Effects of estrogen and progesterone on tibia histomorphometry in growing rats.

The present study was performed to evaluate possible interactions between estrogen and progesterone on peak cancellous bone mass. Ovariectomized (OVX) growing rats were treated with 17beta-estradiol (4.8 microg/day), progesterone (4.8 mg/day), a combination of the two sex steroids, or with vehicle for 14 days beginning 7 days after OVX. The tibiae were removed for histomorphometric analysis of the proximal metaphysis. OVX and growth each resulted in net resorption of cancellous bone at a sampling site adjusted for longitudinal bone growth. Estradiol and progesterone treatment each antagonized bone loss by inhibiting the decrease in trabecular number. Estradiol increased but progesterone had no effect on trabecular thickness. Progesterone did not influence either osteoclast number or the resorption of the pretreatment fluorochrome label. Estradiol reduced osteoclast number and inhibited label resorption, the latter change being accentuated by combination treatment. Estradiol reduced and progesterone enhanced the mineral apposition and bone formation rates. The results indicate that estradiol and progesterone have independent activities on cancellous bone turnover during growth. Whereas estradiol reduced bone turnover, progesterone had a stimulatory effect on bone formation. These findings suggest that progesterone has a role in establishing and maintaining peak cancellous bone volume during growth.

Glucocorticoids regulate the expression of the human osteoblastic endothelin A receptor gene.

The endothelial cell-derived peptide endothelin 1 (ET1) stimulates cell proliferation and differentiated functions of human osteoblastic cells (HOC), and HOC constitutively express the endothelin A receptor (ETRA). Therefore, ET1 may play an important role in the regulation of bone cell metabolism. As glucocorticoids (GC) exert a profound influence on bone metabolism and increase the effects of ET1 on bone cell metabolism in vitro, the effects of GC on ETRA expression in HOC were investigated. Dexamethasone (DEX) increased ETRA mRNA levels in a dose- and time-dependent fashion. The effects of dexamethasone, prednisolone, and deflazacort on the increase of ETRA mRNA levels correlate positively with their binding affinity to the GC receptor. Scatchard analysis of ET1 binding data to HOC revealed that DEX increased the binding capacity for ET1 from 25,300 to 62,800 binding sites per osteoblastic cell, leading to an enhanced mitogenic effect of ET1 on HOC after preincubation with DEX. Transiently transfected primary HOC with a reporter gene construct, containing the 5'-flanking region of the ETRA gene fused to luciferase gene, showed a promoter-dependent expression of the reporter gene and the induction of reporter gene expression by DEX treatment. Total RNA extracts of femoral head biopsies with osteonecrotic lesions from GC-treated patients showed threefold higher ETRA mRNA levels compared with extracts of bone biopsies from patients with traumatically induced osteonecrosis and coxarthrosis. Furthermore, GC treatment increased plasma ET1 levels by 50% compared with pretreatment values. These findings suggest that GC induced upregulation of ETRA, and ET1 plasma levels enhance ET1's anabolic action on bone cell metabolism. Increased ET1 concentrations may also impair bone perfusion by vasoconstriction in a metabolically activated skeletal region.

Gonadal and adrenal androgens are potent regulators of human bone cell metabolism in vitro.

Androgens stimulate bone formation and play an important role in the maintenance of bone mass. Clinical observations suggest that both gonadal and adrenal androgens contribute to the positive impact of androgenic steroids on bone metabolism. We investigated the mechanism of action of the adrenal androgen dehydroepiandrosterone (DHEA) and its sulfated compound dehydroepiandrosterone sulfate (DHEAS) on human osteoblastic cells (HOCs) in vitro. The DHEA- and DHEAS-induced effects were analyzed in parallel with the actions elicited by the gonadal androgen dihydrotestosterone (DHT). There was no qualitative difference between the effects of gonadal and adrenal androgens on HOC metabolism in vitro. Both were stimulatory as regards cell proliferation and differentiated functions, but the gonadal androgen DHT was significantly more potent than DHEA. The actions of DHT and DHEA on HOC proliferation and alkaline phosphatase (ALP) production could be prevented by the androgen receptor antagonist hydroxyflutamide and inhibitory transforming growth factor beta antibodies (TGF-beta ab), respectively, but were not affected by the presence of the 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) and 5-alpha-reductase (5-AR) inhibitor 17 beta-N,N-diethylcarbamoyl-4-methyl- 4aza-5 alpha-androstan-3-one (4-MA). This indicates that DHT and DHEA (1) exert their mitogenic effects by androgen receptor-mediated mechanisms, (2) stimulate ALP production by increased TGF-beta expression, (3) that the action of DHT is not affected by the presence of 4-MA, and that (4) DHEA does not need to be metabolized by 3 beta HSD or 5-AR first to exert its effects on HOCs in vitro.

Short-term effects of high dose estrogen on tibiae of growing male rats.

The short-term effects of estrogen at a single high dose (4 mg/kg body weight/day for 14 days) were determined on tibiae in the normal (noncastrate) growing male rat. In cortical periosteal bone, at a middiaphyseal site devoid of resorbing activity, estrogen suppressed periosteal bone formation and apposition rates, resulting in a smaller cross-sectional area. In middiaphyseal endocortical bone, estrogen had no effect on apposition and formation rates and, because medullary area was unchanged, probably had no effect on endocortical bone resorption. In the proximal tibial metaphysis, estrogen greatly suppressed longitudinal growth rate. In a site within the metaphysis adjusted for the effects of growth, cancellous mineral apposition was greatly reduced by the hormone. Estrogen-treated rats retained more of a fluorochrome label deposited in cancellous bone at the beginning of the study than vehicle-treated animals, indicating a reduced net bone loss. As a result of the lowered resorption induced by estrogen, cancellous bone mass (area and perimeter) were both significantly higher in estrogen-treated rats. No evidence was found for an anabolic action of the hormone in the male rat; indeed, estrogen reduced indices of bone formation.

Lack of an effect of sodium zeolite A on rat tibia histomorphometry.

Cell culture studies suggest that the aluminum silicate polymer sodium Zeolite A (SZA) increases bone cell proliferation and extracellular matrix production. This study in rats investigated the short-term (2 weeks) and long-term (18 weeks) in vivo effects of SZA on growth rate (weight gain) and tibia histomorphometry. In separate short-term experiments, female (experiment 1) or male (experiment 2) Sprague-Dawley rats grown and maintained on normal calcium diets were gavaged daily during a 2 week treatment period with 30 mg/kg, 100 mg/kg, or 500 mg of SZA/kg of rat body weight. In the long-term study (experiment 3) ovariectomized (OVX) rats were fed a low calcium diet containing 0, 1.80, and 9.00 g of SZA/kg of diet (0, 125, and 617 mg/kg of body weight, respectively). Short- and long-term treatment of growing rats with SZA resulted in a dose-dependent increase in bone aluminum. In the first experiment, growing intact female rats showed no significant SZA dose-dependent response in growth rate (weight gain) or histomorphometry of cortical bone in the tibial diaphysis or cancellous bone in the secondary spongiosa of the tibial metaphysis. In the second experiment, growing male rats, with right hind limbs immobilized by unilateral sciatic neurotomy, showed no SZA dose-dependent response in growth rate. The longitudinal growth of cancellous bone in the tibia of the denervated limb and the intact contralateral limb were not influenced by sciatic neurectomy and/or by treatment with SZA. Histomorphometry demonstrated that cortical bone mass and formation was reduced in the sciatic-sectioned limb when compared with the contralateral intact limb of vehicle-treated rats, as evidenced by significant reductions in static measurements of cortical bone area (-8.5%) and cross-sectional area (-4.8%) and in calculations of the periosteal formation rate (-33.8%) and mineral apposition rate (-31.6%), and the endocortical formation rate (-35.5%) and mineral apposition rate (-37.9%). The cancellous bone mass of denervated limbs of vehicle-treated rats was also deficient, as evidenced by decreased cancellous bone area (-39.1%) and perimeter (-31.9%). The bone mineral apposition rate was decreased (-26.7%) indicating reduced osteoblast activity. Treatment with SZA did not influence these indices in the tibiae of either sciatic-sectioned limbs or contralateral intact limbs. In the long-term experiment, OVX resulted in a dramatic 88% decrease in cancellous bone volume which was prevented by treatment with 17 beta-estradiol and not influenced by treatment with Zeolite A. The increases in osteoblast and osteoclast number following OVX were not influenced by SZA. The results indicate that SZA treatment has no anabolic effect on cortical and cancellous bone formation and mass in normal growing female rats and that this compound does not protect against osteopenia due to reduced load bearing in the growing male rat or gonadal hormone deficiency in adult female rats.

Spaceflight results in depressed cancellous bone formation in rat humeri.

Two experiments investigating the effects of short-term spaceflight on cancellous bone turnover were carried out: Physiological Systems Experiment-1 (PSE-1) (a 4-d orbital spaceflight) and PSE-2 (a 10-d flight). Cancellous bone area was not significantly altered by spaceflight in the humerus during either flight. The calculated bone formation rate was unchanged during the 4-d flight but decreased during the 10-d flight. The decrease in calculated bone formation during the longer flight was due to a combination of a statistically significant decrease in mineral apposition rate and a nonsignificant decrease in double label perimeter. The dynamic measurements suggest that spaceflight results in decreases in osteoblast number and activity. Further, the decreases in osteoblast perimeter and osteoid perimeter indicate that a 2-d reloading period was insufficient to restore bone formation to normal. Resorption of the preflight fluorochrome label was not influenced by spaceflight, suggesting that there was no net change in bone resorption. This conclusion is supported by the unremarkable effects of spaceflight on osteoclast number and osteoclast perimeter. Our findings indicate that short-term spaceflight results in depressed osteoblast number and/or activity. Continued decreases in bone formation with normal bone resorption could ultimately lead to cancellous osteopenia and reduced bone strength.

Reduced chondroclast differentiation results in increased cancellous bone volume in estrogen-treated growing rats.

These studies were designed to investigate the role of altered growth processes in mediating estrogen-induced changes in cancellous bone volume in growing female rats. Ovariectomy resulted in an increase in the longitudinal growth rate of the tibia throughout the growth period and estrogen treatment of ovariectomized (OVX) rats resulted in a dose-dependent decrease in longitudinal growth. Estrogen treatment also resulted in decreases in growth plate thickness, chondroclast number in the zone of vascular invasion, and osteoclast number in the secondary spongiosa. There were simultaneous increases in mineralized cartilage in the zone of vascular invasion; total mineralized tissue, mineralized cartilage, and bone in the primary spongiosa; and bone in the secondary spongiosa. Ovariectomy increased and estrogen treatment of OVX rats decreased [3H]thymidine-labeled nuclei in chondroclasts after 24 h, but after 7 days, the labeling indices were similar in the OVX and intact groups and only slightly decreased in the estrogen-treated rats. We interpret these results as evidence that estrogen impairs chondroclast differentiation. We propose that this impairment leads to decreased chondroclast number and reduced resorption of mineralized cartilage in the zone of vascular invasion, which, in turn, results in an increased cancellous bone volume.

Estrogen does not increase bone formation in growing rats.

The purpose of these experiments was to test the hypothesis that estrogen has an anabolic effect on cancellous bone formation in long bones of growing rats. In the present studies, estrogen decreased measurements related to cancellous bone formation, including osteoblast number, fluorochrome-labeled bone perimeter, and incorporation of [3H]proline into osteoid. Steady state mRNA levels for bone matrix proteins and mineral apposition rate were unchanged or reduced. The [3H]thymidine labeling index for osteoblasts was very low and was reduced by estrogen treatment. Finally, estrogen decreased the rate of removal of tetracycline previously incorporated into cancellous bone as well as the growth-dependent decrease in cancellous bone area, indicating that hormone treatment reduced net bone resorption. In summary, no evidence was secured to support the hypothesis that estrogen has an anabolic action on bone formation; the results obtained in these studies consistently demonstrated a pronounced inhibitory action of the hormone on bone turnover.

Mechanism of action of estrogen on cancellous bone balance in tibiae of ovariectomized growing rats: inhibition of indices of formation and resorption.

Ovariectomy results in cancellous osteopenia in rat long bones, a condition that is prevented by treatment with estrogens. The purpose of these studies was to clarify the effects of estrogen on cancellous bone turnover using dynamic bone histomorphometry. Treatment of ovariectomized rats with diethylstilbestrol (DES) reduced the mineral apposition rate, double-label perimeter, osteoblast number, and osteoclast number, suggesting that the hormone had inhibitory effects on bone formation as well as bone resorption. However, we could not estimate the bone formation rate because of rapid resorption of tetracycline-labeled bone in the ovariectomized rat. The magnitude of loss was documented by a time course study: 58% of the tetracycline initially incorporated into the secondary spongiosa of the tibial metaphysis was resorbed after 11 days and 89% was resorbed after 22 days. Similarly, cancellous bone area was decreased by 67% after 11 days and by 88% after 22 days. Administration of either DES or tamoxifen (TAM) dramatically reduced resorption of tetracycline as well as the decrease in cancellous bone area. These results demonstrate that (1) estrogen prevents osteopenia in ovariectomized (OVX) rats, in part by inhibiting bone turnover, (2) TAM is an estrogen agonist on bone resorption, and (3) resorption of tetracycline-labeled bone leads to serious underestimation of the bone formation rate in OVX rats.

Disuse osteopenia is accompanied by downregulation of gene expression for bone proteins in growing rats.

Unilateral sciatic neurectomy (USN) resulted in cortical osteopenia in tibiae from the sciatic nerve-sectioned limb of growing rats. The bone deficit resulted from decreased periosteal addition; there were no changes in the indexes of bone resorption. The periosteal bone formation rate was reduced in the nerve-sectioned limb within 7 days of sciatic neurectomy, and this decrease persisted for at least 56 days. Steady-state mRNA levels for bone proteins were determined in periosteum isolated from tibiae and femurs 7 and 14 days after sciatic nerve section. Nerve section resulted in decreased levels of mRNA for osteocalcin, alkaline phosphatase, and possibly the prepro-alpha (I)-subunit of type I collagen (collagen). The effects were more pronounced in tibiae than femurs, corresponding to the greater degree of immobility induced by USN in the former bone. The results demonstrate that decreased bone formation precedes establishment of disuse cortical osteopenia in growing rats with no evidence for a change in bone resorption. Furthermore, the decreased bone formation is associated with, and may be due to, reduced mRNA levels for matrix proteins and other important bone proteins.

Incorporation of sodium fluoride into cortical bone does not impair the mechanical properties of the appendicular skeleton in rats.

Clinical studies on the use of sodium fluoride (NaF) in osteoporotic patients have demonstrated increased spinal bone mass without a reduction in vertebral fracture incidence, and a trend towards reduced appendicular bone mass with an increase in peripheral fracture incidence. As previous reports have suggested that NaF becomes incorporated into bone's crystal structure, possibly affecting bone strength, we sought to examine the relationship among bone fluoride content, bone mass, and skeletal fragility. Twenty-one-day-old female Sprague-Dawley rats were treated with four different doses of NaF. The tibiae were subjected to histomorphometric and biochemical analyses, and the femora were tested in torsion for the properties of strength, stiffness, energy storage capacity, and angular deformation. The results showed that over 50% of the skeleton in these rats was turned over in the presence of NaF. The four different doses resulted in a linear increase in bone F concentration and suggested excellent absorption and incorporation of this drug. No changes in histomorphometric indices of bone formation or turnover were found. Despite the large fraction of bone formed during NaF treatment, and the linear increase in bone fluoride content in relation to dose, there were no changes observed in any of the mechanical properties. These results suggest that, even extensive incorporation of fluoride into bone, in the absence of an effect on bone mass or remodeling, does not significantly alter its capacity to withstand mechanical loads.

Effects of gallium on bone in the rat.

Gallium nitrate lowers the serum calcium in patients with hypercalcemia caused by malignancy and is available for clinical use. The mechanism for the hypocalcemic action is unknown, however. The present studies were undertaken to determine the effects of gallium on bone metabolism. Normal male rats were implanted subcutaneously with mineralized allogeneic bone matrix. Histomorphometry of the implants and of tibiae was determined after three doses of tetracycline administered at intervals of 1 week. Gallium as nitrate was administered daily by intraperitoneal injection at doses of 0.9, 1.8, and 3.6 mg elemental gallium per kg body weight for 21 days in one study and at 3.5 mg/kg for 33 days in a second study. All the gallium-treated rats gained weight. Rats given gallium at doses of 3.5 mg/kg or more grew at a lower rate than untreated controls (-7 and -10% at doses of 3.5 and 3.6 mg/kg, respectively; p less than 0.05). At a dose of 0.9 mg/kg, gallium did not inhibit bone resorption or lower serum calcium but inhibited bone formation by 32% and bone apposition by 36% at the endosteal surface of the tibia. At a dose of 1.8 mg/kg, gallium produced modest hypocalcemia, prevented a rise in circulating 1,25-dihydroxyvitamin D [1,25-(OH)2D], inhibited bone resorption in implants, and inhibited bone formation by 19% and bone apposition by 18%. At a dose of 3.5 mg/kg, gallium lowered the serum calcium and serum 1,25-(OH)2D, inhibited growth, and accentuated the antiresorptive and antiformative effects seen at the two lower doses.(ABSTRACT TRUNCATED AT 250 WORDS)

Dose-dependent effects of tamoxifen on long bones in growing rats: influence of ovarian status.

Tamoxifen is a nonsteroidal antiestrogen which has been reported by various investigators to have estrogen agonist and antagonist effects on rat bone. These different interpretations may be due to differences in the ovarian status, estrogen levels, and/or tamoxifen levels of the rats. To address this issue, a dose response was determined for the effects of tamoxifen on bone histomorphometry in intact female and ovariectomized (OVX) rats. The results were compared with those obtained after treatment of OVX rats with estrogen alone or a combination of estrogen and tamoxifen. OVX resulted in increases in growth rate (weight gain) and periosteal bone formation rate and decreases in uterine weight and cancellous bone fractional volume (BV/TV). Treatment of OVX rats with estrogen resulted in dose-dependent decreases in growth rate and periosteal bone formation rate as well as dose-dependent increases in uterine weight and BV/TV. Similarly, tamoxifen treatment resulted in dose-dependent decreases in overall growth rate and periosteal bone formation rate in both OVX and intact rats. Tamoxifen treatment prevented the decrease in BV/TV after OVX, although the highest dose of tamoxifen resulted in a small decrease in BV/TV in intact female rats. In contrast to estrogen, tamoxifen treatment prevented the increase in uterine weight in intact female rats as well as the decrease in uterine weight in OVX rats. Tamoxifen treatment did not alter the effects of 17 beta-estradiol on the periosteal bone formation rate in OVX rats, but reduced the increase in BV/TV to values similar to those in intact rats. These results are consistent with tamoxifen behaving as a partial estrogen agonist on rat bone.

Androgen treatment prevents loss of cancellous bone in the orchidectomized rat.

This report describes histomorphometric evidence for an important role of androgens in maintaining cancellous bone balance at a remodeling site in vivo. Rats were orchiectomized (ORX) at 7 weeks of age and received either androgens or vehicle 1 week later (testosterone, 1-dehydrotestosterone, or 5-dihydrotestosterone) by subcutaneous pellet, producing controlled release of the drug for 3 weeks. Intact male rats and untreated ORX animals served as controls. After 4 weeks untreated ORX resulted in undetectable serum testosterone levels and marked atrophy of seminal vesicles compared with intact controls. Histomorphometry revealed severe cancellous osteopenia in the secondary spongiosa of the proximal tibial metaphysis. The length of bone surface lined by apparently "active" osteoblasts and number of osteoclasts per length of cancellous bone surface were increased following ORX. Testosterone treatment at 5 mg (per 21 days) produced subphysiologic serum testosterone levels. In contrast, 10 and 25 mg pellets resulted in serum testosterone ORX, and the degree of protection was dose dependent. 1-Dehydro- and 5-dihydrotestosterones displayed a bone-protective effect similar to that of testosterone. The results demonstrate that gonadal insufficiency results in a cancellous osteopenia that is preventable by testosterone treatment. Further, because a similar protective action was achieved using the nonaromatizable androgen 5-dihydrostestosterone, the results suggest that this bone-sparing effect is mediated by androgen rather than by metabolism of the androgen to an estrogen.

Differential effects of androgens on cortical bone histomorphometry in gonadectomized male and female rats.

The physiological role of gonadal androgens in regulating bone metabolism is not established. To determine if androgens antagonize the changes in cortical bone after gonadectomy, we treated orchiectomized (ORX) rats with testosterone (T) and 5 alpha-dihydrotestosterone (DHT), and ovariectomized (OVX) rats with the afore-mentioned androgens, as well as the synthetic androgen fluoxymesterone (Fl) and the nonsteroidal estrogen diethylstilbestrol (DES). OVX resulted in a rapid, sustained increase in periosteal bone formation at the tibial diaphysis, whereas ORX resulted in decreased bone formation. Androgen treatment stimulated bone formation in ORX rats and suppressed bone formation in OVX rats. A large dose of DES suppressed bone formation in OVX rats to values below the intact controls. The results of these studies demonstrate that androgens counteract the changes in cortical bone formation after gonadectomy in females as well as males, and thereby reestablish the sex difference observed in intact rats.

Dehydroepiandrosterone reduces cancellous bone osteopenia in ovariectomized rats.

The effects of dehydroepiandrosterone (DHA) on cortical bone at the tibial diaphysis and cancellous bone at the tibial metaphysis were determined in intact and ovariectomized rats. Total plasma DHA was reduced at 10 and 16 wk after ovariectomy. Intact and ovariectomized rats were implanted monthly with controlled-release subcutaneous pellets containing 2.5 mg of DHA or placebo. Plasma DHA was elevated in DHA-treated intact rats and was near normal in hormone-treated ovariectomized rats. Ovariectomy resulted in severe cancellous bone osteopenia, which was reduced in DHA-treated animals. Cross-sectional and cortical areas were increased at the diaphysis in ovariectomized rats, and this was not altered by DHA treatment. Bone formation at the endosteal and periosteal surfaces of the tibial diaphysis was decreased 16 wk after ovariectomy, and DHA treatment appeared to antagonize these changes. We interpret these results as evidence that decreased peripheral DHA levels contribute to the skeletal changes in ovariectomized rats.

Efficacy of long-term fluoride and calcium therapy in correcting the deficit of spinal bone density in osteoporosis.

Long-term fluoride therapy for osteoporosis has been shown to increase the thickness of vertebral trabeculae as seen on spinal radiographs. To determine if this qualitative finding represents a measurable increase in spinal bone density, quantitative computed tomography was utilized to measure trabecular vertebral body density (TVBD) in the lumbar spine of 18 female osteoporotic patients, all of whom had been treated with sodium fluoride, 77 +/- 13 mg/day (mean +/- SD), and calcium, 1000 mg/day, for 57 +/- 24 months. TVBD in these fluoride treated osteoporotic patients (132 +/- 82 mg/cm3) was found to be significantly greater than mean TVBD for an age-matched group of untreated female osteoporotic patients (51 +/- 21 mg/cm3, n = 89, p less than 0.001). The value for TVBD in the long-term fluoride treated osteoporotics was not only similar to previously published values for TVBD (104 +/- 30 mg cm3) in normal females of similar age, but was also above the calculated TVBD "fracture threshold" of 100 mg/cm3 for females. Only one of the 18 fluoride treated osteoporotics continued to have spinal fractures during therapy, accounting for 4 fractures per 87.2 patient years of observation, a value which is significantly lower than the published incidence of 76 fractures per 91 patient years for untreated osteoporotic patients (p less than 0.001). Together, these findings demonstrate that long-term fluoride and calcium therapy for osteoporosis increases TVBD in the majority of patients within a reasonable time frame and significantly reduces the risk for spinal fractures.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of tamoxifen on the osteopenia induced by sciatic neurotomy in the rat: a histomorphometric study.

The effects of a 3-week treatment with the nonsteroidal "antiestrogen" tamoxifen were determined on cortical and trabecular bone mass of the tibiae of growing male rats that had undergone unilateral sciatic neurotomy (USN). USN resulted in decreases in cortical area (-11.3%), cross-sectional area (-8.4%), and periosteal bone formation rate (-32.6%) in cortical bone, indicating that the disuse osteopenia results in a decrease in bone formation in cortical bone. USN significantly reduced the amount of trabecular bone in our metaphyseal sampling site (-75%), markedly increasing the amount of bone surface lined by osteoclasts (+65%) without affecting the surface lined by osteoblasts. These results suggest that trabecular bone disuse osteopenia is due, at least in part, to increased bone resorption. Tamoxifen treatment significantly reduced the loss of trabecular bone, restoring resorbing surface length to the control (sham-operated) animal levels. Tamoxifen treatment of sham-operated animals increased trabecular bone area and surface by 35.7% (+/- 10.5) and 41.8% (+/- 7.8), respectively, and reduced resorbing surface by 21.5% (+/- 11.6) compared with sham-operated placebo-treated rats. Tamoxifen had no significant effect on cortical bone parameters in the sham-operated group. The results indicate that tamoxifen is able to reduce the trabecular bone loss that results from USN, but has no effect on cortical bone disuse osteopenia, or on trabecular bone formation. Moreover, tamoxifen treatment of control (intact) animals inhibited the normal levels of bone resorption occurring in these rapidly growing animals.

Tamoxifen inhibits osteoclast-mediated resorption of trabecular bone in ovarian hormone-deficient rats.

The effects of the nonsteroidal antiestrogen tamoxifen were determined on trabecular bone mass in the proximal tibial metaphysis of intact and ovariectomized rats. Rats were ovariectomized at the beginning of the study. On day 7 of the study, 5-mg slow release pellets of tamoxifen or placebo were implanted sc. All of the rats were killed on day 28 of the experiment. Sections of the proximal tibial metaphysis were stained for acid phosphatase and evaluated histomorphometrically. Ovariectomy resulted in marked loss of bone. Compared to the values in sham-operated animals, the trabecular bone at a sampling site in the secondary spongiosa of ovariectomized rats was reduced by more than 60%, the length of trabecular bone surface covered by osteoclasts was increased by 563%, the percentage of trabecular bone surface covered by osteoclasts was increased by 567%, the mean osteoclast size was increased by 84%, and the number of nuclei per osteoclast was increased by 38%. In contrast, treatment of ovariectomized rats for 3 weeks with tamoxifen restored the histomorphometric measurements to values comparable to those in sham-operated animals. 17 beta-Estradiol increased trabecular bone fractional area in ovariectomized and sham-operated rats, and administration of tamoxifen to estrogen-treated, ovariectomized, and sham-operated animals produced a further increase in trabecular bone. In summary, 1) ovariectomy resulted in large increases in both the number and activity of osteoclasts, 2) the increased bone resorption associated with ovariectomy produced a net loss of trabecular bone, and 3) treatment of ovariectomized rats with tamoxifen prevented these skeletal changes. The results indicate that in the rat, tamoxifen mimics the effects of estrogen on trabecular bone at concentrations that are not uterotropic.