EFFECT OF GROWTH-HORMONE ADMINISTRATION AND TREADMILL EXERCISE ON SERUM AND SKELETAL IGF-I IN RATS

Growth factors may be mediators of local and systemic factors that enh ance bone formation. This study examined the effect of treadmill exerc ise and ovine growth hormone administration on levels of insulin-like growth factor I (IGF-I) in serum (ng/ml), long bone, and vertebrae and on bone formation rate. Forty female rats were divided into four grou ps: control; exercise (17 m/min, 1 h/day); growth hormone (0.05 mg.100 g(-1) day(-1)); growth hormone plus exercise. After 9 wk of study, th e serum levels of IGF-I were higher in the intervention groups than in the control group; however, the IGF-T concentration and the periostea l bone formation rate in the long bone were significantly higher only in the exercised rats. The IGF-I concentration and the cancellous bone formation rate in the vertebrae did not differ among the experimental groups. The vertebral and long bone formation rate were correlated wi th bone concentrations of IGF-I. Serum levels of IGF-I were also corre lated with serum osteocalcin and the long bone formation but not with the vertebral bone formation. The association of bone formation with s erum and bone TGF-I supports the suggestion that TGF-I is one of the g rowth factors that regulate bone formation, in particular as a mediato r of the response of bone to exercise.