Dose-response effects of estradiol implants on bone mineral density in ovariectomized ewes

In a longitudinal in vivo study, we studied the effect of two different doses of 17β-estradiol (E2) administered in the form of a subcutaneous implant, on bone mineral density (BMD) of the lumbar vertebrae (L4, L5, L4–L6/L5–L7), the calcaneus (CAL) and the distal radius (DR) in ovariectomized (OVX) ewes. The BMD of various regions of the femur, tibia and humerus were studied at autopsy. Skeletally mature ewes (n = 45) were divided into four groups: sham operated (n = 12), OVX (n = 15), OVX plus one E2 implant (OVXE, n = 12) and OVX plus two E2 implants (OVX2E, n = 6). BMD of L4, L5, L4–L6/L5–L7, CAL and DR was determined at 0, 6 and 12 months using dual-energy X-ray absorptiometry. In-vivo precision of BMD for the last three lumbar vertebrae ranged from 1.4–4.3%, and 1.5% and 3.5% for CAL and DR respectively. In the in vivo study, there were no significant changes in the mean BMD in the sham group at any time point (each group served as its own control). In the OVX group, mean BMD was significantly lower at L5 and DR at 6 months and significantly lower at L4 at 12 months. In the OVXE group, the mean BMD was significantly higher at L5, CAL and DR at 12 months. In the OVX2E group, BMD was significantly higher at CAL but significantly lower at L4 at 12 months. None of the treatments produced significant changes of mean BMD of L4–L6/L5–L7 at any time point. Treatment influenced the rate of change in BMD for L4 and L5 (P = 0.038, 0.041 respectively) but not at other locations between 0 and 12 mo (repeated measures ANOVA). The sham and OVXE groups lost less bone than the OVX and the OVX2E groups (each group served as its own control). After 12 months, ex-vivo measurement of BMD of the proximal and distal femur, proximal tibia and proximal humerus without soft tissues, showed no significant difference between the four treatment groups. The slight decrease in bone mass in the ewe following OVX was expected but we were surprised to see a decrease in BMD of similar magnitude in L4 but increases in BMD of L5, CAL and DR in those animals with two E2 implants with time. We suspect that a continuous supraphysiological dose of E2 may have desensitized the bone by downregulating estrogen receptors. L4, L5 are critical sites where BMD can be measured to evaluate therapies if this model is used. The CAL and DR have not been as promising.