Intermittent administration of human parathyroid Hormone(1-34) prevents immobilization-related bone loss by regulating bone marrow capacity for bone cells in ddY mice.

ddY mice, 6 weeks of age, were neurectomized (Nx) in the right hindlimbs and sham-operated (Sham) in the left limbs for evaluation of the effects of intermittent injections of human parathyroid hormone (hPTH) on trabecular bone turnover and bone marrow cell development in unloaded and loaded limbs. Mice were given subcutaneous injections of hPTH(1-34) five times a week at a dose of 0 (vehicle), 4 (low dose), or 40 (high dose) microg/kg of body weight for 2, 4, or 6 weeks. Histomorphometric analyses of the trabecular bone of the proximal tibiae revealed that high-dose hPTH injections preserved the trabecular bone volume of the Nx limbs, which was reduced after neurectomy, at the same level as that of the contralateral Sham limbs. The mineral apposition rate in the Nx limbs was elevated to values above even that of the Sham limbs by high-dose hPTH injections. The bone formation rate reduced by neurectomy was maintained at the Sham level by low- and high-dose hPTH injections. The neurectomy-induced increase in osteoclast number was suppressed by high-dose hPTH injections. In the bone marrow cells, the numbers of nonadherent and adherent cells per tibia obtained from the Nx and Sham limbs did not change. The hPTH injections decreased the numbers of nonadherent cells and increased those of adherent cells in both the Nx and the Sham limbs, but the effects were less marked in the Nx than in the Sham limbs even at high-dose injections. The formation of osteogenic nodules in the marrow cultures obtained from the Nx limbs was decreased after surgery and was maintained at the level of the Sham limbs by high-dose hPTH injections. The number of osteoclast-like multinucleated cells was reduced in the Sham limbs by high-dose hPTH injections. The value was increased at 2 weeks after neurectomy, but it was maintained at the Sham level by high-dose hPTH injections through the experimental period. The numbers of colony forming units-fibroblastic, which were reduced by neurectomy, and those of colony forming units for granulocytes and macrophages were not altered by hPTH injections. These results demonstrate that intermittent high-dose hPTH administration in the Nx limbs as well as in the contralateral Sham limbs has similar anabolic effects, stimulating osteoblast cell lineage and suppressing osteoclast cell lineage. The anabolic effects at 4 microg were reduced, but the effects at 40 microg seemed to be less affected by unloading due to sciatic neurectomy.