Effects of tiludronate on bone mass, structure, and turnover at the epiphyseal, primary, and secondary spongiosa in the proximal tibia of growing rats after sciatic neurectomy

Abstract

To evaluate the effects of tiludronate on the mass, structure, and turnover of cancellous bone regions in immobilized rat tibiae, we performed a 4 week dosing experiment. The right hindlimbs of 84 Sprague-Dawley rats (5 weeks old) were neurectomized or sham operated. Animals were assigned to seven groups (n = 12 each); group 1 was sham operated, and groups 2-7 were neurectomized. Groups 1 and 2 were given vehicle only (distilled water), and groups 3, 4, and 5 were given tiludronate orally at doses of 25, 50, and 100 mg/kg body weight (BW)/day, respectively, throughout the experimental period. Group 6 was given 100 mg/kg BW/day of the agent for the first 2 weeks only, and group 7 received vehicle only for the first 2 weeks and then 100 mg/kg BW/day of the agent for the last 2 weeks. After tetracycline labeling was performed, the right tibiae were removed from the animals and processed to yield undecalcified sections. Histomorphometry was performed in the epiphyseal, primary, and secondary spongiosa of the proximal tibia. In group 2, trabecular bone volume (BV/TV) and trabecular number (Tb.N) were significantly decreased in the primary and secondary spongiosae, but this did not occur in the epiphyseal spongiosa. Osteoid surface (OS/BS) was decreased and osteoclast surface (Oc.S/BS) was increased in the secondary spongiosa. Tiludronate increased BV/TV and Tb.N in the primary spongiosa by reducing the values for the parameters of osteoclast surface (Oc.S/BS) and osteoclast number (Oc.N/BS). Osteoid surface in this region was not decreased by the agent. In groups 4 and 5, tiludronate prevented bone loss in the secondary spongiosa by reducing both OS/BS and Oc.S/BS. In group 6, BV/TV in the primary spongiosa was maintained at the level of group 1, but Oc.S/BS and Oc.N/BS were elevated. In the secondary spongiosa, bone mass was preserved and the reduction in these parameters was maintained. In group 7, however, BV/TV was increased in the primary spongiosa as a result of a reduction in osteoclastic resorption; in the secondary spongiosa, however, BV/TV was decreased and trabecular turnover was not reduced at the end of the experiment in these growing animals. Mineral apposition rates were not reduced by tiludronate. This study clearly demonstrated that this agent prevented immobilization bone loss by inhibiting resorption. The histomorphometric data suggest that (1) tiludronate begins to act early after administration in the primary spongiosa and rather late in the secondary spongiosa, and (2) the bone matrix is involved in the antiresorbing action of this bisphosphonate.