ABSTRACT

While the osteopenia associated with oestrogen deficiency is thought to arise from a relative defect in bone formation with respect to resorption, oestrogen administration itself leads to a decrease, rather than an increase, in bone formation. This decrease in bone formation, which arises from oestrogen's inhibitory effect on bone turnover, presumably masks any underlying tendency of oestrogen treatment towards stimulation of bone formation. To investigate this further, we have examined the early effect of discontinuing the administration of oestradiol-17β (OE₂; 40 μg/kg on bone formation indices in ovariectomized 13-week-old rats, before the turnover-induced increase in formation occurs. Histomorphometric indices were assessed at the proximal tibial metaphysis 0, 7, 10, 13 and 16 days following discontinuation of OE₂ treatment. Measurements of body weight, uterine weight and longitudinal growth rate confirmed that there were rapid effects of OE₂ deficiency on these parameters.

We could detect no significant increase in bone resorption, as measured by osteoclast surface and number, until 16 days after ending treatment with OE₂; this was coincidental with a reduction in bone volume. Shorter periods of OE₂ deficiency were associated with a marked decrease in bone formation, as assessed by dynamic histomorphometric indices. This inhibition of bone formation was largely due to a reduction in double fluorochrome-labelled trabecular surfaces, which were decreased by approximately 70%. We conclude that ending OE₂ administration in ovariectomized rats caused a striking decrease in trabecular bone formation, if such indices are assessed prior to the subsequent turnover-induced increase in formation. This suggests that oestrogen treatment in ovariectomized rats is associated with a stimulatory effect on bone formation, in addition to its recognized anti-resorptive action.

Journal of Endocrinology (1993) 137, 497–503

ABSTRACT

We have previously found that administration of oestradiol-17β (OE₂) to intact adult female rats of 19 days stimulates cancellous bone formation. However, this effect is not observed following longer periods of OE₂ treatment, suggesting that the responsiveness of the skeleton to oestrogen's anabolic action is reduced after prolonged administration. A possible explanation for this is that oestrogen also suppresses bone resorption, which is an important stimulus for bone formation. We therefore investigated the effect of omitting OE₂ for short periods, on the proximal tibial metaphysis of intact female rats. We found that, unlike continuous treatment with OE₂ (40 pg/kg) for 56 days, omission of OE₂ for 4 days out of every 20 was associated with a significant increase in cancellous bone volume. Although continuous and intermittent OE₂ were both associated with a reduction in osteoclast surface, a decrease in the proportion of double fluorochrome-labelled surface was only seen after continuous OE₂ treatment. We then studied the effects of longer periods of OE₂ omission by giving OE₂ (40 pg/kg) for three repeated cycles of: (1) OE₂ for 16 days/vehicle for 4 days, (2) OE₂ for 12 days/vehicle for 8 days, (3) OE₂ for 8 days/vehicle for 12 days, or (4) OE₂ for 4 days/vehicle for 16 days. We found a significant increase in cancellous bone volume when OE₂ was stopped for either 4 or 8 days at a time. However, longer periods of OE₂ omission did not affect bone volume, possibly because these were associated with an increase in bone resorption and/or a reduction in bone formation during the OE₂-free period. In conclusion, we observed an increase in cancellous bone volume after prolonged treatment with oestrogen only if OE₂ was omitted for short periods. This may be due, at least in part, to bone formation being maintained at a higher rate by such treatment than by either continuous OE₂ administration or by intermittent administration where OE₂ is discontinued for longer periods.

Journal of Endocrinology (1993) 139, 267–273

Abstract

Although short-term administration of oestradiol-17β (OE₂) stimulates cancellous bone formation in the rat, this is replaced by a tendency to suppression after prolonged treatment. Hence, in rats rendered osteopaenic by ovariectomy, OE₂ administration fails either to induce a sustained increase in bone formation or to restore bone volume. A possible explanation for this failure is that OE₂ also inhibits bone resorption, secondarily suppressing bone formation through coupling mechanisms. We therefore investigated whether the effects of OE₂ treatment might be modified by intermittently stimulating bone resorption with retinoic acid (120mg/kg daily) for 4 out of every 20 days. We found, in a preliminary experiment using intact animals, that intermittent retinoic acid reduced cancellous bone volume, consistent with previously documented stimulation of bone resorption by retinoic acid. Rats were then rendered osteopaenic by ovariectomy, and given vehicle, retinoic acid and/or OE₂. We found that animals treated with intermittent retinoic acid and OE₂ showed a substantial increase in cancellous bone volume compared with ovariectomized animals treated with vehicle, retinoic acid alone or OE₂ alone. Therefore, intermittent retinoic acid appears to cause a net increase in bone formation over resorption when given to ovariectomized animals in conjunction with OE₂. We conclude that the effects of OE₂ on cancellous bone are modified by intermittent treatment with retinoic acid, resulting in a substantial increase in bone volume.

Journal of Endocrinology (1994) 142, 61–67

Abstract

Although hormone replacement therapy can prevent postmenopausal bone loss, it does not restore bone mass to normal in patients with established osteoporosis. This might reflect a failure to reproduce certain aspects of gonadal function. One method of investigating this possibility would be to examine the effect of ovarian transplantation on the skeleton of osteopaenic ovariectomized rats. However, ovarian transplantation may not fully restore ovarian function to normal, and it is not known whether transplanted ovaries reproduce the action of native ovaries on the skeleton. Therefore, we investigated whether renal capsular or subcutaneous ovarian transplants prevent the effects of ovariectomy on histomorphometric indices of rat tibiae over 44 days. Daily vaginal smears showed that oestrous cycles returned in all but two of 25 animals receiving ovarian transplants. We found that ovarian transplantation prevented the reduction in cancellous bone volume following ovariectomy. While trabecular number was reduced in ovariectomized animals receiving renal capsular ovarian transplants compared to intact animals, trabecular thickness was increased in both transplant groups. Ovarian transplantation also prevented the increase in cancellous and cortical bone formation, cancellous bone resorption and longitudinal growth rate caused by ovariectomy. We conclude that restoration of ovarian function by ovarian transplantation largely prevents the effects of ovariectomy on histomorphometric indices of rat tibiae, suggesting that transplanted ovaries can substitute for the action of native ovaries on the skeleton.

Journal of Endocrinology (1994) 142, 187–192