A comparison of the skeletal effects of intermittent and continuous administration of calcitonin in ovariectomized rats.

The study was designed to compare the skeletal effects of intermittent and continuous administration of calcitonin (CT) in ovariectomized (OVX) rats. Female rats were sham operated or OVX at 3 months of age and treated for 6 weeks with vehicle or salmon CT. Sham-operated control rats were injected subcutaneously with vehicle on alternate days. One group of OVX rats was treated with vehicle intermittently by subcutaneous injection or continuously via Alzet osmotic minipumps. The remaining OVX rats were treated with CT by either subcutaneous injections (16 U/kg) on alternate days or by continuous infusion via minipumps at a daily dose of 8 U/kg. OVX rats treated with CT continuously were mildly hypocalcemic compared with all other groups. The proximal tibial metaphyses of vehicle-treated OVX rats were osteopenic with a cancellous bone volume at only 28% of the vehicle-treated control level. This bone loss was associated with increased indices of bone turnover such as osteoclast surface, osteoblast surface, and bone formation rate. Cancellous bone volume in OVX rats treated with CT either intermittently or continuously was significantly higher than that of vehicle-treated OVX rats, but lower than that of vehicle-treated control rats. Treatment of OVX rats with intermittent or continuous CT significantly decreased all indices of bone turnover compared with vehicle-treated OVX rats. However, osteoclast and osteoblast surfaces of OVX rats treated with CT continuously were still significantly higher than those of vehicle-treated control rats. These results indicate that intermittent and continuous administration of CT had similar skeletal effects in OVX rats. Both treatment regimens depressed bone turnover and partially prevented cancellous bone loss in the estrogen-deplete skeleton.