Human parathyroid hormone(1-34) increases bone formation and strength of cortical bone in aged rats.

The effect of parathyroid hormone (PTH(1-34)) on mid-diaphyseal femoral cortical bone was studied in 2-year-old male rats. The rats were treated with daily injections of 15 nmol/kg PTH(1-34) or vehicle for 56 days, and labelled with tetracycline and calcein on day 15 and day 40, respectively. The PTH(1-34) treatment did not affect the body weights or the lengths of the femora. Fluorescence microscopy showed large intracortical cavities in the old vehicle-treated rats. After PTH treatment, double labelling and new bone formation filling in these cavities were found. Furthermore, an increased bone formation rate was observed both at the periosteum and at the endosteum. This resulted in an increase in the cross-sectional area and a decrease in the medullary area. Three-point bending analysis revealed an increase in ultimate load, ultimate stiffness, energy absorption and ultimate stress after the PTH(1-34) treatment. No differences were found between the groups regarding the hydroxyproline concentration or apparent and real densities. The ash concentration was, however, slightly reduced after PTH(1-34) treatment. The PTH(1-34) treatment of old rats induced the formation of bone both from the periosteum and endosteum, with a pronounced filling in of intracortical cavities, and, furthermore, a marked increase in the biomechanical competence of the cortical bone.