Anabolic effect of prostaglandin E2 on cortical bone of aged male rats comes mainly from modeling-dependent bone gain

In this study, prostaglandin E₂ (3 mg/kg per day) was administered to 20-month-old male Wistar rats for 10 and 30 days. Histomorphometric analyses were performed on double-fluorescent-labeled undecalcified tibial shaft sections. Thirty days of prostaglandin E₂ (PGE₂) administration increased bone formation rate/total bone surface from undetectable levels to 0.6 μm/day at the periosteal surface and from 0.5 to 2.1 μm/day at the endocortical surface. Endocortical osteoid surface area increased from 2% to 67% at day 10 and decreased to 6% at day 30; woven and lamellar bone formation started at day 0, but was most obvious at day 30, resulting in a 12% increase of total bone mass. The red to yellow marrow ratio was 0.2 in pretreatment controls, and increased to 1.6 by day 10 and 2.4 by day 30 with PGE₂ administration. Intracortical cavity number and area increased after 10 days of PGE₂ treatment, but with forming osteon number and area far exceeding those of resorption cavities at day 30. Endocortical modeling surface/endocortical surface was only 1.5%, and remodeling was 11.1% in pretreatment controls. PGE₂ treatment increased modeling to 24.5% in the 10 day group and 93.7% in the 30 day group, whereas remodeling remained unchanged at 10 days, and decreased to 6.2% at 30 days. Osteoprogenitor cells and osteoblasts could not be detected in pretreatment controls, but increased by day 10, and returned almost to control levels by 30 days. Our data indicate that PGE₂ induced periosteal and endocortical bone formation mainly by modeling-dependent bone gain, accompanied by increases in intracortical remodeling and red bone marrow, and a transient increase in the osteoprogenitor cells adjacent to the endocortical surface. These findings suggest that 20-month-old male Wistar rats were very responsive to the anabolic action of PGE₂ in the tibial shaft, a site consisting mainly of cortical bone and yellow marrow.     

Transient effects of subcutaneously administered prostaglandin E2 on cancellous and cortical bone in young adult dogs

The transient effects of prostaglandin E2 (PGE2) on cancellous and cortical bone in iliac crests and mid-tibial shafts of nine intact young adult dogs were evaluated following 31 days of treatment. Histomorphometric bone changes were characterized from in vivo fluorescent double-labeled undecalcified bone specimens. PGE2 caused an increase in cancellous bone remodeling evidence by increased in activation frequency; increased percent eroded and formation surfaces; increased mineral apposition and bone formation rates; and shortened resorption, formation, and total bone remodeling periods. Activated cancellous bone remodeling did not lead to decreased cancellous bone mass, indicating an imbalance between bone resorption and formation in favor of formation (activation----resorption----stimulated formation; A----R----F increases) at remodeling sites. The PGE2 treatment activated bone modeling in the formation mode (activation----formation; A----F) at the periosteal and endocortical surfaces and increased activation frequency of intracortical bone remodeling in the tibial shaft. Increased modeling activation converted quiescent bone surfaces to formation surfaces with stimulated osteoblastic activity (i.e., increased percent labeled periosteal and endocortical surfaces, mineral apposition rates, and woven and lamellar trabecular bone formation) leading to 9- to 26-fold increases in newly formed bone mass in subperiosteal, subendosteal, and marrow regions, compared to controls. However, increased intracortical bone remodelling elevated remodeling space (i.e., increased cortical porosity), producing a bone loss that partially offsets the bone gain. The combined events lead to a positive bone balance in PGE2-treated cortical bone, compared to a negative bone balance in control bones. Collectively our data suggest that in vivo PGE2 is a powerful activator of cancellous and cortical bone formation, which may be able to build a peak bone mass to prevent and/or correct the skeletal defects to cure osteoporosis.     

Effect of vitamin K2 on cortical and cancellous bones in orchidectomized and/or sciatic neurectomized rats.

We examined the effect of vitamin K2 on cortical and cancellous bones in orchidectomized and/or sciatic neurectomized rats. Ninety male Sprague-Dawley rats, 3 months of age, were randomized by stratified weight method into nine groups with 10 rats in each group: baseline control (BLC), age-matched intact control (IN), IN+vitamin K2 administration (K), orchidectomy (ORX), ORX+K, unilateral sciatic neurectomy (NX), NX+K, ORX+NX (ONX), and ONX+K. Vitamin K2 (menatetrenone) was administered orally twice a week at a dose of 30 mg/kg each. After 10 weeks of feeding, the tibial shaft and proximal tibia were processed for cortical and cancellous bone histomorphometric analyses, respectively. An ORX-induced reduction in maturation-related cortical bone gain and ORX-induced cancellous bone loss were attributable to increased endocortical and trabecular bone turnover, respectively. NX- and ONX-induced reductions in maturation-related cortical bone gain were attributable to decreased periosteal bone formation and increased endocortical bone turnover, while NX- and ONX-induced cancellous bone loss was attributable to increased bone resorption and decreased bone formation. ORX-induced cancellous bone loss was more pronounced when combined with immobilization. Vitamin K2 administration did not significantly alter any parameters in IN rats. Vitamin K2 administration in ORX rats suppressed endocortical bone resorption and trabecular bone turnover, retarding a reduction in maturation-related cortical bone gain and cancellous bone loss. This effect on cancellous bone loss was primarily because of prevention of a reduction of trabecular thickness. Vitamin K2 administration in NX and ONX rats suppressed bone resorption and stimulated bone formation (mineralization), with retardation of a reduction of trabecular thickness without any significant effect on cancellous bone mass, and suppressed endocortical bone resorption, retarding a reduction in maturation-related cortical bone gain. The present study provides evidence indicating that vitamin K2 has the potential to suppress bone resorption or bone turnover and/or stimulate bone formation in vivo in ORX and/or NX rats.     

Trabecular bone volume and microdamage accumulation in the femoral heads of women with and without femoral neck fractures

Prostaglandin E₂ (PGE₂) possesses significant anabolic properties when administered systemically (i.e., it increases bone formation and, consequently, bone mass). We recently characterized the effects of a 3 week administration of 6 mg/kg PGE₂ into young rats and showed it increases cortical and cancellous bone mass and mechanical strength in long bones and bone density in the calvaria. We also found that a single dose of PGE₂ induces the expression of early-response genes (c-fos, c-jun, and egr-1) in bone marrow cells within these two types of bone. These observations, together with findings by others of new cancellous bone formation in PGE₂-treated animals, suggested that recruitment of osteoblasts from their precursors is a major mechanism of the anabolic effect of PGE₂. To test this hypothesis directly, we injected PGE₂ (6 mg/kg) or vehicle into 4-week-old rats for 2 weeks and then assessed the osteogenic potential of bone marrow in an ex vivo culture system. Primary and first-passage bone marrow cultures were established in the presence of β-glycerophosphate, ascorbate, and dexamethasone, and osteogenic differentiation was measured by bone nodule formation and alkaline phosphatase activity. This regimen increased bone mass expressed as femoral ash weight by 4.7% and tibial cancellous bone area by 38.3%. Nodule formation at 21 days was increased in both primary and first-passage cultures from PGE₂-treated rats despite seeding of the same number of marrow cells. Alkaline phosphatase activity was elevated in both primary and first-passage cultures from PGE₂-treated rats beginning 6–10 days after culture initiation. Cell proliferation was only slightly elevated in cultures from PGE₂-treated rats. These data strongly suggest that in vivo administration of PGE₂ induces the proliferation or differentiation of osteoprogenitor cells in bone marrow, and this effect takes a major part in its anabolic effect in vivo.     

Prostaglandin E2 enhances cortical bone mass and activates intracortical bone remodeling in intact and ovariectomized female rats

To assess the efficacy of prostaglandin E2 (PGE2) in augmenting cortical bone mass, graded doses of PGE2 were subcutaneously administered for 30 days to seven-month old sham-ovariectomized (SHAM) and ovariectomized (OVX) rats. Both groups were operated at three months of age. Histomorphometric analyses of double fluorescent labeled tibial shafts were performed on basal control, OVX, and SHAM rats treated with 0, 0.3, 1, 3, and 6 mg PGE2/kg/d for 30 days. Baseline aging data showed increased cortical tissue and cortical bone area and reduced bone formation parameters at the periosteal and endocortical bone envelopes between three and eight months of age. The tibial shafts of OVX rats compared to SHAM controls showed elevated periosteal mineral apposition rate and endocortical bone formation parameters. PGE2 administration to OVX and SHAM rats increased cortical bone by the addition of new circumferential bone on the endocortical and periosteal surfaces, as well as woven cancellous bone in the marrow region. Stimulated osteoblastic recruitment and activity enhanced bone formation at all bone surfaces. The new bone was both lamellar and woven in nature. PGE2 treatment also activated intracortical bone remodeling (not seen in untreated eight-month old rats), creating a porous cortex. Thus, PGE2 administration activated cortical bone modeling in the formation mode (A----F), as well as intracortical bone remodeling (A----R----F). PGE2 administration to OVX rats resulted in more intracortical bone remodeling, periosteal bone formation, and new cancellous bone production than observed in PGE2 treated controls. The findings that PGE2 administration to OVX and intact female rats increases cortical bone mass, coupled with observations that mouse, rat, dog, and man respond similarly to PGE2, suggest that PGE2 administration may be useful in the prevention and treatment of postmenopausal osteoporosis.     

Effect of vitamin K2 on cortical and cancellous bone mass and hepatic lipids in rats with combined methionine-choline deficiency

The present study examined changes of cancellous and cortical bone in rats with combined methionine-choline deficiency (MCD). In addition, the effects of vitamin K2 on cortical and cancellous bone mass and hepatic lipids were investigated in rats with MCD. Six-week-old male Sprague-Dawley rats were randomized into three groups of ten, including an age-matched control (standard diet) group, an MCD diet group, and an MCD diet+vitamin K2 (menatetrenone at 30mg/kg/d orally, 5 times a week) group. After the one-month experimental period, histomorphometric analysis was performed on cortical and cancellous bone from the tibial diaphysis and proximal metaphysis, respectively, while histological examination of the liver was performed after staining with hematoxylin and eosin and Oil Red O. MCD rats displayed weight loss, diffuse and centrilobular fatty changes of the liver, and a decrease of the cancellous bone volume per tissue volume (BV/TV) and percent cortical area (Ct Ar) as a result of decreased trabecular, periosteal, and endocortical bone formation along with increased trabecular and endocortical bone resorption. Administration of vitamin K2 to rats with MCD attenuated weight loss, accelerated the decrease of cancellous BV/TV due to an increase of bone remodeling, and ameliorated the decrease of percent Ct Ar by increasing periosteal and endocortical bone formation. Vitamin K2 administration also prevented MCD-induced diffuse fatty change of the liver. These findings suggest a beneficial effect of vitamin K2 on cortical bone mass and hepatic lipid metabolism in rats with MCD. The loss of cancellous bone mass could possibly have been due to re-distribution of minerals to cortical bone.     

Low-dose estrogen treatment suppresses periosteal bone formation in response to mechanical loading

Estrogen and exercise influence cortical bone formation. Both affect bone during growth, but with complex interactions. We hypothesized that estrogen reduces the osteogenic response caused by exercise at the periosteal surface of bone, while it enhances bone formation on the endocortical surface. To test our hypothesis, 16 young (8 weeks old) male Sprague-Dawley rats were randomized into two groups: (1) low-dose 17- ethynylestradiol treatment + bone loading (EE₂) or (2) vehicle-treated + bone loading (vehicle). We applied controlled loading to the right ulna at a peak force of 17 N, 2 min/day, 3 days/week for 5 weeks to simulate exercise. The left nonloaded ulna served as an internal control for loading. Mechanical loading increased cortical area (7.7%) and bone mineral content (8%) in the vehicle-treated group (P < 0.05) but only slightly increased cortical area in the EE₂ group (P = 0.08). Histomorphometry showed 1 week of mechanical loading increased periosteal bone formation rate by 29% in the vehicle group and this response was reduced (P < 0.05) to only 15% in the EE₂ group. At the endocortical surface, there were no differences in the loading response between the vehicle and EE₂-treated groups. We conclude low-dose EE₂ suppresses the mechanical loading response on the periosteal surface of long bones, but had no effect on the loading response at the endocortical bone surface in growing male rats.     

Significance of time-course changes of serum bone markers after parathyroidectomy in patients with uraemic hyperparathyroidism.

BACKGROUND: The increase of bone mineral density in cortical bone after parathyroidectomy is smaller than that in cancellous bone. Changes of serum bone markers reflect those of bone metabolism both in cortical and cancellous bone after parathyroidectomy. The present study was undertaken to investigate changes of histomorphometric parameters of cortical and cancellous bone together and their correlation with those of serum bone markers. METHODS: Iliac bone biopsy was performed before and 1 week after parathyroidectomy in Group I (n = 13), and before and 4 and 12 weeks after in Group II (n = 11). Moreover, changes of histomorphometric parameters of the endocortical, intracortical and periosteal surfaces as well as in cancellous bone were monitored. Serum levels of intact parathyroid hormone and bone markers were measured simultaneously. Results. In cancellous bone, osteoclast surface (Oc.S/BS) decreased to 0% within 4 weeks after parathyroidectomy, while osteoblast surface (Ob.S/BS) transiently increased at 1 week, followed by a reduction at 4 weeks to levels below the pre-surgical level. In cortical bone, Oc.S/BS was not reduced to 0%, while a significant and temporary increase of Ob.S/BS was observed only on the endocortical and intracortical surfaces at 4 weeks, but not at 1 week. Serum bone resorption markers did not completely disappear and significant and sustained increases of bone formation markers were observed until 4 weeks after parathyroidectomy. CONCLUSIONS: Changes of bone formation markers lagged behind those of histomorphometric parameters in cancellous bone because changes of cortical bone were observed later and were incomplete compared with those of cancellous bone.     

Anabolic effect of prostaglandin E2 on cortical bone of aged male rats comes mainly from modeling-dependent bone gain

In this study, prostaglandin E₂ (3 mg/kg per day) was administered to 20-month-old male Wistar rats for 10 and 30 days. Histomorphometric analyses were performed on double-fluorescent-labeled undecalcified tibial shaft sections. Thirty days of prostaglandin E₂ (PGE₂) administration increased bone formation rate/total bone surface from undetectable levels to 0.6 μm/day at the periosteal surface and from 0.5 to 2.1 μm/day at the endocortical surface. Endocortical osteoid surface area increased from 2% to 67% at day 10 and decreased to 6% at day 30; woven and lamellar bone formation started at day 0, but was most obvious at day 30, resulting in a 12% increase of total bone mass. The red to yellow marrow ratio was 0.2 in pretreatment controls, and increased to 1.6 by day 10 and 2.4 by day 30 with PGE₂ administration. Intracortical cavity number and area increased after 10 days of PGE₂ treatment, but with forming osteon number and area far exceeding those of resorption cavities at day 30. Endocortical modeling surface/endocortical surface was only 1.5%, and remodeling was 11.1% in pretreatment controls. PGE₂ treatment increased modeling to 24.5% in the 10 day group and 93.7% in the 30 day group, whereas remodeling remained unchanged at 10 days, and decreased to 6.2% at 30 days. Osteoprogenitor cells and osteoblasts could not be detected in pretreatment controls, but increased by day 10, and returned almost to control levels by 30 days. Our data indicate that PGE₂ induced periosteal and endocortical bone formation mainly by modeling-dependent bone gain, accompanied by increases in intracortical remodeling and red bone marrow, and a transient increase in the osteoprogenitor cells adjacent to the endocortical surface. These findings suggest that 20-month-old male Wistar rats were very responsive to the anabolic action of PGE₂ in the tibial shaft, a site consisting mainly of cortical bone and yellow marrow.     

Intermittent parathyroid hormone therapy to increase bone formation

Clinical data suggested that parathyroid hormone (PTH) might be effective in improving bone mass in patients with osteoporosis, providing its resorptive effects, which are particularly marked at cortical sites, were kept under control. We reviewed the evidence that intermittent PTH therapy is a valid treatment option whose predominant effect is bone anabolism. In cell culture studies, PTH affected both bone formation and bone resorption, suggesting that the net result of PTH therapy may be either bone gain or bone loss depending on the dosage, mode of administration, bone site, and animal species. Histological studies established that intermittent PTH therapy was associated with an increase in trabecular bone and, importantly, with improvements in trabecular and cortical microarchitectural parameters that have not been reported with antiresorptive drugs. This anabolic effect of intermittent PTH therapy translates into increased biomechanical strength, despite the increase in endocortical porosity seen in humans and nonhuman primates. The biochemical response profile to intermittent PTH therapy in clinical trials indicated a phase of isolated anabolism followed by an overall increase in bone remodeling that predominantly affected bone formation, the result being a large increase in spinal bone mineral density as early as the first treatment year. Thus, intermittent PTH therapy exerts predominantly anabolic effects on bone.     

Comparative effects of orchidectomy and sciatic neurectomy on cortical and cancellous bone in young growing rats

 The purpose of the present study was to compare the effects of orchidectomy and sciatic neurectomy on cortical and cancellous bone in male rats. Fifty male Sprague-Dawley rats, 6 weeks of age, were randomized into five groups, with ten rats in each group: baseline control, age-matched intact control, orchidectomy (ORX), unilateral sciatic neurectomy (NX), and ORX + NX. After 8 weeks of feeding, the tibial shaft and proximal tibia were processed for cortical and cancellous bone histomorphometric analyses, respectively. ORX-induced reductions in maturation-related cortical and cancellous bone gains were attributable to decreased periosteal bone gain and increased trabecular bone resorption, respectively. NX- and ORX + NX-induced reductions in maturation-related cortical bone gain were attributable to decreased periosteal bone formation and increased endocortical bone turnover, while NX- and ORX + NX -induced reductions in maturation-related cancellous bone gain were attributable to increased bone resorption and decreased bone formation. NX more markedly reduced maturation-related cortical and cancellous bone gains than did ORX, and the ORX-induced reductions in maturation-related cortical and cancellous bone gains were more pronounced when combined with NX. The present study demonstrated differences in changes in cortical and cancellous bone following ORX and NX in young rats. The importance of mechanical loading, with or without testosterone deficiency, is emphasized in cortical and cancellous bone growth. Received: October 5, 2002 / Accepted: January 20, 2003 RID="*" ID="*" Offprint requests to: J. Iwamoto     

Changes in cortical width with bone turnover in the three different endosteal envelopes of the ilium in postmenopausal osteoporosis.

Histological indicators of bone turnover were compared in the three endosteal envelopes (cancellous, endocortical, and intracortical) of iliac bone specimens obtained from 82 osteoporotic women, to assess the correlation between bone turnover and bone volume in different remodeling sites. Although there was a significant but weak correlation between the mineral apposition rate (MAR), a histological indicator of bone formation at the basic multicellular unit (BMU) level, and the three endosteal envelopes, the bone formation rate corrected for bone surface (BFR/BS) and mineralizing surface (MS/BS), indicators of the rate of bone formation reflecting activation frequency, in the cancellous and endocortical envelopes was more closely related to the rate in the intracortical envelope. The endocortical BFR/BS and MS/BS were higher than the rate in the cancellous envelope (1.6-2.1 times and 2.0-2.4 times, respectively), indicating a higher turnover rate in the endocortical envelope. According to stepwise regression analysis of the significant determinants contributing to bone mass, several histological determinants relating to bone turnover were identified: (1) trabecular thickness (Tb.Th) was a positive determinant, whereas age and cancellous bone volume referent BFR (BFR/BV) were negatively correlated determinants of the cancellous bone volume (BV/TV) (R2 = 0.50, p < 0.001); and (2) the endocortical wall thickness (W.Th) of the given side and the cortical width (Ct.Wi) of the opposite side were positive determinants, whereas the cancellous osteoid surface (OS/BS), cancellous MAR, and endocortical eroded surface (ES/BS) of the given side were the negatively correlated determinants of the Ct.Wi of the thicker cortex (R2 = 0.62, p < 0.001). In the thinner cortex, the endocortical W.Th of the given side and Ct.Wi of the opposite side were only used as the positive determinants of the Ct.Wi of the given side (R2 = 0.55, p < 0.001). In addition: (3) a significant but weak correlation was found using the intracortical BFR/BV as a positively correlated determinant of the cortical porosity (Ct.Po) in the thicker cortex (R2 = 0.17, p < 0.01). Although these histological determinants do not fully explain the mechanisms of bone loss, an increased rate of bone turnover contributes to bone loss not only in the cancellous and intracortical envelopes, but also in the endocortical envelope, indicating increased endocortical bone resorption in osteoporosis.     

Continuous parathyroid hormone induces cortical porosity in the rat: effects on bone turnover and mechanical properties.

We examined the time course effects of continuous PTH on cortical bone and mechanical properties. PTH increased cortical bone turnover and induced intracortical porosity with no deleterious effect on bone strength. Withdrawal of PTH increased maximum torque to failure and stiffness with no change in energy absorbed. INTRODUCTION: The skeletal response of cortical bone to parathyroid hormone (PTH) is complex and species dependent. Intermittent administration of PTH to rats increases periosteal and endocortical bone formation but has no known effects on intracortical bone turnover. The effects of continuous PTH on cortical bone are not clearly established. MATERIALS AND METHODS: Eighty-four 6-month-old female Sprague-Dawley rats were divided into three control, six PTH, and two PTH withdrawal (WD) groups. They were subcutaneously implanted with osmotic pumps loaded with vehicle or 40 microg/kg BW/day human PTH(1-34) for 1, 3, 5, 7, 14, and 28 days. After 7 days, PTH was withdrawn from two groups of animals for 7 (7d-PTH/7d-WD) and 21 days (7d-PTH/21d-WD). Histomorphometry was performed on periosteal and endocortical surfaces of the tibial diaphysis in all groups. microCT of tibias and mechanical testing by torsion of femora were performed on 28d-PTH and 7d-PTH/21d-WD animals. RESULTS AND CONCLUSIONS: Continuous PTH increased periosteal and endocortical bone formation, endocortical osteoclast perimeter, and cortical porosity in a time-dependent manner, but did not change the mechanical properties of the femur, possibly because of addition of new bone onto periosteal and endocortical surfaces. Additionally, withdrawal of PTH restored normal cortical porosity and increased maximum torque to failure and stiffness. We conclude that continuous administration of PTH increased cortical porosity in rats without having a detrimental effect on bone mechanical properties.     

Simvastatin treatment partially prevents ovariectomy-induced bone loss while increasing cortical bone formation

Statins are commonly prescribed drugs that inhibit hepatic cholesterol synthesis and thereby reduce serum cholesterol concentrations. Some of the statins are thought to possess bone anabolic properties. Effects of statin on tibia, femur, and vertebral cortical and cancellous bone were studied in ovariectomized (OVX) rats. Sixty Wistar female rats, 4 months old, were allocated into four groups: baseline control, sham + placebo group, OVX + placebo, OVX + simvastatin. Simvastatin, 20 mg/kg, or placebo was given twice daily by a gastric tube for 3 months. The rats were labeled with tetracycline at day 11 and calcein at day 4 before sacrifice. Concerning cortical bone, the tibial diaphysis bending strength was increased by 8% and the periosteal bone formation rate (BFR) at the mid-diaphysis increased by twofold in the OVX + simvastatin group compared with the OVX + placebo group, in harmony with increased serum osteocalcin concentrations. Simvastatin did not affect the endocortical bone formation. Concerning cancellous bone, the cancellous bone volumes in the proximal tibia and vertebral body were reduced in both OVX groups, but the reduction was less in the OVX + simvastatin group compared with the OVX + placebo group. This reduction in cancellous bone loss is in agreement with the 36% decreased activity of serum tartrate-resistant-acid-phosphatase 5b (TRAP-5b), indicating decreased osteoclast activity in the OVX + simvastatin group compared with the OVX + placebo group. In conclusion, simvastatin induces a moderate increase in cortical bone formation at the periosteal bone surface. The new cortical bone exhibits a normal lamellar structure, and simvastatin seems to respect the regional pattern of bone formation, bone resorption, and drift; for example, no periosteal bone formation is observed in the vertebral canal. Furthermore, simvastatin reduces the loss of cancellous bone induced by ovariectomy.     

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. Received: 31 December 1995 / Accepted: 3 May 1996     

1 Alpha-hydroxyvitamin D2 and 1 alpha-hydroxyvitamin D3 have anabolic effects on cortical bone, but induce intracortical remodeling at toxic doses in ovariectomized rats

It is well established that vitamin D metabolites have anabolic properties on cancellous bone in rats. However, few data are available on cortical bone effects of vitamin D metabolites. In this study, we examined the effects of the synthetic vitamin D analogs 1alpha-hydroxyvitamin D2 (1alpha(OH)D2) and 1alpha-hydroxyvitamin D3 (1alpha(OH)D3) on cortical bone of the tibial shaft in ovariectomized (OVX) rats using bone histomorphometry. Six-month-old Fischer 344 rats were either OVX or sham-operated (SHAM). OVX rats received vehicle, 1alpha(OH)D2 or 1alpha(OH)D3 orally via the diet in a dose range from 0.025 to 0.2 microg/kg/day. All animals were killed 3 months postsurgery after in vivo fluorochrome labeling. Relative to SHAM rats, vehicle-treated OVX rats showed a reduction in cortical bone area (%) due to expansion of the marrow cavity. Treatment of OVX rats with either 1alpha(OH)D2 or 1alpha(OH)D3 dose-dependently decreased marrow area, and increased cortical area, periosteal perimeter, and periosteal and endocortical bone formation rate compared with OVX vehicle controls. Interestingly, OVX animals receiving the highest doses showed intracortical resorption cavities, a phenomenon only exceptionally observed in rats. The intracortical hole area was significantly lower in 1alpha(OH)D2-treated compared with 1alpha(OH)D3-treated rats. We conclude that 1alpha(OH)D2 and 1alpha(OH)D3 prevent cortical bone loss in OVX rats and have anabolic effects on cortical bone at higher doses. However, very high, toxic doses of both vitamin D analogs induce intracortical remodeling as an untoward side effect.     

Comparative morphometric changes in rat cortical bone following ovariectomy and/or immobilization

Gonadal hormone deficiency following ovariectomy and skeletal unloading by limb immobilization are useful models of osteopenia. The purpose of this study was to compare changes in cortical bone after ovariectomy (OVX) or immobilization (IMM) for 6 and 12 weeks. Comparisons were also made when rats were ovariectomized or immobilized for 6 weeks and then immobilized (OVX/IMM) and ovariectomized (IMM/OVX), respectively, for 6 more weeks. Tibias and femurs were collected and static and dynamic cortical bone indices were determined by morphometric methods. Femurs from animals OVX or IMM for 12 weeks were tested for bone stiffness by torsional testing. Six and 12 weeks after OVX, there were increases in the periosteal perimeter, cortical area, and periosteal bone formation indices, indicating that ovariectomy increased modeling-dependent bone gain on the periosteal envelope, relative to controls. Contrarily, 6 and 12 weeks after IMM, there were decreases, compared with controls, in periosteal perimeter, cortical bone area, and periosteal bone formation indices. This indicates that immobilization decreased modeling-dependent bone gain on the periosteal envelope. These differences in modeling between the animals that were OVX and IMM resulted in a smaller cortical width and minimum cortical width in the IMM compared with the OVX animals. There were significant decreases in cortical bone stiffness and minimum cortical width at the fracture site following mechanical testing in the animals IMM for 12 weeks. Both ovariectomy and immobilization increased endocortical resorption surface, endocortical perimeter and expansion of the marrow cavity. Because of suppressed periosteal bone formation with increased endocortical resorption, immobilization had a greater effect on bone loss and decreased bone stiffness than did ovariectomy. In the OVX/IMM or IMM/OVX groups, there were changes that reflected both conditions. Immobilization mitigated the increase in periosteal bone formation but tended to augment endocortical resorption following ovariectomy. These results show that ovariectomy and immobilization have envelope-specific effects on rat cortical bone.     

Parathyroid hormone and mechanical usage have a synergistic effect in rat tibial diaphyseal cortical bone.

Previous reports showed that bone mass and architecture only partially recovered by remobilization (RM) after immobilization (IM)-induced osteopenia, and that parathyroid hormone (PTH) had an anabolic effect on the skeleton. The aim of this study was to determine whether low doses of PTH could restore IM-induced cortical bone loss and whether a combination of PTH plus loading (RM) treatment would be more effective than the PTH in unloaded (IM) limbs. One hundred and sixty 6-month-old rats were divided into aging and IM groups. The right hindlimb of the rat was immobilized by elastic bandage for 18 weeks, and then groups of rats were either kept IM or RM and treated with 30 microgram or 80 microgram of hPTH(1-38)/kg/day for 2, 10, and 20 weeks. Fluorescent-labeled, undecalcified cross-sections of right tibial shafts were studied. We found that RM for 20 weeks after 18 weeks of IM only partially recovered IM-induced muscle weight loss and PTH had no effect on muscle weight in either IM or RM limbs; that RM for 20 weeks after 18 weeks of IM partially restored some minimal cortical width by stimulating periosteal and endocortical bone formation and decreasing endocortical resorption; that PTH treatment of IM limbs completely restored IM-induced cortical bone loss and added extra bone by stimulating bone formation indices on all bone surfaces and depressing bone resorption on endocortical surface; that PTH treatment of RM limbs produced similar anabolic effects as in IM limbs with 30 microgram/kg/day dose but the 80 microgram/kg/day dose-treated limbs had a higher periosteal bone formation rate, which created a larger cross-sectional area, more cortical bone area, and a thicker cortex than the same dose treated IM limbs; and that PTH 80 microgram/kg/day treatment produced more anabolic effect than the 30 microgram/kg/day in both IM and RM limbs. We concluded that reloading the hindlimb by RM after long-term IM could not recover the cortical bone mass. PTH at employed doses was able to completely restore IM-induced cortical bone loss, and this effect was independent of mechanical stimulation. However, when PTH was combined with mechanical loading (RM), a synergistic anabolic effect on periosteal bone formation occurred which increased the cross sectional area that can increase bone strength.     

Skeletal actions of intermittent parathyroid hormone: effects on bone remodelling and structure

Intermittent administration of parathyroid hormone peptides has anabolic skeletal effects and reduces fracture risk in postmenopausal women with osteoporosis but the cellular and structural mechanisms by which these effects are mediated have not been fully established. In cancellous and endocortical bone, there is evidence that both modelling and remodelling-based formation contribute to the increase in bone mass although the contribution of these at different time points in the response to PTH has not been established. Despite the large increase in spine bone mineral density, however, significant increases in iliac crest cancellous bone volume and trabecular thickness have not been consistently demonstrated, possibly reflecting site-specific differences in PTH-induced skeletal effects and/or the large sampling and measurement variance associated with assessment of iliac crest cancellous bone volume and structure. In iliac crest cortical bone, increased cortical thickness has been demonstrated, due at least in part to increased endosteal bone formation; there is also some evidence for increased formation on periosteal surfaces. At some sites an increase in cortical porosity may also occur and the overall effects on cortical bone strength, particularly at the hip, remain to be established. Studies in iliac crest bone indicate a trend towards a lower mineralisation density of bone matrix and increased heterogeneity of mineralisation, consistent with new bone formation. In addition, there is a reduction in mineral crystallinity and a shift towards more divalent collagen cross-links, indicating a change towards a younger bone profile.

The potential clinical implications of these effects on bone are currently unknown. The stimulatory effect of PTH peptides on bone formation may favour their use in low turnover bone disease and in states of advanced bone loss. Furthermore, if beneficial effects on cortical bone strength are confirmed, efficacy at non-vertebral sites might be superior to those observed with antiresorptive drugs. Better definition of the effects of intermittent PTH administration on cancellous and cortical bone remodelling and structure at different skeletal sites may inform these speculations and is an important area for future research.     

Human parathyroid hormone-(1-34) prevents bone loss and augments bone formation in sexually mature ovariectomized rats

A group of 3-month-old Sprague-Dawley rats were sham operated or ovariectomized and given daily injections of human PTH-(1-34) (8 or 16 micrograms per 100 g body weight) for 5 weeks. At the termination of the study histomorphometric techniques were used to examine changes in cortical and cancellous bone in the diaphysis and proximal metaphysis of the tibia. Ovariectomy resulted in a 50% decrease in cancellous bone that was accompanied by a 41 and 120% increase in osteoclasts and osteoblasts, respectively. In contrast, in the ovariectomized animals treated with PTH, the metaphyseal cancellous bone increased by over 300% to a level in excess of that present in the sham-operated control animals. The increase in cancellous bone induced by PTH was associated with an over 70% increase in osteoblasts and tetracycline-labeled area and an unexpected decrease in trabecular osteoclasts. In the tibial diaphysis PTH also decreased endosteal osteoclasts and at the same time increased osteoblast size and number as well as endosteal and periosteal bone formation; ovariectomy increased only periosteal bone formation. Our findings demonstrate that intermittent administration of PTH prevents ovariectomy-induced bone loss and augments cancellous and cortical bone formation in sexually mature ovariectomized rats. Although the basis of the bone anabolic action of PTH remains elusive, our data indicate that it may involve the uncoupling of bone formation and resorption such that the latter is inhibited as bone formation is enhanced. Our findings are also compatible with the view that intermittent administration of PTH increases bone mass, in part by stimulating the proliferation and differentiation of osteoblast progenitors while inhibiting osteoclast proliferation.