Additive effect of treadmill exercise and 17β-estradiol replacement on prevention of tibial bone loss in adult ovariectomized rat

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Abstract
The effects of 17β-estradiol (E2) and treadmill exercise on tibial bone mass, tibial uptake of 45Ca, and proximal tibia osteoblast and osteoclast cell number were determined in adult ovariectomized rats. Female rats aged 10 months were ovariectomized and divided into five groups: (1) sham-operated; (2) ovariectomized; (3) ovariectomized, given 10 μg E2 biweekly; (4) ovariectomized, trained to exercise on a treadmill daily; and (5) ovariectomized, given E2 and exercised. E2 and/or exercise interventions were started 2 months following surgery and continued for 4 months. The calcium content of the tibial metaphysis and diaphysis and the proximal cancellous bone (BV/TV) were lower in the ovariectomized than in the sham-operated controls 6 months after ovariectomy. This lower bone content was associated with a greater bone uptake of 45Ca and a greater number of osteoblasts and osteoclasts in the proximal tibia compared to the control rats. The meta-physeal calcium content was higher and the 45Ca uptake and osteoblast and osteoclast number were lower in the E2-treated rats than in the nontreated rats. In the exercised group, higher diaphyseal calcium content and proximal cancellous bone were associated with lower bone resorption parameters without a significant effect on bone formation. This study demonstrates that E2 primarily influences tibial cancellous bone of the ovariectomized rat and a positive adaptation to exercise occurs in both cancellous and cortical bone. Under estrogen deficiency, E2 replacement suppresses increased bone formation and resorption; exercise suppresses mainly bone resorption. The effects of E2 replacement and exercise training are independent and additive.