The effect of exercise on systemic and bone concentrations of growth factors in rats

Exercise can prevent bone loss and increase bone density. Growth factors such as insulin-like growth factor-I (IGF-I) and transforming growth factor beta (TGFbeta) are thought to be involved in the local response to mechanical loading, resulting in bone remodelling. We tested the effect of additional weight bearing during exercise on the systemic response of IGF-I and local bone response of IGF-I and TGFbeta. Thirty-four female Wistar rats (aged 3 months, weight 226.9 +/- 20.2 g) were randomly divided in four groups: group 1 baseline controls; group 2 sedentary controls; group 3 ran 15 min a day on a motor-driven exercise belt; group 4 ran 15 min a day with a backpack containing 40 g. The animals ran 5 days a week, for 6 weeks, with an average velocity of 16.6 +/- 4.4 m/min, and a slope of 5 degrees uphill. The serum growth hormone (GH) concentration was significantly higher in the running rats (group 3, P = 0.009) than in the sedentary controls (group 2). The IGF-I and IGF binding protein 3 (IGFBP3) levels in serum and the IGF-I levels in liver were similar in all groups. In the tibia no significant differences were observed in IGF-I, IGFBP3 and TGFbeta concentration. In the humerus, the IGF-I concentration was lower in the running rats (group 3) than in the sedentary controls (P = 0.04), but it was higher in the rats that ran with additional weight than in those without (P = 0.009). The TGFbeta concentration in the humerus was lower in both group 3 (P = 0.001) and 4 (P = 0.03) than in the sedentary controls. The effects in bone caused by mechanical stimulation cannot be explained by changes in serum IGF-I and IGF-I produced in the liver. The concentrations of IGF-I and TGFbeta in bone appeared to be modulated by running exercise.