PTH pre-treatment prior to tibial mechanical loading improves their synergistic anabolic effects in mice

Parathyroid hormone (PTH) increases bone mass and decreases fracture risk. However, the anabolic effects of PTH are limited to a period of approximately 24 months, motivating the need to maximize bone growth during this timeframe. Concurrent mechanical loading with weight-bearing exercise is synergistic with PTH treatment. We sought to determine if priming with PTH prior to initiating mechanical loading would enhance their synergistic effects. We pre-treated 10-week-old, female C57Bl/6J mice with either PTH or saline vehicle (VEH) for six weeks. We subsequently initiated cyclic tibial compression for either two or six weeks while continuing PTH or VEH treatment. We analyzed bone morphology in cortical and cancellous compartments of the proximal tibia. To further explore the effects of PTH and loading in cancellous bone, we measured bone cell presence and changes in bone morphology via histology, immunohistochemistry, and dynamic histomorphometry. Concurrent treatment with PTH enhanced load-induced increases in bone mass in cortical bone but blunted the effects of loading in cancellous bone. PTH pre-treatment further increased load-induced changes in cortical bone mass and rescued the load effects in cancellous bone, returning values to those of VEH-treated animals. Osteoclast populations decreased with loading, independent of PTH treatment. Active osteoblast populations increased with PTH pre-treatment but did not change with loading. Bone formation rate increased with PTH pre-treatment in the 2-week group but did not differ between treatment groups after 6-weeks. Collectively, pre-treating with PTH prior to mechanical loading primed the skeletal tissue and enhanced the anabolic response of concurrent treatment and loading.