Effects of growth hormone on skeletal muscles of aging systems

Growth Hormone (GH) is an anabolic hormone responsible for the somatic growth of young people. However, there is a progressive decline of the level of GH secretion with age where its level in old people is 20% of peak puberty level. Similarly, the level of Insulin-like Growth Factor I (IGF-I), the local mediator of GH action, is also reduced significantly with age. Several studies in the last decade have tried to use GH replacement therapy in attempts to alleviate some of the age-accelerated symptoms. GH administration to elderly people have improved nitrogen retention and reduced urinary excretion of phosphate, sodium and nitrogen. In elderly people over 60 y old, provision of GH for 6 mo improved lean body mass by 8.8% and decreased fat tissue by 14.4%. Other reports also claimed that GH had positive effect on increasing bone cortical strength in aging 24 mo old male rats. One of the main target tissues for GH is the skeletal muscle, especially in the process of differentiation of muscle cells to form postmitotic myotubes and myofibers. Studies on the effects of GH on skeletal muscles of aging systems have led to some mixed results. Some early studies have shown that GH administration to old rats can improve muscle mass and denervation associated with muscle atrophy. Other studies on GH-deficient adult humans also claimed that GH can considerably improve muscle volume in these patients. More recent studies in models of muscle immobilization of old animals have shown that GH had a very positive influence in reducing muscle damage associated with immobilization. However, studies on adult patients suffering from post polio syndrome of muscle weakness have shown no improvement by GH treatment. Similarly, studies on the effect of GH on muscle strength in elderly people subjected to resistance exercise demonstrated that training indeed increases muscle strength. But addition of GH to the regimen of elderly subjects did not further increase the effect of training alone. Thus, it appears that GH may have positive effect on aging and particularly in aging skeletal muscles; however, other studies could not corroborate this positive effect. More work is needed to ascertain the effect and mode of action of GH in aging animals and humans.