PARATHYROID-HORMONE-(1-34) INCREASES VERTEBRAL BONE MASS, COMPRESSIVESTRENGTH, AND QUALITY IN OLD RATS

Human parathyroid hormone 1-34 (PTH) exerts an anabolic effect on bone in younger rats, The aim of the present study was to examine the effe ct of PTH on vertebral bone in 2-year-old male rats, The rats were tre ated with daily injections of 15 nmol/kg PTH or vehicle (V) for 56 day s, Tetracycline and calcein were injected on day 15 and day 40 of the treatment period, respectively, The PTH treatment did not influence th e body weights of the rats, the volumes of whole vertebra, or the vert ebral body heights, However, the PTH treatment induced profound change s in the bone structure, Histomorphometric analyses of the vertebral b odies (L-6) revealed an approximate doubling of the cancellous bone vo lume after PTH treatment from 24.6 +/- 1.3% to 54.9 +/- 2.0% (p < 0.00 1) as well as a doubling of the trabecular thickness while the bone su rface/bone volume decreased by 60%. PTH treatment also increased bone formation as indicated by an increase in mineral apposition rate (from 0.42 +/- 0.01 to 0.89 +/- 0.01 mu m/day, p < 0.01), increased mineral izing surface (from 7.8 +/- 1.4 to 43.8 +/- 1.9%, p < 0.01) and an inc rease in both volume-related and surface-related bone formation rates (5 and 11 times, respectively), The biomechanical properties were anal yzed using standardized bone specimens from the vertebral bodies of L- 4 by applying cranial-caudal compression in a materials testing machin e, The PTH treatment induced a substantial increase in the strength of the vertebral body: ultimate load increased by 66%, ultimate stiffnes s by 47%, and energy absorption by 98%. The increase in vertebral body strength was also evident after normalizing the parameters to the cro ss sectional area and the ash content of the vertebral body specimens, PTH treatment increased ultimate stress from 26 +/- 3 to 44 +/- 3 N p er mm(2) (p < 0.01) and increased ultimate load normalized to ash cont ent per mm specimen height from 59 +/- 4 to 72 +/- 4 N (mm/mg) (p < 0. 05), The PTH treatment induced an increase in dry defatted bone densit y and ash density of both the vertebral body specimen (L-4) and the wh ole vertebra (L-5). In conclusion, PTH showed a remarkable ability to stimulate bone formation in the vertebral body of old rats, Furthermor e, the biomechanical analysis revealed an enhanced compressive bone st rength, even after correction for the increased bone mass, indicating an improved bone quality after the PTH treatment.