Effects of daily administration of prostaglandin E2 and its withdrawal on the lumbar vertebral bodies in male rats

Mechanosensation and transduction in osteocytes

The human skeleton is a miracle of engineering, combining both toughness and light weight. It does so because bones possess cellular mechanisms wherein external mechanical loads are sensed. These mechanical loads are transformed into biological signals, which ultimately direct bone formation and/or bone resorption. Osteocytes, since they are ubiquitous in the mineralized matrix, are the cells that sense mechanical loads and transduce the mechanical signals into a chemical response. The osteocytes then release signaling molecules, which orchestrate the recruitment and activity of osteoblasts or osteoclasts, resulting in the adaptation of bone mass and structure. In this review, we highlight current insights in bone adaptation to external mechanical loading, with an emphasis on how a mechanical load placed on whole bones is translated and amplified into a mechanical signal that is subsequently sensed by the osteocytes.

Different responsiveness to mechanical stress of bone cells from osteoporotic versus osteoarthritic donors

INTRODUCTION

Osteoporosis (OP) and osteoarthritis (OA) are both common diseases in the elderly, but remarkably seldom coexist. The bone defects that are related to both diseases develop with increasing age, which suggests that they are related to some form of imperfect bone remodeling. Current opinion holds that the bone remodeling process is supervised by bone cells that respond to mechanical stimuli. An imperfect response of bone cells to mechanical stimuli might thus relate to imperfect bone remodeling, which could eventually lead to a lack bone mass and strength, such as in OP patients.

MATERIALS

To investigate whether the cellular response to mechanical stress differs between OP and OA patients, we compared the response of bone cells from both groups to fluid shear stress of increasing magnitude. Bone cells from 9 female OP donors (age 60-90 year) and 9 female age-matched OA donors were subjected to pulsating fluid flow (PFF) of low (0.4+/-0.1 Pa at 3 Hz), medium (0.6+/-0.3 Pa at 5 Hz), or high shear stress (1.2+/-0.4 at 9Hz), or were kept under static culture conditions.

RESULTS

We found subtle differences in the shear-stress response of the two groups, measured as nitric oxide (NO) and prostaglandin E2 (PGE2) production. The NO-response to shear stress was higher in the OP than the OA cells, while the PGE2-response was higher in the OA cells.

CONCLUSIONS

Assuming that NO and PGE2 play a role in cell-cell communication during remodeling, these results suggest that slight differences in mechanotransduction might relate to the opposite bone defects in osteoporosis and osteoarthritis.

Prostaglandin E2 increases bone strength in intact rats and in ovariectomized rats with established osteopenia

It is well documented that prostaglandin E2 (PGE2) has the ability to stimulate bone formation, improve bone structure, and increase bone mass in intact or osteopenic rat models. However, the effects of PGE2 on the mechanical properties of bone have not been investigated previously. The purpose of our study was to determine the effects of PGE2 on the mechanical strength of bones in rapidly growing, adult, and ovariectomized rat models. In study I, PGE2 at 3 mg/kg per day, or vehicle, was given by daily subcutaneous injections for 30 days to rapidly growing (3-month-old) intact male rats. Compared with controls, PGE2 significantly increased initial maximal load and stiffness of cancellous bone at the distal femoral metaphysis (DFM) as determined by an indentation test. As determined by a compression test, rats treated with PGE2 showed a significant increase in maximal load, and a nonsignificant increase in stiffness in the fifth lumbar vertebral body (L5) when compared with controls. In study II, PGE2 at 3 mg/kg per day, or vehicle, was given by daily subcutaneous injection for 30 days to mature (10-month-old) intact male rats. PGE2 treatment significantly increased initial maximal load and stiffness of the DFM and L5. PGE2 induced a significant increase in maximal load, but not stiffness, in the femoral neck (FN), as determined by a cantilever compression test. There was an increase in maximal load in a three-point bending test at the femoral shaft (FS) although the increase did not achieve statistical significance. No change in stiffness in the FS was found after PGE2 treatment. In study III, 3-month-old female rats were sham-operated or ovariectomized (ovx) for 30 days. Thereafter, PGE, at 1 or 3 mg/kg, or vehicle, were given by daily subcutaneous injection to these rats for 30 days. After 30 and 60 days, ovx induced a significant decrease in initial maximal load and stiffness of cancellous bone at the DFM as compared with sham controls. In ovx rats with established osteopenia, PGE2 at 1 mg/kg per day nonsignificantly increased the initial maximal load and stiffness, whereas, at 3 mg/kg per day, PGE2 completely restored the initial maximal load and stiffness of DFM to sham control levels. Similarly, maximal load and stiffness of L5 decreased significantly in ovx rats compared with sham controls at 30 days postsurgery. PGE2 at 1 mg/kg per day partially restored the maximal load, whereas, at 3 mg/kg per day, it completely restored the maximal load and stiffness of L5 in the established osteopenia, ovx rats. At the FS, PGE2 at 3 mg/kg per day nonsignificantly increased maximal load (+11%) and significantly increased stiffness (+25%) compared with ovx controls. Neither ovx nor PGE2 treatment caused a significant change in the maximal load and stiffness of the FN in this study. These results reveal that PGE2 significantly increased the mechanical strength at various skeletal sites in rapidly growing and mature male rats, although the increase in femoral shafts was not statistically different. Furthermore, PGE2 completely restored mechanical strength to the cancellous bone in ovx rats with established osteopenia.

Effects of prostaglandin E2 and F2 alpha on the skeleton of osteopenic ovariectomized rats

This article contains the histomorphometric evaluation of the effects of prostaglandin F2 alpha (PGF2 alpha) on cancellous bone from the lumbar vertebra and cortical bone from the tibial shaft of ovariectomized, osteopenic rats. These effects were then compared with those of prostaglandin E2 (PGE2). Three-month-old rats were either ovariectomized (ovx) or sham-ovx. Then, either PGF2 alpha or PGE2 in doses of 1 and 3 mg/kg/day was given subcutaneously for 21 days at 150 days post ovx. Histomorphometric analysis was performed separately on both the primary and secondary spongiosae of the fourth lumbar vertebral bodies (LVB) and on tibial shafts. The ovx rats exhibited osteopenia in both primary (-23% to -37%) and secondary (-20%) spongiosae of the LVB, but not in the tibial shafts at 150 and 171 days post ovx. In the LVB, PGE2 in doses of 1 or 3 mg/kg/day for 21 days restored trabecular bone volume to the levels of sham-ovx controls in the primary spongiosa. However, in the secondary spongiosa, the treatments only thickened the trabeculae. The effects of the PGF2 alpha treatment were similar to those of the PGE2 in both the primary and the secondary spongiosae. While both PGF2 alpha and PGE2 treatments stimulated bone formation in the LVB as indicated by the increases in labeled perimeter, tissue and bone area-based bone formation rates, PGE2 is about 10 times more potent than PGF2 alpha in these effects. The PGE2 treatment also elevated activation frequency in the LVB, while the PGF2 alpha treatment did not. The treatments differed in that PGE2 at these dose levels did not alter the eroded surface in the LVB while PGF2 alpha decreased it significantly. Thus, the increase of the ratio of labeled to eroded perimeter in the LVB in PGF2 alpha-treated animals was much more than that in PGE2-treated animals. In the tibial shafts, PGE2 in doses of 1 and 3 mg/kg/day produced new marrow trabeculae in 2 of 6 and 3 of 6 of the ovx rats. However, no new trabecula was found in PGF2 alpha-treated tibial shafts. Higher doses of PGE2 also increased periosteal labeled perimeter, MAR, and BFR/BS, while PGF2 alpha did not produce any significant change in these parameters. Both PGE2 and PGF2 alpha in doses of 1 and 3 mg/kg/day increased the labeled perimeter, MAR and BFR/BS and decreased the eroded perimeter in the endocortical surface. We concluded that both PGF2 alpha and PGE2 in doses of 1 and 3 mg/kg/day for 21 days exhibited anabolic bone effects. The effects were mostly confined to an increase in trabecular volume in the primary spongiosa of the LVB and in the endocortical surface of tibial shafts. The tissue level mechanism behind this appears to be that PGE2 and PGF2 alpha can both stimulate osteoblast recruitment and activity. Overall, we found PGE2 to be more potent than PGF2 alpha at the same dose level at the endocortical surface. Furthermore, new marrow trabecular bone formed only after PGE2 treatment. PGF2 alpha differed from PGE2 by significantly reducing the trabecular eroded surface in ovx rats.

Intermittent treatments of prostaglandin E2 plus risedronate and prostaglandin E2 alone are equally anabolic on tibial shaft of ovariectomized rats

Effects of intermittent administration of prostaglandin E2 (PGE2), risedronate (Ris) and their combination on bone mass were studied on the cortical bone of tibial shafts of ovariectomized (ovx) rats. Six month old ovx rats were treated immediately after operation with subcutaneous injections of 6 mg PGE2/kg/d, 5 micrograms Ris/kg/2x/wk or their combination for 60 days each of an on/off/on cycle. PGE2 alone and in combination with Ris added the same amount of new bone in the first 60 days on period. During the 60 days off period, newly added endocortical and marrow trabecular bone disappeared in PGE2 alone treatment groups. In co-treatment group, the marrow trabeculae were only partly lost. There was no difference in total bone area between co-treatment and PGE2 alone groups after another 60 days on treatment. Our findings indicate that co-treatment was better in the maintenance of newly added endocortical and marrow trabecular bone during the off period; however, it formed less bone than PGE2 alone during the second treatment period, and ended up with the same amount of bone with both treatments.

Risedronate plus prostaglandin E2 is superior to prostaglandin E2 alone in maintaining the added bone after withdrawal in a non-growing bone site in ovariectomized rats

Effects of risedronate and prostaglandin E2 (PGE2) alone or in combination on the distal tibia, a non-growing bone site with closed epiphysis at 3 months of age, were studied in ovariectomized (ovx) rats. Six-month-old Sprague-Dawley female rats were either ovx or sham-ovx. Rats were treated immediately after operation either with risedronate (5 micrograms/kg/2x/wk), PGE2 (6 mg/kg/d), or risedronate+PGE2 for 60 days (on-groups) and followed by 60 days without treatment (off-groups). Trabecular area, width and numbers were determined in metaphyseal cancellous bone of the distal tibia. No significant bone loss or structural changes were observed in the distal tibial metaphysis after 120 days of ovx. Risedronate alone did not produce any effect on bone mass during the treatment and the withdrawal periods. PGE2 alone increased the trabecular bone mass associated with thickened trabeculae and increased trabecular numbers. However, some of the newly formed bone was lost at the end of 60 days withdrawal. Combination of risedronate and PGE2 treatment added the same amount of bone mass as PGE2 alone, and the added new bone was maintained during the 60 days withdrawal. These results indicate that treatment with risedronate and PGE2 can preserve the anabolic effect of PGE2 on bone mass for at least 60 days after treatment.

Maintenance therapy for added bone mass or how to keep the profit after withdrawal of therapy of osteopenia

Since continuous therapy for osteoporosis can be expensive and may have detrimental effects, there is a need to develop a strategy to maintain bone mass after withdrawal of treatment. The bone maintained by estrogen and calcitonin therapies and exercise, but the added bone induced by anabolic agents disappears upon cessation of treatment. To avoid this pitfall, the concepts of activation, restore and maintain (ARM) or loss, restore and maintain (LRM), the on/off administration of combined anabolic agent with an antiresorptive or antiactivation agent, and cyclical treatment of the two regimes have been employed successfully in "keeping the profit" (maintaining bone) in preclinical studies. The data for the disappearance of bone upon cessation of certain osteopenic treatments, its mechanism of loss and the development of maintenance concept and subsequent preclinical studies indicate that there was no need for costly continuous therapy in the treatment strategy for osteoporosis.

Prostaglandin E2 administration prevents bone loss induced by orchidectomy in rats

The objects of this study were to investigate whether prostaglandin E2 (PGE2) can prevent orchidectomy (ORX)-induced cancellous bone loss in growing male rats, and to determine the differential effects of PGE2 on sham-operated (sham) and ORX male rats. Fourteen-week-old Sprague-Dawley male rats were divided into groups of basal, vehicle-treated sham, PGE2-treated sham, vehicle-treated ORX, and PGE2-treated ORX rats for either 3 or 9 weeks. PGE2 was given at 6 mg/kg body weight daily by subcutaneous injection. Static and dynamic cancellous bone histomorphometry were performed on double-fluorescent labeled undecalcified proximal tibial metaphyseal sections. No effect was observed by ORX on body weight or longitudinal bone growth rate when compared with sham-operated controls. However, androgen deficiency caused significant increases in percent eroded perimeter, mineral apposition rate, and bone turnover (bone-volume-referent-bone formation rate), which resulted in a significant decrease in trabecular bone number, increase in trabecular separation, and a nonsignificant decrease in trabecular bone area by 3 weeks of ORX. After 9 weeks of ORX, trabecular bone area and number were significantly decreased, and trabecular separation, percent eroded perimeter, and the index of bone turnover (bone-volume-referent-bone formation rate) remained significantly increased while the index of bone formation (tissue-volume-referent-bone formation rate) was nonsignificantly decreased when compared with sham controls. When 6 mg PGE2/kg/day was given for 3 and 9 weeks, similar anabolic effects were observed in sham and ORX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-dependent morphometric change in the lumbar vertebrae of male and female rats: comparison with the femur

The morphological parameters, bone area, marrow area, bone-to-bone+marrow ratio, periosteal-to-total bone surface ratio, and surface-to-volume ratio, were studied in the fourth and fifth lumbar vertebrae of male and female rats (Heiligenberg strain) between birth and the end of lifespan. With increasing age, the bone area and marrow area increased for all ages, whereas the bone-to-bone + marrow ratio, periosteal-to-total bone surface ratio, and surface-to volume ratio decreased during the first 150 days. Afterwards, the bone-to-bone + marrow ratio decreased, whereas the periosteal-to-total bone surface ratio and surface-to-volume ratio were nearly constant. Modelling data were measured by use of the vital labeling technique with calcein. From the stained bone area the bone formation and the bone resorption rates were calculated. The bone formation rate, about 8300%/year, was highest after birth and decreases continuously with increasing age to 14%/year. The bone resorption rate, about 1100%/year, was highest after birth and decreased continuously with increasing age to 9%/year, whereas for all ages the bone formation rate was higher than the bone resorption rate, which led to an increase in bone area. The values obtained for the lumbar vertebra are compared with literature data and with the corresponding data for the distal femur obtained under identical conditions.

Anabolic effect of high doses of human parathyroid hormone (1-38) in mature intact female rats

Adult female retired breeder Wistar rats were treated with placebo or with synthetic human parathyroid hormone (hPTH) fragment (1-38) by subcutaneous injection at doses ranging from 50 to 1000 micrograms/kg, 5 days per week, for 2-8 weeks. One group of rats received hPTH-(1-38), 400 micrograms/kg, 5 days per week for 4 weeks, followed by 4 weeks placebo. The effects on bone mass and structure were evaluated by histomorphometric analysis of cross sections of femoral metaphyses. The effects on bone formation were evaluated by measuring uptake of [3H]tetracycline into femora and by qualitative evaluation of histologic sections after labeling with calcein and tetracycline. All doses of hPTH-(1-38) were anabolic. Lamellar bone formation was stimulated on all surfaces, and cancellous bone mass was increased as a result of trabecular thickening. Small quantities of woven bone were also formed in marrow cavities after treatment with 400 or 1000 micrograms/kg. Treatment with 1000 micrograms/kg had an effect on measured parameters similar to that with 400 micrograms/kg, and there was no evidence for a diminution of the anabolic effect at high doses. Animals treated with hPTH-(1-38) for 4 weeks followed by 4 weeks placebo tended to have less bone than those that received hPTH-(1-38) for 8 weeks but had significantly more bone than placebo-treated rats. In conclusion, daily injection of hPTH-(1-38) for 2-8 weeks at doses ranging from 50 to 1000 micrograms/kg had an anabolic effect on both cortical and trabecular bone in the femur of mature female intact rats. After withdrawal of treatment, the bone formation rate fell below levels in untreated animals, but bone mass remained higher.

Local delivery of prostaglandin E1 induces periodontal regeneration in adult dogs

Formation of cementum, alveolar bone and periodontal ligament was produced in 18 sites on buccal surfaces of mandibular premolars and molars of 11 adult dogs near to sites of local delivery of prostaglandin E1 (PGE) for three weeks. Mineralizing bone and cementum were labelled with fluorescent dyes and polarizing microscopy showed periodontal ligament fibers between these new mineralized tissues. These observations extend recent demonstrations that local application of PGE causes formation of new bone on the mandible and suggest the potential for predictable, site-directed periodontal regeneration.

Prostaglandin E2 adds bone to a cancellous bone site with a closed growth plate and low bone turnover in ovariectomized rats

The objects of this study were to determine the responses of a cancellous bone site with a closed growth plate (the distal tibial metaphysis, DTM) to ovariectomy (OVX) and OVX plus a prostaglandin E2 (PGE2) treatment, and compare the site's response to previous findings reported for another site (the proximal tibial metaphysis, PTM). Thirty-five 3-month-old female Sprague-Dawley rats were divided into five groups: basal, sham-OVX, and OVX + 0, +1, or +6 mg PGE2/kg/d injected subcutaneously for 3 months and given double fluorescent labels before sacrifice. Cancellous bone histomorphometric analyses were performed on 20-microns-thick undecalcified DTM sections. Similar to the PTM, the DTM showed age-related decreases in bone formation and increases in bone resorption, but it differed in that at 3 months post-OVX, there was neither bone loss nor changes in formation endpoints. Giving 1 mg PGE2/kg/d to OVX rats prevented most age-related changes and maintained the bone formation histomorphometry near basal levels. Treating OVX rats with 6 mg PGE2/kg/d prevented age-related bone changes, added extra bone, and improved microanatomical structure by stimulating bone formation without altering bone resorption. Furthermore, after PGE2 administration, the DTM, a cancellous bone site with a closed growth plate, increased bone formation more than did the cancellous bone in the PTM.

Influence of age on cyclosporin A-induced alterations in bone mineral metabolism in the rat in vivo

Cyclosporin A (CsA) administered to actively growing young rats produces a high-turnover osteopenia. We investigated and compared the effect of CsA on the bone mineral metabolism in young rats with that of older rats, which have a lower rate of bone turnover. A group of 24 young (9 weeks) and 24 older (9 months) male Sprague-Dawley rats were orally administered 15 mg/kg of CsA or placebo daily for 24 days. Rats were weighed and serum assayed serially for bone gla protein (BGP), parathyroid hormone, ionized calcium, blood urea nitrogen, creatinine, and 1,25-dihydroxyvitamin D [1,25-(OH)2D]. After sacrifice, histomorphometric analysis was performed on undecalcified proximal tibial metaphysis with double-fluorescent labeling. Serum BGP levels were significantly elevated in both young and older rats administered CsA, and 1,25-(OH)2D levels were significantly elevated in CsA-treated young rats more than in older rats. Body weight was significantly reduced in CsA-treated older rats. There were mild but significant alterations in renal function in both groups receiving CsA. In the most comprehensive examination to date of the effects of CsA on bone histomorphometry, both young (-44%) and older rats (-20%) lost significant amounts of trabecular bone compared to their respective controls. Bone loss in young rats was mainly due to a reduced number of trabecular; older rats lost mainly trabecular thickness. Microanatomic nodal studies were consistent with these results. These data demonstrate that although cancellous bone loss induced by CsA is more marked in young rats, older rats with slower bone turnover are also at risk.

Anabolic responses of an adult cancellous bone site to prostaglandin E2 in the rat

The objects of this study were to determine: (1) the response of a non-growing cancellous bone site to daily prostaglandin E2 (PGE2) administration; and (2) the differences in the effects of daily PGE2 administration in growing (proximal tibial metaphysis, PTM) and non-growing cancellous bone sites (distal tibial metaphysis, DTM). Seven-month-old male Sprague-Dawley rats were given daily subcutaneous injections of 0, 1, 3 and 6 mg PGE2/kg per day for 60, 120 and 180 days. The static and dynamic histomorphometric analyses were performed on double-fluorescent labeled undecalcified distal tibial metaphyses (DTM). No age-related changes were found in static and dynamic histomorphometry of DTM cancellous bone between 7 and 13 months of age. The DTM of 7-month-old (basal controls) rats consisted of a 24.5 +/- 7.6%-metaphyseal cancellous bone mass, and a thick trabeculae (92 +/- 12 micron). It also had a very low tissue-base bone formation rate (3.0 +/- 7.3%/year). Exogenous PGE2 administration produced the following transient changes in a dose-response manner between zero and 60 days: (1) increased trabecular bone mass and improved architecture (increased trabecular bone area, width and number, and decreased trabecular separation); (2) increased trabecular interconnections; (3) increased bone formation parameters; and (4) decreased eroded perimeter. A new steady state with more cancellous bone mass and higher bone turnover was observed from day 60 onward. The elevated bone mass induced by the first 60 days of PGE2 treatment was maintained by another 60 and 120 days with continuous daily PGE2 treatment. When these findings were compared to those previously reported for the PTM, we found that the DTM was much more responsive to PGE2 treatment than the PTM. Percent trabecular bone area and tissue-based bone formation rate increased significantly more in DTM as compared to PTM after the 60 days of 6 mg PGE2 treatment. These observations indicate that a non-growing cancellous bone site is more responsive than growing bone site to long-term daily administration of PGE2.

Prostaglandin E2 increased rat cortical bone mass when administered immediately following ovariectomy

To investigate the effects of ovariectomy and the simultaneous administration of prostaglandin E2 (PGE2) on rat tibial shaft cortical bone histomorphometry, thirty-five 3-month-old female Sprague-Dawley rats were either ovariectomized (OVX), or sham ovariectomy (sham-OVX). The OVX rats were divided into three groups and treated with 0, 1 and 6 mg PGE2/kg/day for 90 days. The double fluorescent labeled undecalcified tibial shaft cross sections (proximal to the tibiofibular junction) of all the subjects were used for histomorphometry analysis. No differences in cross-sectional area and cortical bone area were found between sham-OVX and OVX controls, but OVX increased marrow area, intracortical porosity area and endocortical eroded perimeter. Periosteal and endocortical bone formation rates decreased with aging yet OVX prevented these changes. These OVX-induced increases in marrow area and endocortical eroded perimeter were prevented by 1 mg PGE2/kg/day treatment and added bone to periosteal and endocortical surfaces and to the marrow cavity. At the 6 mg/kg/day dose level, PGE2-treated OVX rats increased total tissue area, cortical bone area, marrow trabecular bone area, minimal cortical width and intracortical porosity area, and decreased marrow area compared to basal, sham-OVX and OVX controls. In addition, periosteal bone formation was elevated in the 6 mg PGE2/kg/day-treated OVX rats compared to OVX controls. Endocortical eroded perimeter increased from basal and sham-OVX control levels, but decreased from OVX control levels in the 6 mg PGE2/kg/day-treated OVX rats. Our study confirmed that ovariectomy does not cause osteopenia in tibial shaft cortical bone in rats, but it does stimulate endocortical bone resorption and enlarges marrow area. The new findings from the present study demonstrate that PGE2 prevents the OVX-induced increases in endocortical bone resorption and marrow area and adds additional bone to periosteal and endocortical surfaces and to marrow cavity to increase total bone mass in the tibial shaft of OVX rats when given immediately following ovariectomy.

Greater bone formation induction occurred in aged than young cancellous bone sites

We have determined the differences in the effects of continual prostaglandin E2 (PGE2) treatment in aged (non-growing) and young (growing) cancellous bone sites in 7-month-old Sprague-Dawley rats. The sites involved are the aged distal tibial metaphysis (DTM) with a closed epiphysis and the young proximal tibial metaphysis (PTM) with a slow growing, open epiphysis. The study involved rats treated with 0, 1, 3 or 6 mg PGE2/kg/d for 60, 120 and 180 days. Static and dynamic histomorphometry of percent trabecular area, and tissue-referent bone formation rate (BFR/TV) were determined in both DTM and PTM. In pretreatment controls, the secondary spongiosa of the two metaphyses contain the same amount of cancellous bone (11% in DTM vs. 13% in PTM), but markedly less bone formation in DTM (0.6%/y in DTM vs. 41.5%/y in PTM). After 60 days of 6 mg PGE2/kg/d treatment, %Tb.Ar was increased 607% in DTM and 199% in PTM, BFR/TV was increased to nearly 14 fold in DTM and only 5 fold in PTM. These results indicated the aged metaphysis of the DTM was much more responsive to PGE2 treatment than young, growing metaphysis of the PTM. The results of 120 and 180 days treatment did not significantly differ from 60 days treatment in both sites, indicating that the effect of continuous daily PGE2 treatment were in equilibrium after 60 days. We concluded that aged metaphysis was much more responsive to PGE2 treatment than young growing metaphysis.

Maintaining restored bone with bisphosphonate in the ovariectomized rat skeleton: dynamic histomorphometry of changes in bone mass

This experiment contains the crucial data for the Lose, Restore and Maintain (LRM) concept, a practical approach for reversing existing osteoporosis. The LRM concept uses ovariectomy (ox) to lose bone, an anabolic agent to restore bone mass and then switches to an antiresorptive agent to maintain bone mass. We ox'd or sham-ox'd rats for 150 days (Loss Phase), treated them with 6 mg PGE2/kg/d for 75 days to restore lost cancellous bone mass (Restore Phase) and then stopped PGE2 treatment and began treatment with 1 or 5 micrograms/kg Risedronate, a bisphosphonate twice a week for 60 days (Maintain Phase). During the Loss Phase, cancellous bone volumes of the proximal tibial metaphysis (PTM) in the ox'd rat fell to 19% of initial controls. During the Restore Phase, the PTM bone volume in ox'd rats doubled. However, when PGE2 treatment was stopped, the PGE2-induced cancellous bone disappeared. In contrast, 5 micrograms of Risedronate inhibited the bone loss and maintained it at the PGE2 treatment level. The key dynamic histomorphometry value for the restore (R) and maintenance (M) phases was the ratio of bone formation to resorption rates. The ratio was elevated to 5.8 in the R phase and depressed to 0.4 for no and 1 microgram Risedronate treated M phase and to a ratio of near unity of 1.1 for the 5 micrograms Risedronate treatment. These findings indicate that we were successful in maintaining the new PTM bone induced by PGE2 after discontinuing PGE2 by administering enough Risedronate, a resorption inhibitor.(ABSTRACT TRUNCATED AT 250 WORDS)

Prostaglandin E2 prevents ovariectomy-induced cancellous bone loss in rats

The object of this study was to determine whether prostaglandin E2 (PGE2) can prevent ovariectomy-induced cancellous bone loss. Thirty-five 3-month-old female Sprague-Dawley rats were divided into two groups. The rats in the first group were ovariectomized (OVX) while the others received sham operation (sham-OVX). The OVX group was further divided into three treatment groups. The daily doses for the three groups were 0, 1 and 6 mg PGE2/kg for 90 days. Bone histomorphometric analyses were performed on double-fluorescent-labeled undecalcified proximal tibial metaphysis (PTM). We confirmed that OVX induces massive cancellous bone loss (-80%) and a higher bone turnover (+143%). The new findings from the present study demonstrate that bone loss due to ovarian hormone deficiency can be prevented by a low-dose (1 mg) daily administration of PGE2. Furthermore, a higher-dose (6 mg) daily administration of PGE2 not only prevents bone loss but also adds extra bone to the proximal tibial metaphyses. PGE2 at the 1-mg dose level significantly increased trabecular bone area, trabecular width, trabecular node density, density of node to node, ratio of node to free end, and thus significantly decreased trabecular separation from OVX controls. At this dose level, these same parameters did not differ significantly from sham-OVX controls. However, at the 6-mg dose level PGE2, there were significant increases in trabecular bone area, trabecular width, trabecular node density, density of node to node, and ratio of node to free end, while there was significant decrease in trabecular separation from both OVX and sham-operated controls. The changes in indices of trabecular bone microanatomical structure indicated that PGE2 prevented bone loss as well as the disconnection of existing trabeculae. In summary, PGE2 administration to OVX rats decreased bone turnover and increased bone formation parameters resulting in a positive bone balance that prevented bone loss (in both lower and higher doses) and added extra bone to metaphyses of OVX rats (in higher dose). These findings support the strategy of the use of bone stimulation agents in the prevention of estrogen depletion bone loss (postmenopausal osteoporosis).