Variation in the Short-Term Changes in Bone Cell Activity in Three Regions of the Distal Femur Immediately Following Ovariectomy

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Abstract
The effect of ovariectomy (OVX) on cancellous bone in the rat is not uniform at all sites of the skeleton. We report variation in the short-term effects of adult OVX in three regions of the distal femur: the diaphysis (DIA), the metaphysis (META), and the epiphysis (EPI). Cancellous bone parameters were estimated in the three separate zones of the femora and compared with changes in bone cell activity, as estimated by osteoclast surface (Oc.S) and bone formation rate (BFR). Changes were studied for 30 days in a series of rats either sham-operated (Sham) or ovariectomized (OVX) at 7 months of age. Oc.S and BFR were elevated following OVX in all regions. The time course for the OVX-induced changes differed between regions: DIA, both Oc.S and BFR were elevated at day 9; META, Oc.S was also elevated at day 9, while the rise in BFR was delayed until day 21; EPI, Oc.S remained stable but increased relative to ovary-intact rats by day 18 due to reduced levels in the latter, but BFR did not rise until day 28. These changes in bone cell activity following OVX produced a 71% reduction of cancellous bone in the DIA and a 35% reduction in the META. In contrast, no OVX-induced bone loss was observed in the EPI. This study shows that bone cell activity increases in each region of the distal femur within the first 30 days following OVX, independent of bone loss. However, the time course of increased bone cell activity is not uniform. These data highlight the role of local factors in the response to ovarian hormone deficiency.