Anabolic action of parathyroid hormone on cortical and cancellous bone differs between axial and appendicular skeletal sites in mice

The mouse is being increasingly used to study the anabolic action of parathyroid hormone (PTH) on the skeleton. The efficacy of intermittent PTH treatment on bone varies widely among tested strains of mice with differences in peak bone mass and structure. We have therefore examined the responses of skeletal sites with high or low cancellous bone mass to PTH treatment in a single strain with genetically low bone mass. Mature C57BL/6 mice were ovariectomized (ovx) or sham operated and, after 4 weeks, treated with PTH(1-34) (40 microg/kg/day, 5 days/week sc) or vehicle for 3 or 7 weeks. Two doses of fluorescent labels were given to the animals 9 and 3 days before euthanasia. Histomorphometry was performed on sections of the proximal tibia, tibial diaphysis, and vertebral body. The results indicate that 4 to 11 weeks of ovx induced a approximately 44% loss of cancellous bone in the proximal tibia and a approximately 25% loss of cancellous bone in the vertebra with impaired trabecular architecture and high bone turnover. In the intact animals, PTH increased cancellous bone volume to a greater extent in the vertebral body than in the proximal tibia, a site with lower cancellous bone volume at the outset. In the ovx mice, PTH increased cancellous bone volume to a greater extent in the vertebral body, a site displaying moderate cancellous bone loss, than in the proximal tibia, a site with severe cancellous bone loss. Conversely, the treatment added a little cortical bone to the tibia, a highly loaded site, but did not significantly increase cortical width of the vertebral body, a less loaded site. We conclude that, for intermittent PTH treatment to be maximally effective, there must be an adequate number of trabeculae present at the beginning of treatment, regardless of estrogen status. Our results also support an interaction between PTH anabolic action and mechanical loading.