Effects of growth hormone on muscle regeneration and IGF‐I concentration in old rats

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Abstract
A study was made of regeneration of skeletal muscle in young adult and old rats and of the effects of administration of growth hormone to old rats. Regeneration was achieved by subjecting the right extensor digitorum longus muscle to ischaemic necrosis. The level of insulin‐like growth factor I in serum was markedly decreased in old rats of 27 months of age compared with young adult rats aged 6 months. Daily injections of 4 IE recombinant human growth hormone during the 10 weeks of regeneration increased the level of insulin like growth factor I to almost the same levels as in rats aged 6 months. The body weight increased by 13% in the old rats treated with recombinant human growth hormone, while the weight of the untreated old rats was constant. The regenerating right and the normal left extensor digitorum longus muscles were studied 10 weeks after the ischaemic necrosis. The absolute and relative weights of regenerating muscle were decreased in 27‐month‐old rats compared with the rats aged 6 months. No significant difference between normal old rats and old rats treated with recombinant human growth hormone was observed either in muscle weight or in maximum contraction force. The results show that old rats have reduced levels of circulating insulin‐like growth factor I. Administration of recombinant human growth hormone raises the level of insulin‐like growth factor I to that of young adult rats, and causes an increased body weight. Muscle regeneration after ischaemic necrosis is impaired in old rats apparently by the formation of large amounts of connective tissue. Supplementation with growth hormone does not reverse this impairment.