REGIONAL RESPONSIVENESS OF THE TIBIA TO INTERMITTENT ADMINISTRATION OF PARATHYROID-HORMONE AS AFFECTED BY SKELETAL UNLOADING

To determine whether the acute inhibition of bone formation and defici t in bone mineral induced by skeletal unloading can be prevented, we s tudied the effects of intermittent parathyroid hormone (PTH) administr ation (8 mu g/100 g/day) on growing rats submitted to 8 days of skelet al unloading, Loss of weight bearing decreased periosteal bone formati on by 34 and 51% at the tibiofibular junction and tibial midshaft, res pectively, and reduced the normal gain in tibial mass by 35%, Treatmen t with PTH of normally loaded and unloaded animals increased mRNA for osteocalcin (+58 and +148%, respectively), cancellous bone volume in t he proximal tibia (+41 and +42%, respectively), and bone formation at the tibiofibular junction (+27 and +27%, respectively), Formation was also stimulated at the midshaft in unloaded (+47%, p < 0.05), but not loaded animals (-3%, NS), Although cancellous bone volume was preserve d in PTH-treated, unloaded animals, PTH did not restore periosteal bon e formation to normal nor prevent the deficit in overall tibial mass i nduced by unloading, We conclude that the effects of PTH on bone forma tion are region specific and load dependent, PTH can prevent the decre ase in cancellous bone volume and reduce the decrement in cortical bon e formation induced by loss of weight bearing.