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Previous reports showed that bone mass and architecture only partially recovered by remobilization (RM) after immobilization (IM)-induced osteopenia, and that parathyroid hormone (PTH) had an anabolic effect on the skeleton. The aim of this study was to determine whether low doses of PTH could restore IM-induced cortical bone loss and whether a combination of PTH plus loading (RM) treatment would be more effective than the PTH in unloaded (IM) limbs. One hundred and sixty 6-month-old rats were divided into aging and IM groups. The right hindlimb of the rat was immobilized by elastic bandage for 18 weeks, and then groups of rats were either kept IM or RM and treated with 30 µg or 80 µg of hPTH(1-38)/kg/day for 2, 10, and 20 weeks. Fluorescent-labeled, undecalcified cross-sections of right tibial shafts were studied. We found that RM for 20 weeks after 18 weeks of IM only partially recovered IM-induced muscle weight loss and PTH had no effect on muscle weight in either IM or RM limbs; that RM for 20 weeks after 18 weeks of IM partially restored some minimal cortical width by stimulating periosteal and endocortical bone formation and decreasing endocortical resorption; that PTH treatment of IM limbs completely restored IM-induced cortical bone loss and added extra bone by stimulating bone formation indices on all bone surfaces and depressing bone resorption on endocortical surface; that PTH treatment of RM limbs produced similar anabolic effects as in IM limbs with 30 µg/kg/day dose but the 80 µg/kg/day dose-treated limbs had a higher periosteal bone formation rate, which created a larger cross-sectional area, more cortical bone area, and a thicker cortex than the same dose treated IM limbs; and that PTH 80 µg/kg/day treatment produced more anabolic effect than the 30 µg/kg/day in both IM and RM limbs. We concluded that reloading the hindlimb by RM after long-term IM could not recover the cortical bone mass. PTH at employed doses was able to completely restore IM-induced cortical bone loss, and this effect was independent of mechanical stimulation. However, when PTH was combined with mechanical loading (RM), a synergistic anabolic effect on periosteal bone formation occurred which increased the cross sectional area that can increase bone strength. (J Bone Miner Res 1999;14:439-448)

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