Growth hormone administration potentiates the effect of treadmill exercise on long bone formation but not on the vertebrae in middle-aged rats.

To determine whether growth hormone administration would potentiate bone response to the stimulation of exercise, 80 female rats aged 14 months were divided into control (CON), ovine growth hormone administration (0.5 mg/kg daily) (GH), treadmill exercise (17 m/minute, 60 minutes daily) (EX), and GH+EX groups for 9 and 16 weeks. Static and dynamic histomorphometry were measured on the tibial shaft and (L-5) vertebral cortical bone. The periosteal and endocortical bone formation rate of the tibial shaft were higher in both EX and GH+EX than in the CON group in the 9-week study. There is a synergistic interaction between the two interventions in both cortical surfaces. After 16 weeks of study, the cortical bone area and periosteal bone formation rate were higher only in the EX than in the CON group. In the L-5 vertebra, the labeled surface on the periosteum was higher in the EX and the bone formation rate on the endocortical surface was higher in the GH than in the CON group. However, there was a negative interaction when the two interventions were combined. We conclude that a low-dose of growth hormone administration could initially potentiate long bone response to exercise. However, from the present study, long-term treatment with low-dose growth hormone administration does not enhance the increase in bone mass from exercise.