Effects of estrogen on the concentration of insulin-like growth factor-I in rat bone matrix

Insulin-like growth factor-I (IGF-I) plays an important role in bone metabolism, but data on the regulation of IGF-I in bone tissue in vivo are still limited. In the present study, we examined the effects of ovariectomy (ovx) and estrogen replacement on the skeletal concentration of IGF-I in the femur shaft of 6-10 week-old female rats. Ovx had no consistent effect on bone matrix IGF-I concentration regardless of animal age at ovx. In contrast, administration of estradiol in doses that exceeded physiological replacement (50 and 150 nmol/kg per day, subcutaneously) significantly increased the bone matrix IGF-I concentration. These are the first in vivo data which demonstrate that estrogens are capable of increasing the concentration of IGF-I in bone tissue. However, this stimulatory effect appears to be limited to supraphysiological estrogen concentrations.