A comparison of the anabolic effects of rat and bovine parathyroid hormone (1-34) in ovariectomized rats

The current study was designed to compare the skeletal effects of comparable doses of rat parathyroid hormone 1-34 (rPTH) and bovine parathyroid hormone 1-34 (bPTH) in ovariectomized (OVX) rats. Female Sprague-Dawley rats were OVX or sham-operated at 6 months of age and maintained untreated for 28 days after surgery. Baseline control and OVX rats were sacrificed at the beginning of treatment. Beginning 28 days post-OVX, the remaining rats were subcutaneously injected daily with rPTH or bPTH at 0, 5, 25, or 50 microg/kg/d for 28 days. Bone area, bone mineral content (BMC), and bone mineral density (BMD) of the distal femoral metaphyses were determined ex vivo using dual energy X-ray absorptiometry. Quantitative bone histomorphometry was performed on undecalcified longitudinal sections of the proximal tibia from each rat. Baseline OVX rats exhibited osteopenia as demonstrated by their significantly reduced femoral BMD and proximal tibia cancellous bone volume compared with those of baseline sham controls. Both rPTH and bPTH restored bone in OVX rats by markedly stimulating bone formation in a dose-dependent manner. However, a difference in potency between the two forms of PTH was evident. The percentage increases of BMC, BMD, cancellous bone volume, trabecular thickness, mineralizing surface, and bone formation rate in the OVX rats treated with bPTH at 5 microg/kg/d were the same as or above those treated with rPTH at the 25 microg/kg/d dose level. A relative potency analysis showed that bPTH was approximately 4- to 6-fold relatively more potent than rPTH in increasing distal femoral BMD as well as cancellous bone volume, mineralizing surface, and bone formation rate of proximal tibial metaphyses at comparable dose levels and a given time. These results may serve as a reference for in vivo study design when rPTH or bPTH are to be the agents for studies on bone anabolism.