COMPARISON OF THE EFFECTS OF PROGESTERONE AND ESTROGEN ON ESTABLISHED BONE LOSS IN OVARIECTOMIZED AGED RATS

The effects of estrogen and progesterone therapy on established bone loss were assessed in aged, ovariectomized (OVX) rats. One year old Sprague Dawley rats were ovariectomized and followed without treatment for 20 weeks to induce bone loss. At this point, hormone replacement with either 17-beta-estradiol (E2) or progesterone (PROG) was started and continued for 20 weeks. A significant decrease in femur, tibia and fourth lumbar vertebra ash weight and calcium content was observed at 20 weeks as well as at 40 weeks post-OVX as compared to corresponding sham-operated controls. Treatment with either E2 or PROG resulted in an increase in the bone mineral in the vertebra and tibia of the OVX rats. In the femur, bone mineral and strength was increased significantly in OVX progesterone treated, but not in OVX estrogen treated animals as compared to OVX rats, suggesting that cortical bone may be more responsive to progesterone. One group of sham-operated rats was treated with progesterone. There were no changes in either bone mineral or strength in this group as compared to sham-operated untreated rats. These studies demonstrate that both E2 and progesterone treatment has beneficial effects on bone in OVX aged rats, even when started after osteopenia has occurred. Human studies to evaluate the effects of progesterone in established postmenopausal osteoporosis are indicated.