Calcitonin Provides Complete Protection Against Cancellous Bone Loss in the Femoral Neck of Ovariectomized Rats

Calcitonin (CT) has been found to partially prevent cancellous bone loss in the proximal tibia of ovariectomized (OVX) rats. The current study was designed to determine whether CT has similar bone protective effects in the femoral neck, a skeletal site with a slower rate of bone loss after ovariectomy than the proximal tibia. Female Sprague Dawley rats were sham-operated or ovariectomized at 3 months of age. Groups of OVX rats were injected s.c. with vehicle or CT at a dose of 16 U/kg body weight on alternate days for 30, 60, or 90 days. Sham-operated control rats were treated with vehicle alone on alternate days. The proximal femur from each rat was processed undecalcified for quantitative bone histomorphometry. Cancellous bone volume in the femoral neck of vehicle-treated OVX rats was significantly less than that of vehicle-treated control rats at all time points. This cancellous osteopenia induced by ovariectomy was associated with increased indices of bone turnover such as osteoclast surface, osteoblast surface, and bone formation rate (tissue level, total surface referent). In contrast, cancellous bone volume in the femoral neck of CT-treated OVX rats was nearly identical of that of vehicle-treated control rats throughout the study. In addition, CT treatment of OVX rats decreased all indices of bone turnover to near the level of vehicle-treated control rats. The results indicate that CT treatment depresses bone turnover and provides complete protection against moderate cancellous osteopenia in the femoral neck of OVX rats. Since previous studies have shown that CT only partially protects against more pronounced cancellous bone loss in the proximal tibia of OVX rats, our findings suggest that CT has a greater bone protective effect at a skeletal site with a slower rate of cancellous bone loss (femoral neck) than at a skeletal site with a rapid rate of cancellous bone loss (proximal tibia).