INFLUENCE OF AGING ON CORTICAL AND TRABECULAR BONE RESPONSE TO ESTRADIOL TREATMENT IN OVARIECTOMIZED RATS

Three groups (n = 15/group) of 6-, 12- and 30-month-old (mature, old a nd senescent animals, respectively) female Wister rats on a diet (6 g/ 100 g BW/ day) containing 0.8% calcium and 0.8% inorganic phosphorus w ere studied. Within each group, 10 rats were ovariectomized surgically and 5 injected s.c. with 17 beta-estradiol (E rats, 10 mu g/kg BW/48 h) and 5 with solvent alone (OVX rats) from day 2 until day 60 after o variectomy. Five other rats were sham-operated (SH rats) and received solvent only. All rats were killed by exsanguination 60 days after ova riectomy. Neither ovariectomy nor estradiol treatment had a significan t effect upon tibial mechanical properties in 6-. 12- and 30-month-old animals. Bone mineral density (BMD) and bone mineral content (BMC) of the distal femur and BMC of the whole femur were decreased by ovariec tomy in 6- and 12-month-old rats, but were not different in the SH and E groups. In senescent animals, in which the lowest BMD and BMC were measured, estradiol treatment was more effective in increasing these p arameters than in adult and old rats. Image analysis of the distal fem oral diaphysis showed that estradiol treatment prevented trabecular bo ne loss induced by senescence and/or ovariectomy. In each group, urina ry deoxypyridinoline excretion and plasma osteocalcin concentration we re higher in the OVX animals than in the controls, consistent with inc reased bone turnover in the estrogen-deficient state. Both biochemical turnover markers were reduced in the estrogen-treated groups. These r esults indicate that 17 beta-estradiol is particularly effective at pr eventing high-turnover-induced osteopenia in 30-month-old animals.