Discordance between bone turnover and bone loss: Effects of aging and ovariectomy in the rat

Mechanical strain maintains bone architecture even under conditions of incr eased bone turnover such as occurs with ovarian hormone deficiency. The rat distal femur contains two sites that apparently experience different level s of mechanical strain and therefore the rat is a suitable model for invest igating such effects. The femoral epiphysis experiences higher strain energ y compared with the metaphysis and we report the effects of aging between 7 and 12 months and the postovariectomy effects over the same time period on cancellous bone variables measured at these two sites, Age-related bone lo ss in sham-operated (Sham) animals occurred in both regions, with a greater fall in the metaphysis than in the epiphysis (trabecular bone volume [BV/T V, %] Mean [SEM] Metaphysis: day 0, 25.9 [2.4]; day 150, 8.8 [1.3]: Epiphys is: day 0, 44.8 [1.7]; day 150, 36.7 [1.4] [p < 0.0001]). With ovariectomy (OVX) there was a 73% reduction in cancellous bone at the metaphysis compar ed with no specific loss at the epiphysis (BV/TV [%] OVX: Metaphysis: day 1 50, 2.4 [0.4] [p < 0.01 compared with Sham]: Epiphysis: day 150 29.3 [2.7] [NS]), Osteoblast cell activity and osteoclast surface were increased after ovariectomy in both regions. The mineral apposition rate decreased at 9.5 months of age in both regions (p < 0.0001), independent of ovariectomy, and was coincident with a reduction in trabecular number in the epiphyses of b oth operative groups and in the metaphysis of the ovary-intact group, These data suggest that local mechanical strain governs bone balance with aging and that architectural changes resulting from age-related bone loss may mir ror those following estrogen deficiency but occur via a different cellular mechanism.