High-Dose gestagens modulate bone resorption and formation and enhance estrogen-induced endosteal bone formation in the ovariectomized mouse

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Abstract
To determine if gestagens of two separate classes have differing skeletal actions, we studied the effects of pharmacologic doses of norethisterone acetate (NETA), a 19‐nortestosterone, and megestrol acetate (MA), a 17α‐hydroxyprogesterone, on bone formation and resorption in intact and in ovariectomized mice. In the same set of experiments, we also attempted to determine if these gestagens can alter the skeletal activity of 17β‐estradiol (E2). Experimentally, the skeletons of 78 female BALB/c mice were prelabeled with [3H]tetracycline (3H‐T). The animals were randomized to 13 groups of 6 mice each 3 days after the final 3H‐T injection. Ovariectomies (OVX) were performed on 8 groups and sham operations (SO) on 5 groups. To study the skeletal effects of the gestagens, 4 groups each of the OVX and SO mice were treated with controlled‐release pellet implants calculated to deliver 80 or 250 μg of NETA or MA per day. To study gestagen interactions with E2, 3 groups of OVX mice were treated with either 40 μg/day of E2 or 40 μg/day of E2 plus 250 μg/day of NETA or MA. One group of OVX and one group of SO animals received placebo pellets. Fluorochrome labels were administered 10 and 11 and 3 and 4 days before sacrifice to allow histomorphometric evaluation of bone formation. At the end of the 60 day protocol, tibiae and thoracic vertebrae were removed and processed for quantitating the levels of bone resorption based on the amounts of 3H‐T retained in the bones. The femora were fixed and embedded for comparison of diaphyseal bone histomorphometry, and the humeri and lumbar vertebrae were prepared for bone density determinations. Reflecting an increase in bone resorption, 3H‐T levels in tibiae and vertebrae were decreased in placebo‐treated OVX animals compared to the placebo‐treated SO group (p < 0.01). Treatment of both SO and OVX mice with NETA decreased bone resorption in a dose‐dependent manner, but MA had no significant effects on vertebral bone resorption and increased bone resorption in the tibiae (p < 0.01). E2 treatment of OVX mice reduced bone resorption, but there were no significant interactions between the E2 and gestagen treatments on resorptive activity. Based on bone histomorphometry of in vivo fluorochrome labels, both gestagens increased periosteal bone formation rates but had no effect on endosteal bone formation (BFRe). In contrast, E2 treatment of the OVX mice stimulated bone formation at the endosteal surface. However, even though the gestagens alone had no effect on BFRe, NETA enhanced E2‐stimulated endosteal bone formation 36%, and the combination of MA with E2 increased BFRe 2.3‐fold compared to E2 alone (p < 0.01). These results indicate that pharmacologic doses of gestagens can independently influence bone formation and resorption in the mouse. Furthermore, the capacity of gestagens to enhance E2 stimulation of BFRe appears to indicate that gestagens may play an important role in the regulation of estrogen action in murine skeletal tissues.