Osteoglycin, a novel coordinator of bone and glucose homeostasis.

OBJECTIVE: The skeleton, which is strongly controlled by endocrine factors, has recently been shown to also play an active endocrine role itself, specifically influencing energy metabolism. However, much less is known about this role. Therefore, we sought to identify novel endocrine factors involved in the regulation of both bone mass and whole-body glucose homeostasis. METHODS: We used transcriptomic and proteomic analysis of Y1 receptor deficient osteoblasts combined with the generation of a novel osteoglycin deficient mouse model and performed comprehensive in vivo phenotype profiling, combined with osteoglycin administration in wildtype mice and human studies. RESULTS: Here we identify a novel role for osteoglycin, a secreted proteoglycan, in coordinating bone accretion with changes in energy balance. Using an osteoglycin knockout mouse model, we show that at a whole body level, osteoglycin acts to suppress bone formation and modulate whole body energy supplies by altering glucose uptake through changes in insulin secretion and sensitivity, as well as by altering food intake through central signaling. Examining humans following gastric surgery as a model of negative energy balance, we show that osteoglycin is associated with BMI and lean mass as well as changes in weight, BMI, and glucose levels. CONCLUSIONS: Thus, we identify osteoglycin as a novel factor involved in the regulation of energy homeostasis and identify a role for it in facilitating the matching of bone acquisition to alterations in energy status.

Central RANK signalling in NPY neurons alters bone mass in male mice.

RANKL signalling known to be important for the control of bone mass, has recently also been implicated in the brain to control thermoregulation, however, it is not known which neuronal pathways are involved and whether other aspects of energy homeostasis are also affected. Here we show that selective deletion of RANK from NPY neurons down-regulated NPY mRNA expression in the hypothalamus. While comprehensive phenotyping of germline-induced NPY neuron specific RANK deficient mice revealed no significant changes in physical or metabolic parameters, adult onset deletion of RANK from NPY neurons led to a significant increase in fat mass and a decrease in whole body bone mineral content and bone mineral density. Intriguingly, when these conditional knockout mice were placed on a high fat diet, body weight and fat mass did not differ to control mice. However, they were able to significantly increase their bone mass to match their increased body weight, an ability that was lacking in control mice. Taken together, results from this study demonstrate that RANK signalling in NPY neurons is involved in modulating NPY levels and through that matching bone mass to body weight.

Neuropeptide y attenuates stress-induced bone loss through suppression of noradrenaline circuits.

Chronic stress and depression have adverse consequences on many organ systems, including the skeleton, but the mechanisms underlying stress-induced bone loss remain unclear. Here we demonstrate that neuropeptide Y (NPY), centrally and peripherally, plays a critical role in protecting against stress-induced bone loss. Mice lacking the anxiolytic factor NPY exhibit more anxious behavior and elevated corticosterone levels. Additionally, following a 6-week restraint, or cold-stress protocol, Npy-null mice exhibit three-fold greater bone loss compared to wild-type mice, owing to suppression of osteoblast activity. This stress-protective NPY pathway acts specifically through Y2 receptors. Centrally, Y2 receptors suppress corticotropin-releasing factor expression and inhibit activation of noradrenergic neurons in the paraventricular nucleus. In the periphery, they act to control noradrenaline release from sympathetic neurons. Specific deletion of arcuate Y2 receptors recapitulates the Npy-null stress response, coincident with elevated serum noradrenaline. Importantly, specific reintroduction of NPY solely in noradrenergic neurons of otherwise Npy-null mice blocks the increase in circulating noradrenaline and the stress-induced bone loss. Thus, NPY protects against excessive stress-induced bone loss, through Y2 receptor-mediated modulation of central and peripheral noradrenergic neurons.

Bariatric surgery, bone loss, obesity and possible mechanisms.

Bariatric surgery remains the most effective treatment for severely obese patients. However, the potential long-term effects of bariatric surgical procedures on health, including bone health, are only partially understood. The goal of this review was to present data on the impact of bariatric surgery on bone metabolism and to analyse possible reasons for the loss of bone mass that frequently occurs after bariatric surgery. Such factors include nutritional deficiencies, rapid weight loss per se, effects of fat-derived adipokines and gut-derived appetite-regulatory hormones. However, the relative roles of these factors in skeletal regulation and the mechanisms by which they work are not yet fully defined. Our review was focussed on the complex relationship between body weight, fat mass and bone mass, as well as peripheral and central mediators potentially involved in the dual regulation of both energy and bone homeostasis. We also review the data on the inverse relationship between central obesity, bone marrow fat and osteoporosis. As the number of bariatric operations increases, it is imperative to recognize mechanisms responsible for bariatric surgery-induced bone loss, with careful monitoring of bone health including long-term fracture incidence in patients undergoing these procedures.

Neuropeptide Y mediates the short-term hypometabolic effect of estrogen deficiency in mice.

BACKGROUND: Estrogen deficiency increases body weight or total and central adiposity and decreases energy expenditure. Hypothalamic neuropeptide Y (NPY) expression is altered by estrogen deficiency in rodents, but the long-term consequences on energy homeostasis are unknown. OBJECTIVE: To investigate the role of NPY in the changes in energy expenditure and physical activity, as well as the associated changes in body weight and composition in response to short-term and long-term estrogen deficiency. DESIGN: Sham and ovariectomy (OVX) operations were performed at 8 weeks of age in wild-type (WT) and NPY(-/-) mice. Energy expenditure, physical activity, body composition and weight, as well as food intake were measured at 10-18 days (short-term) and 46-54 days (long-term) after OVX. RESULTS: OVX influences energy homeostasis differently at early compared with later time-points. At the early but not the late time point, OVX in WT mice reduced oxygen consumption and energy expenditure and tended to reduce resting metabolic rate. Interestingly, these effects of short-term estrogen deficiency were ablated by NPY deletion, with NPY(-/-) mice exhibiting significant increases in energy expenditure and resting metabolic rate. In addition to these hypermetabolic effects, OVX NPY(-/-) mice exhibited significantly lower body weight and whole-body fat mass relative to OVX WT controls at the short-term but not the long-term time point. Food intake and physical activity were unaltered by OVX, but NPY(-/-) mice exhibited significant reductions in these parameters relative to WT. CONCLUSION: The effects of estrogen deficiency to reduce energy metabolism are transient, and NPY is critical to this effect as well as the early OVX-induced obesity.

NPY modulates PYY function in the regulation of energy balance and glucose homeostasis.

AIMS: Both the neuronal-derived neuropeptide Y (NPY) and the gut hormone peptide YY (PYY) have been implicated in the regulation of energy balance and glucose homeostasis. However, despite similar affinities for the same Y receptors, the co-ordinated actions of these two peptides in energy and glucose homeostasis remain largely unknown. METHODS: To investigate the mechanisms and possible interactions between PYY with NPY in the regulation of these processes, we utilized NPY/PYY single and double mutant mouse models and examined parameters of energy balance and glucose homeostasis. RESULTS: PYY(-/-) mice exhibited increased fasting-induced food intake, enhanced fasting and oral glucose-induced serum insulin levels, and an impaired insulin tolerance, - changes not observed in NPY(-/-) mice. Interestingly, whereas PYY deficiency-induced impairment in insulin tolerance remained in NPY(-/-) PYY(-/-) mice, effects of PYY deficiency on fasting-induced food intake and serum insulin concentrations at baseline and after the oral glucose bolus were absent in NPY(-/-) PYY(-/-) mice, suggesting that NPY signalling may be required for PYY's action on insulin secretion and fasting-induced hyperphagia. Moreover, NPY(-/-) PYY(-/-) , but not NPY(-/-) or PYY(-/-) mice had significantly decreased daily food intake, indicating interactive control by NPY and PYY on spontaneous food intake. Furthermore, both NPY(-/-) and PYY(-/-) mice showed significantly reduced respiratory exchange ratio during the light phase, with no additive effects observed in NPY(-/-) PYY(-/-) mice, indicating that NPY and PYY may regulate oxidative fuel selection via partly shared mechanisms. Overall, physical activity and energy expenditure, however, are not significantly altered by NPY and PYY single or double deficiencies. CONCLUSIONS: These findings show significant and diverse interactions between NPY and PYY signalling in the regulation of different aspects of energy balance and glucose homeostasis.

Distal tibial fracture repair in a neurofibromatosis type 1-deficient mouse treated with recombinant bone morphogenetic protein and a bisphosphonate.

Congenital pseudarthrosis of the tibia is an uncommon manifestation of neurofibromatosis type 1 (NF1), but one that remains difficult to treat due to anabolic deficiency and catabolic excess. Bone grafting and more recently recombinant human bone morphogenetic proteins (rhBMPs) have been identified as pro-anabolic stimuli with the potential to improve the outcome after surgery. As an additional pharmaceutical intervention, we describe the combined use of rhBMP-2 and the bisphosphonate zoledronic acid in a mouse model of NF1-deficient fracture repair. Fractures were generated in the distal tibiae of neurofibromatosis type 1-deficient (Nf1(+/-)) mice and control mice. Fractures were open and featured periosteal stripping. All mice received 10 µg rhBMP-2 delivered in a carboxymethylcellulose carrier around the fracture as an anabolic stimulus. Bisphosphonate-treated mice also received five doses of 0.02 mg/kg zoledronic acid given by intraperitoneal injection. When only rhBMP but no zoledronic acid was used to promote repair, 75% of fractures in Nf1(+/-) mice remained ununited at three weeks compared with 7% of controls (p < 0.001). Systemic post-operative administration of zoledronic acid halved the rate of ununited fractures to 37.5% (p < 0.07). These data support the concept that preventing bone loss in combination with anabolic stimulation may improve the outcome following surgical treatment for children with congenital pseudarthosis of the tibia and NF1.

Synergistic effects of genetic beta cell dysfunction and maternal glucose intolerance on offspring metabolic phenotype in mice.

AIMS/HYPOTHESIS: Diabetes in pregnancy is linked to development of obesity in the offspring, but the mechanisms are not fully understood. Gestational diabetes mellitus (GDM) occurs when beta cells are unable to compensate for the normal insulin resistance of late pregnancy. In this study, we used a murine model of beta cell dysfunction to examine the effects of maternal GDM on phenotype in male offspring with and without an inherited predisposition for beta cell dysfunction. METHODS: Beta cell-specific aryl-hydrocarbon receptor nuclear translocator-null (ßArnt) mice develop GDM from beta cell dysfunction. ßArnt and control female mice were used to induce GDM and non-diabetic pregnancies, respectively. RESULTS: Offspring from GDM pregnancies became spontaneously obese on a normal-chow diet. They were heavier than offspring from non-diabetic pregnancies, with increased body fat. Respiratory exchange ratio (RER) was higher, indicating decreased capacity to switch to lipid oxidation. Metabolic rate in GDM offspring was decreased prior to onset of obesity. The phenotype was more pronounced in ßArnt GDM offspring than in GDM offspring of control genotype, demonstrating an interaction between genotype and pregnancy exposure. ßArnt GDM offspring had increased hypothalamic neuropeptide Y (Npy) and decreased pro-opiomelanocortin (Pomc) expression. Weight, body fat, insulin sensitivity and RER in all mice, and hypothalamic Npy in ßArnt mice were significantly correlated with AUC of maternal late pregnancy glucose tolerance tests (p < 0.01), but not with litter size, maternal weight, triacylglycerol or pre-pregnancy glycaemia. CONCLUSIONS/INTERPRETATION: In ßArnt mice, exposure to GDM and inheritance of genetic beta cell dysfunction had additive effects on male offspring obesity; severity of the offspring phenotype correlated with maternal glycaemia.

Additive actions of the cannabinoid and neuropeptide Y systems on adiposity and lipid oxidation.

AIMS: Energy homeostasis is regulated by a complex interaction of molecules and pathways, and new antiobesity treatments are likely to require multiple pharmacological targeting of anorexigenic or orexigenic pathways to achieve effective loss of excess body weight and adiposity. Cannabinoids, acting via the cannabinoid-1 (CB1) receptor, and neuropeptide Y (NPY) are important modulators of feeding behaviour, energy metabolism and body composition. We investigated the interaction of CB1 and NPY in the regulation of energy homeostasis, hypothesizing that dual blockade of CB1 and NPY signalling will induce greater weight and/or fat loss than that induced by single blockade of either system alone. METHODS: We studied the effects of the CB1 antagonist Rimonabant on food intake, body weight, body composition, energy metabolism and bone physiology in wild-type (WT) and NPY knockout (NPY(-/-)) mice. Rimonabant was administered orally at 10 mg/kg body weight twice per day for 3 weeks. Oral Rimonabant was delivered voluntarily to mice via a novel method enabling studies to be carried out in the absence of gavage-induced stress. RESULTS: Mice with dual blockade of CB1 and NPY signalling (Rimonabant-treated NPY(-/-) mice) exhibited greater reductions in body weight and adiposity than mice with single blockade of either system alone (Rimonabant-treated WT or vehicle-treated NPY(-/-) mice). These changes occurred without loss of lean tissue mass or bone mass. Furthermore, Rimonabant-treated NPY(-/-) mice showed a lower respiratory exchange ratio than that seen in Rimonabant-treated WT or vehicle-treated NPY(-/-) mice, suggesting that this additive effect of dual blockade of CB1 and NPY involves promotion of lipid oxidation. On the other hand, energy expenditure and physical activity were comparable amongst all treatment groups. Interestingly, Rimonabant similarly and transiently reduced spontaneous and fasting-induced food intake in WT and NPY(-/-) mice in the first hour after administration only, suggesting independent regulation of feeding by CB1 and NPY signalling. In contrast, Rimonabant increased serum corticosterone levels in WT mice, but this effect was not seen in NPY(-/-) mice, indicating that NPY signalling may be required for effects of CB1 on the hypothalamo-pituitary-adrenal axis. CONCLUSIONS: Dual blockade of CB1 and NPY signalling leads to additive reductions in body weight and adiposity without concomitant loss of lean body mass or bone mass. An additive increase in lipid oxidation in dual CB1 and NPY blockade may contribute to the effect on adiposity. These findings open new avenues for more effective treatment of obesity via dual pharmacological manipulations of the CB1 and NPY systems.

Peripheral neuropeptide Y Y1 receptors regulate lipid oxidation and fat accretion.

OBJECTIVE: Neuropeptide Y and its Y receptors are important players in the regulation of energy homeostasis. However, while their functions in feeding regulation are well recognized, functions in other critical aspects of energy homeostasis are largely unknown. To investigate the function of Y1 receptors in the regulation of energy homeostasis, we examined energy expenditure, physical activity, body composition, oxidative fuel selection and mitochondrial oxidative capacity in germline Y1(-/-) mice as well as in a conditional Y1-receptor-knockdown model in which Y1 receptors were knocked down in peripheral tissues of adult mice. RESULTS: Germline Y1(-/-) mice of both genders not only exhibit a decreased respiratory exchange ratio, indicative of increased lipid oxidation, but interestingly also develop late-onset obesity. However, the increased lipid oxidation is a primary effect of Y1 deletion rather than secondary to increased adiposity, as young Y1(-/-) mice are lean and show the same effect. The mechanism behind this is likely because of increased liver and muscle protein levels of carnitine palmitoyltransferase-1 (CPT-1) and maximal activity of key enzymes involved in beta-oxidation; beta-hydroxyacyl CoA dehydrogenase (betaHAD) and medium-chain acyl-CoA dehydrogenase (MCAD), leading to increased mitochondrial capacity for fatty acid transport and oxidation. These effects are controlled by peripheral Y1-receptor signalling, as adult-onset conditional Y1 knockdown in peripheral tissues also leads to increased lipid oxidation, liver CPT-1 levels and betaHAD activity. Importantly, these mice are resistant to diet-induced obesity. CONCLUSIONS: This work shows the primary function of peripheral Y1 receptors in the regulation of oxidative fuel selection and adiposity, opening up new avenues for anti-obesity treatments by targeting energy utilization in peripheral tissues rather than suppressing appetite by central effects.

The control of bone remodeling by neuropeptide Y receptors.

An important role for the neuropeptide Y receptor system in the regulation of bone formation was recently revealed with a significant elevation in trabecular bone formation and bone volume following germline or hypothalamus-specific deletion of neuropeptide Y2 receptors in mice. Subsequent studies have now demonstrated that this central pathway is distinct from that of the other centrally regulated bone formation pathway mediated by leptin. This review discusses these recent findings and outlines how these new pathways could translate into potential novel targets for the treatment of bone disease.

Genetic determinants of bone mass.

PURPOSE OF REVIEW: This review examines recent advances in the analysis of genetic determinants of bone mass. It addresses both human and animal linkage studies as well as genetic manipulations in animals, inbred mouse models, and candidate gene analyses. RECENT FINDINGS: Recent studies have implicated novel regulatory pathways in bone biology including both the neuroendocrine system and metabolic pathways linked to lipid metabolism. Variations in the lipoprotein receptor-related protein 5 (LRP5), part of the Wnt-frizzled pathway, were independently identified by linkage in high and low bone mass families. Subsequently, other high bone mass syndromes have been shown to have mutations in this gene. Neural studies have shown the skeletal regulatory activity of leptin and neuropeptide Y receptors via the hypothalamus. Subsequently, the beta-adrenergic pathway has been implicated, with important changes in bone mass. The lipoxygenase 12/15 pathway, identified through inbred mouse models and through pharmacologic studies with specific inhibitors, has also been shown to have important effects on bone mass. These studies exemplify the value of genetic models both to identify and then confirm pathways by mutational study and pharmacologic interventions. Continuing candidate gene studies often performed with multiple loci complement such discoveries. However, these studies have not focused on the clinical endpoint of fracture and few have included large enough groups to engender confidence in the associations reported, as such studies may require thousands of individuals. Interestingly, results often differ by ethnicity, age, or gender. A small proportion have examined whether relevant genes influence response to treatment. SUMMARY: The combinations of human and animal genetic linkage studies have advanced understanding of the regulation of bone mass. Studies ranging from linkage to pharmacology provide optimism for new targets and treatments for osteoporosis.

Targeted overexpression of vitamin D receptor in osteoblasts increases calcium concentration without affecting structural properties of bone mineral crystals.

Increased cross-sectional area and strength of long bones has been observed in transgenic mice with 2-fold (OSV9) and 3-fold (OSV3) elevation of osteoblast vitamin D receptor (VDR) levels. In the present study, mineralization density distributions, including typical calcium content (Ca(Peak)) and homogeneity of mineralization (Ca(Width)) of femoral bone and growth plate cartilage, were determined by quantitative backscattered electron imaging (qBEI). Fourier-transform infrared (FTIR) microspectroscopy was used to examine mineral content, collagen and crystal maturation, and scanning small angle X-ray scattering (scanning-SAXS) for studying mineral particle thickness and alignment. In addition, X-ray diffraction (XRD) of distal tibiae revealed mineral particle c-axis size. In trabecular bone, the increase in Ca(Peak) was significant for both OSV9 (+ 3.14%, P = 0.03) and OSV3 (+ 3.43%, P = 0.02) versus controls with 23.61 +/- 0.45 S.D. wt% Ca baseline values. In cortical bone, Ca(Peak) was enhanced for the OSV3 mice (+ 1.84%, P = 0.02) versus controls with 26.61 +/- 0.28 S.D. wt% Ca, and OSV9 having intermediate values. Additionally, there was significantly increased homogeneity of mineralization as denoted by a reduction of Ca(Width) (-8.4%, P = 0.01) in primary spongiosa. FTIR microspectroscopy, with the exception of an increased collagen maturity in OSV3 trabecular bone (+ 9.9%, P = 0.02), XRD, and scanning-SAXS indicated no alterations in the nanostructure of transgenic bone. These findings indicate that elevation of osteoblastic vitamin D response led to formation of normal bone with higher calcium content. These material properties, together with indications of decreased bone resorption in secondary spongiosa and increased cortical periosteal bone formation, appear to contribute to the improved mechanical properties of their long bones and suggest an important physiological role of the vitamin D-endocrine system in normal bone mineralization.

Increased formation and decreased resorption of bone in mice with elevated vitamin D receptor in mature cells of the osteoblastic lineage.

The microarchitecture of bone is regulated by complex interactions between the bone-forming and resorbing cells, and several compounds regulate both actions. For example, vitamin D, which is required for bone mineralization, also stimulates bone resorption. Transgenic mice overexpressing the vitamin D receptor solely in mature cells of the osteoblastic bone-forming lineage were generated to test the potential therapeutic value of shifting the balance of vitamin D activity in favor of bone formation. Cortical bone was 5% wider and 15% stronger in these mice due to a doubling of periosteal mineral apposition rate without altered body weight or calcium homeostatic hormone levels. A 20% increase in trabecular bone volume in transgenic vertebrae was also observed, unexpectedly associated with a 30% reduction in resorption surface rather than greater bone formation. These findings indicate anabolic vitamin D activity in bone and identify a previously unknown pathway from mature osteoblastic cells to inhibit osteoclastic bone resorption, counterbalancing the known stimulatory action through immature osteoblastic cells. A therapeutic approach that both stimulates cortical anabolic and inhibits trabecular resorptive pathways would be ideal for treatment of osteoporosis and other osteopenic disorders.

Discordance between bone turnover and bone loss: effects of aging and ovariectomy in the rat.

Mechanical strain maintains bone architecture even under conditions of increased bone turnover such as occurs with ovarian hormone deficiency. The rat distal femur contains two sites that apparently experience different levels of mechanical strain and therefore the rat is a suitable model for investigating such effects. The femoral epiphysis experiences higher strain energy compared with the metaphysis and we report the effects of aging between 7 and 12 months and the postovariectomy effects over the same time period on cancellous bone variables measured at these two sites. Age-related bone loss in sham-operated (Sham) animals occurred in both regions, with a greater fall in the metaphysis than in the epiphysis (trabecular bone volume [BV/TV, %] Mean [SEM] Metaphysis: day 0, 25. 9 [2.4]; day 150, 8.8 [1.3]: Epiphysis: day 0, 44.8 [1.7]; day 150, 36.7 [1.4] [p < 0.0001]). With ovariectomy (OVX) there was a 73% reduction in cancellous bone at the metaphysis compared with no specific loss at the epiphysis (BV/TV [%] OVX: Metaphysis: day 150, 2.4 [0.4] [p < 0.01 compared with Sham]: Epiphysis: day 150 29.3 [2. 7] [NS]). Osteoblast cell activity and osteoclast surface were increased after ovariectomy in both regions. The mineral apposition rate decreased at 9.5 months of age in both regions (p < 0.0001), independent of ovariectomy, and was coincident with a reduction in trabecular number in the epiphyses of both operative groups and in the metaphysis of the ovary-intact group. These data suggest that local mechanical strain governs bone balance with aging and that architectural changes resulting from age-related bone loss may mirror those following estrogen deficiency but occur via a different cellular mechanism.

Comparison of three methods for estimation of bone resorption following ovariectomy in the distal femur and the proximal tibia of the rat.

The study of estrogen-deficient bone loss requires accurate assessment of bone resorption; however, there has been considerable variance in the values reported for this variable. We compared three techniques for estimating bone resorption in the adult rat at four anatomical sites, the metaphysis and epiphysis of the distal femur and the proximal tibia. The techniques include an osteoclast-morphology-based method (VK-Oc), a bone-surface-based method (Pit-Oc), and an enzymatic method (AP-Oc). Thirty 6-month-old rats were either ovariectomized (ovx) or sham operated (sham) and killed at 0, 9, or 18 days postoperation. Each method was analyzed for variance in the ovary-intact groups and for the ability to detect the increase in osteoclast surface known to occur following ovariectomy. A 50-fold variation for the estimation of extent of osteoclast surface was entirely accounted for by these three methods for osteoclast estimation. The VK-Oc method was the most consistent for discriminating levels of osteoclast surface between ovary-intact and ovariectomized rats, detecting an increase at three of the four sites. There was no difference between the methods in their ability to produce consistent values for ovary-intact groups. The Pit-Oc method produced the largest numerical difference between the two operation groups with a 2.5-3-fold increase compared with 1.25-1.5-fold for the other methods (p < 0.001). However, the greater variance associated with this method limited the ability to detect the increase in osteoclast surface following ovariectomy. The AP-Oc method lacked the sensitivity of the other two methods. The oophorectomy-induced increase of osteoclast surface in the metaphysis of distal femur was larger and more consistently demonstrated than increases at the other sites.

Variation in the short-term changes in bone cell activity in three regions of the distal femur immediately following ovariectomy.

The effect of ovariectomy (OVX) on cancellous bone in the rat is not uniform at all sites of the skeleton. We report variation in the short-term effects of adult OVX in three regions of the distal femur: the diaphysis (DIA), the metaphysis (META), and the epiphysis (EPI). Cancellous bone parameters were estimated in the three separate zones of the femora and compared with changes in bone cell activity, as estimated by osteoclast surface (Oc.S) and bone formation rate (BFR). Changes were studied for 30 days in a series of rats either sham-operated (Sham) or ovariectomized (OVX) at 7 months of age. Oc.S and BFR were elevated following OVX in all regions. The time course for the OVX-induced changes differed between regions: DIA, both Oc.S and BFR were elevated at day 9; META, Oc.S was also elevated at day 9, while the rise in BFR was delayed until day 21; EPI, Oc.S remained stable but increased relative to ovary-intact rats by day 18 due to reduced levels in the latter, but BFR did not rise until day 28. These changes in bone cell activity following OVX produced a 71% reduction of cancellous bone in the DIA and a 35% reduction in the META. In contrast, no OVX-induced bone loss was observed in the EPI. This study shows that bone cell activity increases in each region of the distal femur within the first 30 days following OVX, independent of bone loss. However, the time course of increased bone cell activity is not uniform. These data highlight the role of local factors in the response to ovarian hormone deficiency.

Prepubertal oophorectomy limits the accumulation of cancellous bone in the femur of growing rats with long-term effects on metaphyseal bone architecture.

Bone loss after oophorectomy of adult rats is more rapid and complete in the metaphysis than in the epiphysis of the femur, particularly in the proximal region of the metaphysis distant from the growth plate. This study was undertaken to determine the effects of prepubertal oophorectomy, on femoral cancellous bone acquisition during growth. Rats were oophorectomized (OVX) or sham operated at 3 weeks of age and killed at intervals up to 78 weeks for scanning electron microscopy and histomorphometry of the distal femur. Differences in cancellous bone architecture between the two groups was evident after 6 weeks of age. Relatively minor differences were found in the part of the metaphysis near the growth plate and in the epiphysis, with less trabeculae in the primary spongiosa and 1 to 2 less trabeculae/mm in the secondary spongiosa. However, as metaphyseal growth proceeded, trabeculae were present for a greater distance up the femoral shaft in controls than in OVX rats, with mean BV/TV in the proximal part of the metaphysis increasing from 1.4% at 6 weeks to 13.4% at 20 weeks in controls, with no increase in the OVX rats. We find that the lack of ovarian hormones increases the rate of destruction of trabeculae near the metaphyseal-diaphyseal junction.