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 th e 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 Fac tor I (IGF-I), the local mediator of GH action, is also reduced signif icantly with age. Several studies in the last decade have tried to use GH replacement therapy in attempts to alleviate some of the age-accel erated symptoms. GH administration to elderly people have improved nit rogen retention and reduced urinary excretion of phosphate, sodium and nitrogen. In elderly people over 60 y old, provision of GH for 6 mo i mproved lean body mass by 8.8% and decreased fat tissue by 14.4%. Othe r 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 d ifferentiation of muscle cells to form postmitotic myotubes and myofib ers. 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 as sociated with muscle atrophy. Other studies on GH-deficient adult huma ns also claimed that GH can considerably improve muscle volume in thes e patients. More recent studies in models of muscle immobilization of old animals have shown that GH had a very positive influence in reduci ng muscle damage associated with immobilization. However, studies on a dult patients suffering from post polio syndrome of muscle weakness ha ve shown no improvement by GH treatment. Similarly, studies on the eff ect 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 h ave positive effect on aging and particularly in aging skeletal muscle s; however, other studies could not corroborate this positive effect. More work is needed to ascertain the effect and mode of action of GH i n aging animals and humans.