influenced by the branch chain amino acids, notably leucine, which may act as signals of nutrient availability to peripheral tissues. The role of individual amino acids in GH and IGF-1 responsiveness is not yet known. Other amino acids, notably those transported into cells by Na(+)-dependent systems, e.g., glutamine, may also directly effect protein turnover, probably by mechanisms involving changes in cellular hydration. (C) 1998 Elsevier Science B.V. All rights reserved.