Alfacalcidol treatment increases bone mass from anticatabolic and anabolic effects on cancellous and cortical bone in intact female rats

It has been reported that alfacalcidol had an anticatabolic and anabolic effect on bone in ovariectomized and aged male rat models, but this has not been tested on intact female rats. The current study was to determine the effects of alfacalcidol on cancellous and cortical bone in intact female rats with or without exercise. Seventy-four, 8.5-month-old, intact female rats were orally treated with 0, 0.005, 0.025, 0.05, or 0.1 microg/kg alfacalcidol alone or in combination with raised cage (RC) exercise for 3 months. In vivo peripheral quantitative computerized tomography (pQCT) of the proximal tibial metaphyses (PTM) and ex vivo histomorphometric analyses of the PTM and tibial shaft (TX) were performed. Only the 0.1 microg alfacalcidol/kg dose proved to be anabolic. pQCT analysis showed that this dose increased total and cortical bone mineral content and density and trabecular bone mineral density. Histomorphometrically, it induced an anabolic response by increased trabecular mass and microarchitecture from stimulated cancellous bone and bone bouton formations, and suppressed bone resorption more than bone formation on the trabecular and endocortical surfaces, to produce a positive bone balance. A positive correlation between trabecular connectivity and bone bouton numbers occurred. These findings suggest alfacalcidol treatment augments bone mass by increased cancellous bone mass and improved trabecular architecture through its anticatabolic and anabolic properties in the intact adult female rat. Last, raised cage exercise alone or the combination of raised cage and alfacalcidol was no more effective than alfacalcidol alone.

Review of 239Pu and 226Ra effects in beagles

A long term biological study has been completed that was designed to assess the predicted effects in humans of internally deposited 239Pu by comparison with 226Ra in beagles. Herein we summarize for the first time results of several previous reports about the effects of these two radionuclides in our beagles in an attempt to elucidate what has been learned since the beginning of the study in the early 1950's. Perhaps the most important finding was that bone surface-seeking plutonium is more toxic at equal mean skeletal radiation doses (<3 Gy for 239Pu, <20 Gy for 226Ra) than bone volume-seeking radium for the induction of skeletal malignancy by about a factor of 16 for a single intravenous injection of monomeric 239Pu. In addition, ancillary studies have shown that when plutonium transfers continuously onto bone surfaces from a depot of particulate 239Pu in phagocytic cells, its relative toxicity per Gy average skeletal dose is enhanced by about a factor of 2. Juvenile animals or dogs injected as mature adults were only about half as sensitive for equal mean skeletal doses as dogs injected as young adults. Male and female dogs were about equally sensitive to radiation of the skeleton by either radionuclide. Findings about radiation-induced fractures are summarized as well as data on the induction of soft-tissue malignancies by 239Pu or 226Ra. Natural survival was not affected at the lower dosage levels of either 226Ra or 239Pu as compared with control dogs given no radioactivity, but the survival of animals at higher levels was reduced. No additional life-shortening effects beyond those attributable to occurrence of radiation-induced malignancies or other radiation-induced effects were suggested by analysis of data for low dosage levels.

Synthesis and biological evaluation of prostaglandin-F alkylphosphinic acid derivatives as bone anabolic agents for the treatment of osteoporosis

A series of novel C(1) alkylphosphinic acid analogues of the prostaglandin-F family have been evaluated at the eight human prostaglandin receptors for potential use in the treatment of osteoporosis. Using molecular modeling as a tool for structure-based drug design, we have discovered that the phosphinic acid moiety (P(O)(OH)R) behaves as an isostere for the C(1) carboxylic acid in the human prostaglandin FP binding assay in vitro and possesses enhanced hFP receptor selectivity when compared to the parent carboxylic acid. When evaluated in vivo, the methyl phosphinic acid analogue (4b) produced a bone anabolic response in rats, returning bone mineral volume (BMV) [corrected], to intact levels in the distal femur in the ovariectomized rat (OVX) model. These results suggest that prostaglandins of this class may be useful agents in the treatment of diseases associated with bone loss.

Bipedal stance exercise enhances antiresorption effects of estrogen and counteracts its inhibitory effect on bone formation in sham and ovariectomized rats

In this study we employed a raised cage model in combination with estrogen to observe their effects on the proximal tibial metaphysis (PTM) and tibial shaft (TX) in sham-operated or ovariectomized rats. A total of 105 6-month-old female Sprague-Dawley rats were used in the study. Bilateral sham ovariectomy or ovariectomy was performed at day 0 and the rats were housed in normal height or raised cages (RCs) and injected subcutaneously twice per week with 10 microg/kg of 17beta-estradiol (E2) or vehicle for 4 and 8 weeks. Because the time course of bone loss or bone gain distribution was not uniform in the metaphyses of the tibia, we subdivided the PTM into three zones (medial, central, and lateral) to observe the different bone loss or bone gain patterns after ovariectomy and/or raised cages. We found that: (1) E2 alone did not alter bone area or architecture in sham rats, whereas RC alone increased trabecular thickness and area of PTM, but had no effects on TX; (2) Ovx induced most bone loss from the central zone of the PTM and endocortical surface of TX, accompanied by decreased trabecular number and increased bone resorption; (3) E2 alone prevented ovx-induced bone loss by preserving trabecular number and depressing bone resorption; (4) RC alone partially compensated for bone loss following ovx by thickening the surviving trabeculae in lateral and medial zones, and tended to stimulate bone formation and decrease bone resorption; and (5) RC plus E2 increased trabecular bone area by having an additive effect on bone resorption and bone turnover. RCs helped to prevent the depressive effect of estrogen on periosteal bone formation. In conclusion, early and rapid bone loss occurred in the central zone of the metaphysis and endocortical surface after ovx. Estrogen replacement therapy prevented this loss. Raised cages partially compensated for bone loss following ovx by thickening the trabeculae in the lateral area of the metaphysis and decreased endocortical erosion. Combination treatment added bone to the PTM and prevented the decrease of periosteal bone formation after estrogen administration.

Transforming growth factor-beta administration modifies cyclosporine A-induced bone loss

Cyclosporine A (CsA), a potent immunosuppressant used in transplantation, induces increased formation with excess resorption in the rat with resultant osteopenia. These findings are confirmed in the human model. Transforming growth factor-beta (TGF-beta) is reported to be involved in the coupling of bone formation with resorption and in vivo and in vitro stimulates osteoblasts, and in vitro inhibits osteoclasts. CsA stimulates secretion of TGF-beta1 in humans, which, while improving immunosuppression, may also contribute to renal toxicity. This study was performed determine whether exogenously administered TGF-beta would modify the bone effects of CsA. Male Sprague-Dawley rats, 6 months of age, were randomized to receive: TGF-beta and CsA vehicle (group A); TGF-beta 5 microg/kg three times per week and CsA vehicle (group B); TGF-beta vehicle and CsA 10 mg/kg (group C); or TGF-beta 5 microg/kg three times per week and CsA 10 mg/kg (group D). These were compared with control over 28 days. CsA, but not TGF-beta, increased serum 1,25(OH)(2)D levels throughout the study. CsA increased osteocalcin (BGP), but TGF-beta negated this effect. Histomorphometry confirmed the known effects of CsA, whereas TGF-beta alone had no effect. However, in combination, TGF-beta blocked CsA's effect and increased osteoblast recruitment and activity, as reflected by increased percent mineralizing surface, percent osteoid perimeter, bone formation rate (bone volume referent), and activation frequency. Thus, it appears as if TGF-beta administration may have potential in modulating the deleterious bone effects of CsA.

Effect of the interaction of parathyroid hormone and cyclosporine a on bone mineral metabolism in the rat

Cyclosporine A (CsA) induces high turnover osteopenia in the rat and there is evidence for this in humans. Recent studies suggest that increases in parathyroid hormone (PTH) may be involved in posttransplantation bone loss. However, human studies are difficult to interpret since transplant patients usually receive a cocktail of immunosuppressants and have underlying disease. Our aim was to try to resolve the influence of the absence or presence of PTH on CsA-induced bone disease. Male Sprague Dawley rats aged 7-9 months, either sham operated or parathyroidectomized (PTX), were randomly divided into vehicle and CsA groups. All PTX rats were given oral calcium supplementation ad libitum. The rats were divided into groups: basal, sham/vehicle, sham/CsA, PTX/vehicle, and PTX/CsA. Serial biochemistry was performed 0, 14, and 28 days after the start of the experimental period; bone histomorphometry was performed 28 days after the start of the experimental period. Statistical analysis consisted of group comparisons and factorial analyses. The results showed that CsA alone produced a high turnover osteopenia consistent with previous studies. In the PTX animals there was an increase in bone mass. PTX also decreased osteoblast activity and recruitment, and serum 1,25OH2D levels. Serum levels of osteocalcin (BGP) were unaffected by PTX. The combination group (PTX/CsA) did not differ statistically from the controls in most of the histomorphometric parameters measured, with the exception of reduced mineral apposition and bone formation rates, reflecting the effects of PTX. Serum BGP and 1,25OH2D levels did not differ, but PTH was reduced from the control. Explanations for these results are (1) CsA and PTX exert their effects via separate mechanisms, negating each other; (2) in the absence of PTH, CsA managed to cause bone loss, and thus PTH may not be essential for CsA-induced bone loss; or (3) the profound accelerated bone loss produced by CsA in normal rats requires PTH. These findings may help explain the discrepancies found in clinical studies where bone loss occurs with either elevated or normal PTH levels.

Overview: animal models of osteopenia and osteoporosis

Prior to initiating a clinical trial in a post-menopausal osteoporosis study, it is reasonable to recommence the evaluation of treatment in the 9-month-old ovariectomized female rat. A female rat of this age has reached peak bone mass and can be manipulated to simulate clinical findings of post-menopausal osteoporosis. Ample time exists for experimental protocols that either prevent estrogen depletion osteopenia or restore bone loss after estrogen depletion. More time can be saved by acceleration of the development of the osteopenia by combining ovariectomized (OVX) plus immobilization (IM) models. Methods like serum biochemistry, histomorphometry and densitometry used in humans are applicable in rats. Like most animal models of osteopenia, the rat develops no fragility fractures, but mechanical testing of rat bones substitutes as a predictor of bone fragility. Recent studies have shown that the prevailing activity in cancellous and cortical bone of the sampling sites in rats is remodeling. The problems of dealing with a growing skeleton, the site specificity of the OVX and IM models, the lack of trabecular and Haversian remodeling and the slow developing cortical bone loss have been and can be overcome by adding beginning and pre-treatment controls and muscle mass measurements in all experimental designs, selecting cancellous bone sampling sites that are remodeling, concentrating the analysis of cortical bone loss to the peri-medullary bone and combining OVX and IM in a model to accelerate the development of both cancellous and cortical bone osteopenia. Not to be forgotten is the distal tibia site, an adult bone site with growth plate closure at 3 months and low trabecular bone turnover and architecture similar to human spongiosa. This site would be most challenging to the action of bone anabolic agents. Data about estrogen-deplete mice are encouraging, but the ovariectomized rat model suggests that developing an ovariectomized mouse model as an alternative is not urgent. Nevertheless, the mouse model has a place in drug development and skeletal research. In dealing with drug development, it could be a useful model because it is a much smaller animal requiring fewer drugs for screening. In skeletal research mice are useful in revealing genetic markers for peak bone mass and gene manipulations that affect bone mass, structure and strength. When the exciting mouse glucocorticoid-induced bone loss model of Weinstein and Manolagas is confirmed by others, it could be a significant breakthrough for that area of research. Lastly, we find that the information generated from skeletal studies of nonhuman primates has been most disappointing and recommend that these expensive skeletal studies be curtailed unless it is required by a regulatory agency for safety studies.

Long-term effects of aging and orchidectomy on bone and body composition in rapidly growing male rats

The purpose of this study was to assess the long-term effects of aging and sex hormone deficiency on skeletal metabolism and body composition in rapidly growing male rats. Sprague-Dawley male rats were sham-operated (sham) or orchidectomized (ORX) at 3 months of age. Eight sham rats and eight ORX rats at each time point were serially sacrificed at 3, 4, 8, 12, 15, and 23 months of age. Bone mass in sham rats rapidly increased until 8 months of age, then slightly increased between 8 to 12 months of age; thereafter, an age-related decrease in bone mass was found between 12 to 23 months of age. In sham rats, bone formation parameters decreased between 3 and 8 months, and maintained at the lower level between 8 and 23 months of age, while bone resorption parameters decreased between 3 and 12 months, and thereafter, increased with age between 12 and 23 months of age. ORX significantly inhibited age-related gain in body weight, lean body mass, and cancellous and cortical bone mass and decreased peak bone mass (approximately 20% less versus sham). Further, we found that the lower bone and lean body mass in ORX rats was due to the lack of age-related gain rather than the net loss from basal controls. These data suggest that sex hormones are important factors for the accumulation of peak bone and lean body mass in male rats.

A novel method to 'exercise' rats: making rats rise to erect bipedal stance for feeding - raised cage model

We employed a novel method to exercise rats: making them rise to bipedal stance for feeding using raised cages. We studied its effects on the skeletons of 6 and 10-month-old intact or orchidectomized (ORX) rats. Body and hindlimb muscle weights, tibial BMC and periosteal cortical bone formation increased after housing in raised cages, but more so in 6-month-old animals than in 10-month-old ones. In 6-month-old orchidectomized rats, raised cages partially prevented ORX-induced bone loss by stimulating periosteal cortical bone (TX) formation and decreased bone resorption next to marrow. In 10-month-old male orchidectomized rats, raised cages also decreased the endosteal and trabecular bone resorption, but not enough to prevent completely ORX-induced net bone losses. Because the osteogenic effects of raised cages alone were only partial, we also studied the interaction between raised cage and prostaglandin E(2) (PGE(2)) in 10-month-old retired female breeders. When treated with combined raised cage and PGE(2), both cortical (TX) and trabecular bone mass of the proximal tibial metaphysis and lumbar vertebral body increased over either raised cages or PGE(2) treatment alone, that was accompanied by dramatic increased bone formation at periosteal and endosteal surfaces. Thus making rats rise to erect bipedal stance for feeding helps to prevent bone loss after orchidectomy; it amplifies the anabolic effects of PGE(2), and it provides an inexpensive, non-invasive and reliable way to increase mechanical loading of certain bones of the rat skeleton.

Lumbar vertebral cancellous bone is capable of responding to PGE2 treatment by stimulating both modeling and remodeling-dependent bone gain in aged male rats

Previously we found that PGE2 3 mg/kg in 20-month-old male rats induced massive bone formation mainly by modeling dependent bone gain in cortical bone. It is not known whether cancellous bone will respond similarly; thus, we evaluated the effect of PGE2 on cancellous bone of the same aged rats. Thirty-four 20-month-old Wistar male rats were given PGE2 (3 mg/kg/day) or vehicle subcutaneously for 10 and 30 days. Double fluorescent labels were injected 9 and 2 days prior to the sacrifice. Histomorphometry was performed on 1% toluidine blue stained and unstained sagittal sections of lumbar vertebral bodies. The results demonstrated that 10-day PGE2 treatment increased osteoprogenitor cells, osteoblasts (x 2-fold), osteoid (x 4.5-fold), woven bone formation (0.04%), and 40% more trabecular area; it stimulated modeling (x 2-fold) and remodeling-dependent (x 1.5-fold) bone formation with increase of mineralization lag time (MLT, x 7.5-fold). Thirty-day treatment sustained increases in osteoblast numbers, modeling and remodeling-dependent bone formation and further stimulated woven bone formation (6.6%), turnover (x 3-fold), and trabecular area and number (x 2-fold). Osteoprogenitor cells were undetectable along with 70% less osteoid area compared with 10-day treatment but still was 1.5-fold higher than aging controls. MLT returned to aging control level. It was concluded that the aged cancellous bone of 20-month-old male Wistar rat retains a capability of responding to the anabolic effect of PGE2. Osteoblastogenesis and enhanced modeling and remodeling-dependent woven or lamellar formation contribute to this anabolic action. Bone formation differed in that the endocortical surface of cortical bone was stimulated mainly by modeling whereas both modeling and remodeling-dependent bone gain were equally stimulated at the trabecular surface of the lumbar vertebral body.

Cancellous bone of aged rats maintains its capacity to respond vigorously to the anabolic effects of prostaglandin E2 by modeling-dependent bone gain

The present study examined the early effects of prostaglandin (PG)E2 on proximal tibial metaphyses of 20-month-old Wistar male rats. PGE, was given to intact rats for 10 and 30 days at 3mg/kg/day. After multiple in vivo fluorochrome labeling, undecalcified longitudinal sections were subjected to analysis of bone histomorphometry and classification of the contour of the cement line in bone formation units. The latter was used to classify bone formation units into modeling, remodeling and uncertain units. After 10 days of treatment, there was a 2% increase in woven bone formation with the appearance of osteoprogenitor cells and increases in the number of osteoblasts (649%) and osteoid (375%) surfaces. Remodeling and modeling units increased by 56% and 429%. respectively. After 30 days of treatment, there was an increase of 212% of total trabecular bone mass, 60% of which was woven bone. In addition, there were increases in labeling surface (147%), mineral apposition rate (760%), bone formation rates tissue area (BFR/T.Ar, 1920%; BFR/B.Pm, 343%), and bone turnover (BFR/B.Ar, 426%). Osteoblasts and osteoid production at 30 days were 29% and 58% less than at 10 days post-treatment. Modeling and remodeling activity did not differ from that seen at 10 days. In addition, PGE2 treatment tended to stimulate the closing of growth plates and decrease the fatty marrow area. We conclude that the aged skeleton was able to respond vigorously to PGE2 treatment. Massive osteoprogenitors cells, and osteoid and osteoblast formations were observed within 10 days. and dramatic woven and lamellar bone formation was seen at 30 days post-treatment. The anabolic effects were driven mainly by modeling.

Predictors of bone mass by peripheral quantitative computed tomography in early adolescent girls

This cross-sectional study used peripheral quantitative computed tomography (pQCT) to evaluate the influences of age, body size, puberty, calcium intake, and physical activity on bone acquisition in healthy early adolescent girls. The pQCT technique provides analyses of volumetric bone mineral density (vBMD) (mg/cm(3)) for total as well as cortical and trabecular bone compartments and bone strength expressed as polar strength strain index (mm(2)). Bone mass of the nondominant distal and midshaft tibia by pQCT and lumbar spine and hip by dual X-ray absorptiometry (DXA) were measured in 84 girls ages 11-14 yr. Pubertal stage, menarche status, anthropometrics, and 3-d food intake and physical activity records were collected. Total and cortical bone mineral content and vBMD measurements by pQCT were significantly related to lumbar spine and femoral neck BMD measurements by DXA. We did not note any significant determinants or predictors for trabecular bone mass. Body weight was the most important predictor and determinant of total and cortical bone density and strength in healthy adolescent girls. Menarche, calcium intake, height, body mass index, and weight-bearing physical activity level age were also identified as minor but significant predictors and determinants of bone density and strength. Bone measurements by the pQCT technique provide information on bone acquisition, architecture, and strength during rapid periods of growth and development. Broader cross-sectional studies using the pQCT technique to evaluate the influence of age, gender, ethnicity, puberty, body size, and lifestyle factors on bone acquisition and strength are needed.

Immunosuppressant use without bone loss--implications for bone loss after transplantation

Cyclosporine A (CsA) is associated with posttransplantation bone disease. Immunosuppressant drugs such as sirolimus (SRL), which are more potent and less deleterious than CsA, are being developed. Previous experiments have shown that SRL although immunosuppressive, is relatively bone sparing. The use of low doses of CsA and SRL in combination has displayed in vivo synergism. This study was initiated to examine the effect of low-dose CsA and SRL on bone metabolism, thereby hopefully providing a bone sparing immunosuppressive regimen for transplant recipients. One hundred and nineteen rats were divided into groups: basal, vehicle, CsA high dose, CsA low dose, SRL low dose, and combination low-dose CsA and SRL. The basal group was killed on day 0 for histomorphometry. The experimental groups were weighed and bled on days 0, 28, 56, and 84 and were killed on day 84 for histomorphometry. Serial assays for blood urea nitrogen (BUN), creatinine, and osteocalcin were performed. Osteocalcin was raised on days 28 and 56 in the high dose CsA group. Histomorphometry showed osteopenia with high-dose CsA. Low-dose CsA was relatively bone sparing, while low-dose SRL and combined low-dose CsA did not cause bone loss. In conclusion, the synergistic combination of low-dose CsA and SRL has the potential of providing both bone sparing and immunosuppressive benefits.

Anabolic agents and osteoporosis: quo vadis?

There are preclinical studies and limited clinical experiences with bone and muscle anabolic agents (e.g., parathyroid hormone (PTH), sodium fluoride (NaF), prostaglandins (PGs), growth hormones (GH), etc.) that show they have significant advantages over antiremodeling agents in patients with established osteoporosis. The strength of anabolic therapy is as follows: it rapidly reverses bone loss in laboratory animal models and humans, the quality of bone with some agents is believed to be normal, an increase in bone strength in animal models, and a reduction of spinal fracture rate with PTH. The weaknesses of this therapy are high cost, poor understanding of mechanism of action, parenteral mode of administration, rapid bone loss following termination of treatment, abnormal quality of bone, lack of tissue specificity, and undesirable side effects. Both animal and clinical studies have shown one can preserve the bone gain following termination of treatment with antiremodeling agents or exercise based on the lose, restore and maintain (LRM) concept. However, the more important efficacy issues which need to be addressed are tissue specificity and reduction of undesirable side effects. This report will address these issues with the suggestions that the potentiation of the mechanical loading osteogenic response by anabolic agents can overcome the disadvantages which accompany the use of anabolic agents. In addition, the possible role of nitric oxide (NO), an agent required for mechanical loading-induced bone formation, will be discussed.

Erect bipedal stance exercise partially prevents orchidectomy-induced bone loss in the lumbar vertebrae of rats

This study investigates the responses of the fourth and fifth lumbar vertebral bodies of 6-month-old male Sprague-Dawley (SD) rats to orchidectomy (orx) and to erect bipedal stance for feeding for 12 weeks in specially designed raised cages (RC) for which the heights were raised from 20 cm to 35.5 cm. A total of 30 rats were divided into groups of: baseline; sham + housed in normal height cage (NC); orx + NC; sham + RC; and orx + RC. Bone histomorphometry was performed on the triple-labeled undecalcified fourth sagittal (LVL-4) and fifth transverse (LVX-5) sections. We found that orchidectomy induced high-turnover trabecular and cortical bone loss in the lumbar vertebrae. Forcing the rats to rise to erect stance for feeding reduced trabecular and cortical bone loss caused by orx. Apparently, depressing the elevated bone resorption next to the marrow induced by orx, and stimulating bone formation at the ventral periosteal surfaces, caused these effects. Orchidectomy and raised cage had similar effects on the two vertebrae except that the percentage of trabecular bone loss was greater in the LVL-4 than in LVX-5, and that bipedal stance exercise increased the total tissue area and mineral apposition rates (0-80 day interval) of ventral periosteal and dorsal endocortical surfaces of LVX-5 to a greater extent than it did in LVL-4. Such findings suggest that forcing rats to rise to an erect bipedal stance for feeding helps prevent loss of trabecular and cortical bone "mass," and presumably bone strength, in orchidectomized rats. This method also provides an inexpensive, noninvasive, reliable model to increase in vivo vertebral loading in rats that is similar in humans.

Principles in bone physiology

The view that nonmechanical agents dominate control of osteoblasts and osteoclasts and thus postnatal changes in bone strength and mass (agent --> effector cells --> disease) is obsolete. Nonmechanical agents include hormones, calcium, vitamin D, cytokines, gender, genetics, etc. This paradigm overlooks all tissue level features, biomechanics and relationships found after 1960. This more recent information led to the Utah paradigm of skeletal physiology, proposed by Harold Frost in 1995. The Utah paradigm's view is that mechanical factors dominate control of the biologic mechanisms that control changes in postnatal bone and mass. Nonmechanical agents could help or hinder the influence of the mechanical factors but could not replace them. The simplified scheme is as follows: [reaction: see text] New evidence supports the Utah paradigm which we view as a supplement to many former views, not as a negation of them.

Making rats rise to erect bipedal stance for feeding partially prevented orchidectomy-induced bone loss and added bone to intact rats

The objectives of this study were to investigate the different effects on muscle mass and cancellous (proximal tibial metaphysis [PTM]) and cortical (tibial shaft [TX]) bone mass of sham-operated and orchidectomized (ORX) male rats by making rats rise to erect bipedal stance for feeding. Specially designed raised cages (RC) were used so that the rats had to rise to erect bipedal stance to eat and drink for 12 weeks. Dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computerized tomography (pQCT) were used to estimate the lean leg mass and bone mineral. Static and dynamic histomorphometry were performed on the triple-labeled undecalcified sections. We found that making the intact rats rise to erect bipedal stance for feeding increased muscle mass, cortical bone volume, and periosteal bone formation. Orchidectomy increased net losses of bone next to the marrow by increasing bone turnover. Making the ORX rats rise to erect bipedal stance increased muscle mass, partially prevented cancellous bone loss in the PTM, and prevented net cortical bone loss in TX induced by ORX by depressing cancellous and endocortical high bone turnover and stimulating periosteal bone formation. The bone-anabolic effects were achieved mainly in the first 4 weeks in the PTM and by 8 weeks in the TX. These findings suggested that making the rats rise to erect bipedal stance for feeding helped to increase muscle mass and cortical bone mass in the tibias of intact rats, increase muscle mass, and partially prevented cancellous and net cortical bone loss in ORX rats.

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 microm/day at the periosteal surface and from 0.5 to 2.1 microm/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.

Interferon-alpha, unlike interferon-gamma, does not cause bone loss in the rat

Interferons (IFN) are a group of related glycoproteins. IFN-gamma, in vitro, has been shown to inhibit resorption; however, an in vivo experiment showed that it had the opposite effect, resulting in bone loss that was comparable to that caused by cyclosporine A. IFN-alpha has numerous clinical applications but is used most extensively in the treatment of chronic hepatitis B and chronic hepatitis C. Research into the effects of IFN-alpha on bone mineral metabolism has been very sparse, and the majority of studies reflect in vitro models. Like IFN-gamma, there exists discordance between in vitro and in vivo studies on IFN-alpha. Both in vivo and in vitro studies demonstrate that IFN-alpha decreases bone resorption, whereas osteoblasts may or may not be affected in vivo. This study was designed to provide information on the in vivo effects of IFN-alpha in the rat model, because we feel that, given its widespread clinical use, this is an extremely important issue. Rats were given low dose IFN-alpha (1.6 x 10(6) IU/m2), intermediate dose IFN-alpha (5.35 x 10(6) IU/m2), and high dose IFN-alpha (30 x 10(6) IU/m2) three times per week for 28 days. Serum osteocalcin (bone gla protein, or BGP) and parathyroid hormone (PTH) were measured serially and, after double labeling, the bones were examined histomorphometrically. IFN-alpha did not alter any of the histomorphometric parameters measured and did not affect PTH. However, it produced a disparate BGP response. Low dose IFN-alpha resulted in a statistically significant increase in serum BGP on days 14 and 28, whereas intermediate and high doses of IFN-alpha did not. Overall, these results provide no evidence of a deleterious effect of IFN-alpha on bone metabolism and confirm the limited clinical study.

Animal models of immobilization osteopenia

There are numerous animal and human models to study the effects upon the skeleton of eliminating mechanical usage by immobilization (IM). Care must be taken in employing the various models. They all have the same pattern of bone loss, but they respond slightly differently in the location and the amount of bone loss, depending upon the degree of unloading. Experimental immobilization studies involving local disuse-induced bone loss models can lose up a baseline level of 60% of their trabecular bone mass. This baseline level suggests a physiological threshold where basal hormonal and cellular activity determined by genetics is reached in the absence of mechanical stimuli. During the transient, immediate, or acute phase of immobilization, remodeling-dependent bone loss occurs in bone adjacent to marrow and from an increase in bone resorption and a decrease in formation. At steady state, or chronic phase, bone mass has plateaued at baseline with cellular activities back to normal levels. Further lowering of immobilization-induced bone loss from immobilization baseline levels can occur with nonmechanical factors like estrogen deficiency. A reasonable working model of immobilization-induced bone loss uses Frost's mechanostat theory and skeletal adaptation to mechanical usage highway to show the IM-induced bone loss, the mechanical usage feedback from the overloaded osteopenic bone and the modification of the IM-induced baseline response with non-mechanical agents. Lastly, an investigator should employ the combination of IM plus OVX model for evaluating agents in the prevention and healing of osteoporosis because it combines the skeletal response of estrogen deficiency in postmenopausal osteoporosis and age-related bone loss due to decreased physical activity.

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.

Droloxifene does not blunt bone anabolic effects of prostaglandin E2, but maintains prostaglandin E2-restored bone in aged, ovariectomized rats

Droloxifene (DRO) is a selective estrogen receptor modulator that prevents bone loss by inhibition of bone turnover associated with estrogen deficiency in both growing and aged female rats. The purposes of this study were to test: (a) whether DRO can maintain prostaglandin E2 (PGE2)-restored bone after discontinuation of PGE2 in aged, ovariectomized (ovx) rats; (b) if an inhibition of bone turnover by DRO reduces bone anabolic effects of PGE2; and (c) whether bone mass restored by PGE2 plus DRO can be maintained after discontinuation of both agents. Female rats at 12 months of age were sham-operated (sham) or ovx. Three months postsurgery, ovx rats were treated with either PGE2 (3 mg/kg per day, subcutaneously [s.c.]) alone, or PGE2 plus DRO (10 mg/kg per day, per os [p.o.]) for 2 months. Thereafter, the PGE2 or PGE2 plus DRO treatment was withdrawn and the rats were then treated with either vehicle or DRO for another 1.5 months. Using dual-energy X-ray absorptiometry (DXA), total lumbar vertebral bone mineral density (LV-BMD) was determined in vivo at months 0, 3, 5, and 6.5. At the end of the study, the rats were autopsied, and BMD of total femur, femoral shaft, distal femoral metaphysis, and proximal femur was determined ex vivo by DXA. Standard static and dynamic bone histomorphometric parameters were determined on the fourth lumbar vertebral body (L-4). At 3, 5, or 6.5 months postsurgery, LV-BMD decreased significantly (-15%, -19%, and -19%, respectively) in the vehicle-treated ovx rats compared with sham. Beginning at 3 months post-ovx, PGE2 alone or in combination with DRO for 2 months completely restored LV-BMD back to the sham level. There was no difference in LV-BMD in PGE2 alone or PGE2 plus DRO. Upon cessation of PGE2 treatment, a significant decrease in LV-BMD was observed in the PGE2-alone group (-12%). On the other hand, when DRO treatment was given after discontinuation of PGE2, the PGE2-restored LV-BMD was completely maintained. In the PGE2 plus DRO group, no loss in LV-BMD was observed after cessation of either PGE2 alone or both PGE2 and DRO. However, treatment with DRO following 2 months of PGE2 plus DRO further increased LV-BMD (+10%). At the end of the study, ex vivo femoral BMD data confirmed the observation in lumbar vertebrae. Histomorphometric results of L-4 indicated that loss in bone mass after cessation of PGE2 in PGE2 alone group was associated with increased bone turnover. Treatment with DRO in the maintenance phase inhibited bone turnover and prevented bone loss induced by withdrawal of PGE2. Trabecular bone mass was maintained in the PGE2 plus DRO followed by vehicle group and further increased in the PGE2 plus DRO followed by DRO groups. We found that: (a) DRO is efficacious in maintaining PGE2-restored bone after discontinuation of PGE2; (b) DRO did not blunt the anabolic effects of PGE2; (c) bone loss occurred after cessation of treatment in the PGE2-alone group, whereas it was maintained after cessation of treatment in PGE2 plus DRO group; and (d) an additional anabolic effect was found in ovx rats treated with PGE2 plus DRO followed by DRO.

Relative radiosensitivity of bone tumor induction among beagles as a function of age at injection of 239Pu or 226Ra

A comparison was made of the response to induction of skeletal malignancy from exposure of beagles to monomeric 239Pu or to 226Ra as juveniles (3 mo of age), young adults (1.5 y of age), or mature adults (5 y of age). This indicated that of these age groups, animals injected as young adults are most sensitive per Gy of average skeletal dose evaluated at 1 y before death. Dogs exposed either as juveniles or as mature adults appeared to be less sensitive. Relative radiosensitivities (RRS) of juvenile and mature beagles ranged between about 0.3 and 0.7 that of dogs injected as young adults. Mean values of RRS for both radionuclides were about 0.5, but RRS values derived from dogs given monomeric 239Pu appeared to be most reliable and were 0.27+/-0.09 for dogs injected as juveniles and 0.41+/-0.13 for animals exposed as mature adults.

Effects of CP-336,156, a new, nonsteroidal estrogen agonist/antagonist, on bone, serum cholesterol, uterus and body composition in rat models

We have discovered a new, nonsteroidal, potent estrogen agonist/antagonist, CP-336,156. CP-336,156 binds selectively and with high affinity to the human estrogen receptor-alpha with a half-inhibition concentration of 1.5 nM, which is similar to that seen with estradiol (4.8 nM). When given orally to immature (3-week-old) female Sprague-Dawley rats for 3 days at doses of 0.1, 1.0, 10, or 100 microg/kg x day, unlike 17alpha-ethynyl estradiol, CP-336,156 had no effect on uterine wet or dry weight. Similarly, no uterine hypertrophy was observed in aged (17-month-old) female rats treated (p.o.) with CP-336,156 at 10 or 100 microg/kg x day for 28 days. We also found that CP-336,156 decreased total serum cholesterol and fat body mass and had no effect on lean body mass in these aged female rats. In 5-month-old ovariectomized (OVX) Sprague-Dawley female rats, CP-336,156 completely prevented OVX-induced increases in body weight gain, total serum cholesterol, and serum osteocalcin at doses between 10 and 1000 microg/kg x day after 4 weeks. At these doses, CP-336,156 completely prevented OVX-induced bone loss and inhibited the increased bone turnover associated with estrogen deficiency in lumbar vertebrae, proximal tibiae, and distal femora. Similar to estrogen, CP-336,156 induced apoptosis and p53 expression with a concomitant decrease in the number of tartrate-resistant acid phosphatase-positive multinuclear cells in rat bone marrow cell cultures in vitro, suggesting that the induction of apoptosis may be a mechanism for the estrogenic activities of CP-336,156 in bone. In summary, CP-336,156 is a new, orally active, nonsteroidal, potent estrogen agonist/antagonist that has similar effects in bone as estradiol but without the uterine-stimulating effects associated with estradiol in rats.

Mycophenolate mofetil: a promising new immunosuppressant that does not cause bone loss in the rat

BACKGROUND

Posttransplantation bone disease is a well-described phenomenon; among its etiology is immunosuppressant-induced bone disease. Mycophenolate mofetil (MMF) has emerged as a promising new immunosuppressant. Our study was designed to investigate the effect of MMF on in vivo bone mineral metabolism.

METHODS

Twenty-four 6-month-old male Sprague-Dawley rats were randomized into two groups to receive either MMF vehicle daily for 28 days or 30 mg/kg MMF daily for 28 days. The serum was assayed for osteocalcin and 1,25-dihydroxy vitamin D3. Subsequent to double-labeling, the right tibiae were removed on day 28 for histomorphometry.

RESULTS

MMF suppressed bone gla protein (osteocalcin) levels on days 14 and 28 (P < 0.05). Except for percentage osteoid perimeter, there was no difference in bone histomorphometry between the two groups.

CONCLUSION

In this relatively short-term study, MMF did not cause osteopenia in the rat model, but the suppressed bone gla protein merits further study.

Estrogen and bone-muscle strength and mass relationships

The largest voluntary loads on bones come from muscles. To adapt bone strength and mass to them, special strain threshold ranges determine where modeling adds and strengthens bone, and where remodeling conserves or removes it, just as different thermostat settings control the heating and cooling systems in a house. If estrogen lowers the remodeling threshold, two things should occur. First, at puberty in girls, bone mass should begin to increase more than in boys with similar muscle strengths, owing to reduced remodeling-dependent bone losses, while gains from longitudinal bone growth and bone modeling continue normally. That increase in bone mass in girls should plateau when their muscle strength stops increasing, since their stronger bones could then reduce bone strains enough to turn modeling off, but could let remodeling keep conserving existing bone. Second, decreased estrogen secretion [or a related factor(s)], as during menopause, should raise the remodeling threshold and make remodeling begin removing that extra bone. That removal should also tend to plateau after the remaining and weaker bone lets bone strains rise to the higher threshold. Postmenopausal bone loss shows the second effects. Previously unremarked relationships in the data of a 1995 Argentine study showed the first effects. This supports the idea that estrogen can affect human bone strength and mass by lowering the remodeling threshold, and loss of estrogen would raise the threshold and help cause postmenopausal bone loss even if other factors help to do it. The Argentine study also suggested ways to study those things and the roles of muscle strength and other factors in controlling bone strength and mass in children and adult humans. Those factors included, in part, hormones, vitamins, calcium, diet, sex, race, age, medications, cytokines, genetic errors, gene expression patterns, and disease.

Intermittent on/off prostaglandin E2 and risedronate are equally anabolic as daily PGE2 alone treatment in cortical bone of ovariectomized rats

In this study, we evaluated the rat cortical bone changes after a two-cycle, 60-day each (ON/OFF/ON/OFF) treatment with either prostaglandin E2 (OVX/c-PGE2) alone or in combination with risedronate (OVX/c-PGE2+Ris), in comparison with daily treatment with PGE2 for 240 days (OVX/PGE2-240d) in ovariectomized (OVX) rats. At the end of the study, we found that: (1) the overall effectiveness of the treatment on bone mass in the tibial shaft indicates the following ranking: OVX/PGE2-240d = OVX/c-PGE2+Ris > OVX/c-PGE2 > OVX/c-Ris > or = OVX = aging; (2) the same bone mass and architecture were produced in the OVX/PGE2-240d and the OVX/c-PGE2+Ris groups, but the histomorphometric profiles differed in that the former exhibited a higher bone turnover and index of resorption; (3) OVX/c-PGE2+Ris treatment prevented endocortical bone loss and minimized trabecular bone loss during the OFF periods; and (4) the OVX/c-PGE2 alone treatment resulted in the accumulation of less total bone than OVX/PGE2-240d and OVX/c-PGE2+Ris because it could not maintain most of the new subendocortical and marrow trabecular bone generated earlier. In summary, both continuous daily PGE2 and two cycles ON/OFF combined PGE2 and Ris treatments result in more bone mass than two cycles ON/OFF PGE2 alone and Ris alone in estrogen-deficient rats. This study showed that the anabolic effects of PGE2 can be induced and maintained either by continuous administration or by cyclical PGE2+Ris.

Effects of short-term treatment with the bisphosphonates zoledronate and pamidronate on rat bone: a comparative histomorphometric study on the cancellous bone formed before, during, and after treatment

To study the anti-resorptive effects of zoledronate and pamidronate on growing long bones we have performed a histomorphometric analysis of the three regions of the proximal tibial cancellous bone of bone formed before, during, and after drug treatment. Male rats (190-220 g) were treated subcutaneously for 10 days with zoledronate (0.028-2.8 microg/kg) or pamidronate (3.7-370 microg/kg) and sacrificed 5 days later. To delineate the three regions of cancellous bone, and for dynamic bone histomorphometry, calcein and demeclocycline were injected at various times. Both bisphosphonates caused a dose-dependent suppression of cancellous bone turnover and resorption to produce an increase in cancellous bone, but zoledronate was 100 times more potent than pamidronate. The increase in the bone amount and connectivity was more pronounced in the bone formed during treatment where transient bone resorption and normal bone formation led to a positive bone balance. In the bone formed before treatment, inhibition of bone resorption associated with reduced bone formation produced a net gain in amount of bone. Although both bone regions showed a positive bone balance, more bone accumulated in the bone formed during treatment probably because its trabecular bone surface was three times greater. In the primary spongiosa formed after treatment, a moderate increase in the bone amount and connectivity was observed only at the highest dose of both bisphosphonates. The bone formed before, during, and after treatment with bisphosphonates responds differently due to differences in bone architecture, rates of modeling and remodeling, and period of drug exposure.

Aging- and ovariectomy-related skeletal changes in spontaneously hypertensive rats

BACKGROUND

The skeletal impact of estrogen deficiency on subjects with hypertension has not been studied previously. In this study, we examined the skeletal characteristics of female spontaneously hypertensive rats (SHR) and their normotensive genetic control Wistar-Kyoto rat (WKY). We aimed to reveal: 1) the skeletal characters of female SHR, and 2) the response of SHR to ovariectomy (ovx) when compared to WKY and other strains.

METHODS

Undecalcified double-fluorescent labeled cancellous (proximal tibial metaphysis, PTM) and cortical (tibial shaft, TX) bones from 23-weeks-old, and from rats 2 and 8 weeks post-ovx were studied.

RESULTS

The SHR showed lower body weight, higher heart rate, and higher blood pressure than the WKY. Female SHR possessed more percent cancellous bone, less net cortical bone, smaller tissue area, and thinner cortex than WKY. Furthermore, SHR exhibited an age-related cancellous (-18%) and cortical (-7%) bone loss associated with a decrease in the longitudinal growth rate and bone balance and a decrease in periosteal bone formation in cortical bone. In contrast, the WKY maintained most of these parameters unchanged at their 23-week-old levels. Ovariectomy induced earlier and greater cancellous bone loss in the SHR than in the WKY, with greater increases in bone turnover rate, eroded surface, activation frequency, and a decrease in the ratio of labeled to eroded perimeter in PTM at 2 weeks postsurgery. However, the two groups exhibited no differences in bone loss at 8 weeks after ovx in PTM and TX.

CONCLUSIONS

Spontaneously hypertensive rats were highly sensitive to estrogen deficiency. This might have clinical relevance to those postmenopausal women who suffer from hypertension, in that they may be more susceptible to osteopenia. If so, preventive measures should be initiated sooner than otherwise.

Bone tumor location in dogs given skeletal irradiation by 239Pu or 226Ra

Statistical analyses have indicated that there was a significant difference between dogs injected with bone volume-seeking 226Ra as compared to those given bone surface-seeking 239Pu with respect to location within the skeleton of 334 radiation-induced primary bone malignancies. Corresponding differences also were evident when dogs given bone volume-seeking 90Sr or bone surface-seeking 241Am, 228Th, (249,252)Cf, or 224Ra (which decays mostly on bone surfaces because of its short, 3.6 d half time) were included along with the 226Ra or 239Pu, respectively (562 total tumors). Further analysis suggested that higher values of percent red marrow (M) and bone turnover rate (R) are correlated with increased probability of tumor appearance at a particular location within the skeleton for the surface seekers. Proportionately higher values of M and R are associated with skeletal sites containing mostly trabecular bone as compared to those with mostly compact (cortical) bone. Coefficients of determination (r2) for the relationship between percent of total tumors vs the combination of percent red marrow and turnover rate (= MR) was about 0.7 for the surface seekers but only about 0.1 for the volume seekers. This indicates that the neoplastic effects of surface seekers, but not volume seekers, are associated with the presence of trabecular bone at the various sites of radionuclide deposition within the skeleton.

Relationship of natural incidence and radiosensitivity for bone cancer in dogs

A comparison of the risk coefficients for 239Pu- or 226Ra-induced bone cancer in two canine breeds, one with a relatively low (beagle) and the other with a very high (St. Bernard) natural incidence, indicated only slightly higher risk in the giant breed. The differences in risk for skeletal malignancy in 239Pu and 226Ra dogs were nonsignificant (p > 0.05). Likewise, the values of the 239Pu:226Ra "toxicity ratios" for these respective breeds, using bone cancer as the endpoint, were not significantly different at the 0.05 level. The anatomical distribution of the radiation-induced bone tumors tended to be a function of both the bone mass and the skeletal distribution of the radionuclide, not the site of predilection for naturally occurring bone neoplasia. Although the etiology of the higher natural incidence of bone cancer in the St. Bernard was not determined, several possible factors, including a higher osteoblastic activity level in the St. Bernards, are presented. These data suggest that making extrapolations of radiation-induced bone cancer risk from animals to humans is valid.

Impact of antihypertensive therapy on the skeleton: effects of moexipril and hydrochlorothiazide on osteopenia in spontaneously hypertensive ovariectomized rats

Skeletal effects of moexipril, an angiotensin-converting enzyme (ACE) inhibitor, and hydrochlorothiazide (HCTZ), a thiazide diuretic, were studied in ovariectomized (OVX) spontaneously hypertensive rats (SHR). Moexipril (10 mg/kg per day), HCTZ (10 mg/kg per day), alone or in combination, as well as 17 alpha-estradiol (30 micrograms/kg per day) were given to OVX SHR immediately after surgery and studied for short- and long-term effects (14 and 56 days respectively). All drugs were given orally. Histomorphometric data on the secondary spongiosa of proximal tibial metaphyses (cancellous bone) and tibiofibular junctions of tibial shafts (cortical bone) were analyzed. Ovariectomy induced cancellous bone loss in SHR by inducing negative bone balance. Estrogen prevented ovariectomy-induced cancellous bone loss in the SHR by reducing bone turnover and partially suppressing the coupling of bone formation to resorption on the endocortical surface. HCTZ reduced blood pressure after 1 week of treatment, yet this effect was no lower than that seen in controls after 3 weeks of treatment. Two weeks of HCTZ transiently prevented ovariectomy-induced increases in bone turnover rate and eroded surface. This delayed ovariectomy induced trabecular bone loss in the proximal tibial metaphysis, but had no effect on the tibial shaft. Like HCTZ, moexipril also reduced blood pressure after the first week of treatment but it had no apparent effect on either the proximal tibial metaphysis or the tibial shaft. A combination of moexipril and HCTZ exhibited a much more potent hypotensive effect and had the same effect on bone mass and dynamic end-points as HCTZ alone. Our data indicate that (1) HCTZ treatment has some transient beneficial effects on both antihypertension and osteoprotection in hypertensive osteopenic rats, (2) the combination of moexipril with HCTZ improved the antihypertensive effect but did not potentiate or hamper the osteoprotective effect of HCTZ, and (3) the skeletal effect of estrogen is not impacted by the hypertensive state. These findings are relevant for the use of ACE inhibitor and thiazide diuretics, alone or in combination, in antihypertensive therapy in postmenopausal women.

Alendronate prevents cyclosporin A-induced osteopenia in the rat

Post-transplantation bone disease is an increasingly recognized clinical entity whose etiology is multifactorial. The immunosuppressant agent cyclosporine-A (CsA) has repeatedly been shown experimentally to induce a high-turnover osteopenic state. Alendronate (Alen.) is a new generation bisphosphonate having far greater antiresorptive potency than previous bisphosphonates. It inhibits osteoclast resorption in vitro and in vivo without adversely affecting bone mineralization. This study was designed to investigate whether alendronate could prevent CsA-induced osteopenia in the rat. Forty-eight 8-month-old male Sprague Dawley rats were randomized into four groups to receive the following for 28 days: (1) CsA vehicle (veh.) p.o. daily and alendronate vehicle subcutaneously (s.c.) twice/week, (2) CsA 15 mg/kg p.o. daily and Alen. veh. s.c. twice/week, (3) Alen. 70 micrograms/kg s.c. twice/ week and CsA veh. p.o. daily, and (4) CsA 15 mg/kg p.o. daily and Alen. 70 micrograms/kg s.c. twice/week. Rats were weighed and bled and serum was assayed serially for calcium, PTH, 1,25(OH)2vit.D, and osteocalcin. Tibiae were removed following sacrifice on day 28, after double demeclocycline and calcein labeling, for histomorphometric analysis. Treated groups were compared to the vehicle-treated control. We confirmed previous findings that CsA produces elevated 1,25(OH)2 vitamin D and serum osteocalcin levels. Alendronate treatment by itself decreased osteocalcin by day 28 and resulted in a marginal decrease in serum total calcium on day 14. The histomorphometry findings reconfirmed that the administration of CsA induces a state of high-turnover osteopenia. Alendronate prevented CsA's adverse effects, particularly in maintaining trabecular bone volume, presumably by decreasing bone remodeling. Alendronate would seem to hold therapeutic promise in post-transplantation bone disease.

The role of testosterone in cyclosporine-induced osteopenia

Our laboratory has demonstrated that the immunosuppressants Cyclosporin A (CsA) and tacrolimus (FK506), in vivo in the rat, produce a high-turnover osteopenia. CsA is known to decrease serum testosterone (Test) levels both in the rat and in human transplant patients. Less is known of FK506's effect on androgens. CsA-induced hypogonadism may contribute to the aforementioned bone loss because hypogonadism itself is a risk factor for osteoporosis and fracture. The aim of this study was to assess serum androgen levels following CsA and FK506 therapy and to see wether Test replacement therapy, in the form of 28-day controlled release subcutaneous pellet implants, could prevent CsA-induced osteopenia. Two experiments were conducted. In experiment I, four groups of 6-month-old male Sprague-Dawley rats received the following: (A) CsA vehicle and placebo pellet, (B) Test 15 mg pellet and CsA vehicle, (C) CsA 10 mg/kg and placebo pellet, (D) Test 15 mg pellet and CsA 10 mg/kg. In experiment II, two groups of rats received (E) FK506 vehicle and (F) FK506 4 mg/kg. CsA, FK506, and vehicles were given for 28 days by daily oral gavage. The rats were weighted and bled on days 0, 14, and 28. All rats received double fluorescent labeling, and on day 28 the tibiae were removed for histomorphometry. Whole blood was assayed for CsA and FK506 levels. Serum was assayed for total and free Test as well as for osteocalcin (BGP), blood urea nitrogen (BUN), creatinine, and calcium. Whole blood monoclonal CsA levels measured by fluorescent immunoassay were in the therapeutic range, while a drug concentration profile showed good absorption of FK506. Those rats receiving Test and FK506 lost weight, while those receiving CsA remained constant. BUN was only marginally elevated in the CsA-treated groups on day 28 (p < 0.05), while creatinine was unchanged. On day 28, total and free Test was significantly reduced in the CsA-treated rats versus control (p < 0.05), while Test replacement therapy maintained total Test levels above vehicle (p < 0.01) and free Test levels similar to vehicle on day 28. FK506 did not lower total or free Test levels. BGP levels were significantly increased in the CsA (p < 0.01) and FK506 (p < 0.001) groups on day 28. BGP in the groups receiving Test alone and in combination with CsA remained similar to vehicle. Histomorphometry confirmed CsA- and FK506-induced high-turnover osteopenia. The Test alone group marignally increased bone formation. Test replacement failed to prevent the CsA-induced bone loss. In conclusion, immunosuppressive doses of CsA, but not FK506, lowers serum total and free Test. Hypoandrogenemia does not seem to be a major factor in CsA-induced osteopenia because bone loss occurs despite Test replacement.

Prostaglandin E2 (PGE2) and risedronate was superior to PGE2 alone in maintaining newly added bone in the cortical bone site after withdrawal in older intact rats

The objects of this study were (1) to determine the effects of risedronate (Ris) and prostaglandin E2 (PGE2) alone and in combination, on tibial diaphyses of older intact female rats; and (2) to observe the fate of any extra bone if formed after withdrawal of the treatment. Nine-month-old female Sprague-Dawley rats were treated with 6 mg of PGE2/kg/day, 1 or 5 micrograms of Ris/kg twice a week, or 6 mg of PGE2/kg/day plus 1 or 5 micrograms of Ris/kg twice a week for the first 60 days and followed by vehicle injections for another 60 days. Cross-sections of double fluorescent labeled, undecalcified tibial diaphyses proximal to the tibiofibular junction were processed for histomorphometry. We found that: (1) neither the 1 microgram nor the 5 micrograms of Ris treatment in the 60-day on/60-day off group showed any histomorphometric differences from age-related controls; (2) while the 60 days of PGE2 treatment added extra cortical bone (6%) on the tibial shaft (due to stimulation of periosteal, endocortical, and marrow trabecular bone formation), the new endocortical and most of the new marrow trabecular bone were lost when treatment was withdrawn; however, the new periosteal bone remained; (3) PGE2 with Ris added the same amount of new bone to tibial diaphysis as did PGE2 alone and upon withdrawal, new marrow trabecular bone was lost but new periosteal and endocortical bones were preserved in PGE2 + 1 microgram of Ris on/off group. In contrast, all the new bone was maintained in the PGE2 + 5 micrograms of Ris on/off group; (4) PGE2 + Ris cotreatment failed to block the increase in cortical bone porosity induced by PGE2; and (5) in the PGE2 alone and PGE2 + 1 microgram of Ris on/off groups bone turnover was higher than that in the PGE2 + 5 micrograms of Ris on/off group. These results indicate that on/off treatment with PGE2 and Ris is superior to PGE2 alone in that it forms the same amount of new bone during treatment, but preserves more cortical bone during withdrawal. Depression of bone resorption and turnover were the tissue mechanisms responsible for this protection.

Prostaglandin E2 increases the skeletal response to mechanical loading

The study tested the influence of prostaglandin E2 (PGE2) on the skeletal response to increased in vivo mechanical loading through a four-point bending device. One hundred and twenty Sprague-Dawley female rats (6 months old, 354 +/- 34 g) were divided into 12 groups to accommodate all possible combinations of doses of loads (25, 30, or 35 N) and PGE2 (0, 0.1, 0.3, or 1 mg/kg). Rats received subcutaneous injections of PGE2 daily and in vivo loading of the right tibia every Monday, Wednesday, and Friday for four weeks. Histomorphometric analysis of the periosteal and endocortical surfaces following in vivo dual fluorochrome labeling was performed on both the loaded region of the right tibial diaphysis and a similar region of the left tibial diaphysis. Without PGE2, the threshold for loading to stimulate bone formation was 30 N (peak strain 1360 mu epsilon) at the periosteal surface and 25 N (peak strain 580 mu epsilon) at the endocortical surface. Without loading, the minimum dose of PGE2 to stimulate bone formation at all surfaces was 1 mg/kg/day. When 1 mg/kg/day PGE2 was combined with the minimum effective load, an additive effect of PGE2 and loading on bone formation was observed at the endocortical surface, but a synergistic effect was noted at the periosteal surface. No combined effect of ineffective doses of loading and PGE2 was found. A synergistic effect at peak strains of approximately 1625 mu epsilon on the periosteal surface could suggest either the involvement of locally produced growth factors or autoregulation of endogenous synthesis of PGE2 by exogenously administered PGE2.

Comparison of internal emitter radiobiology in animals and humans

Investigations of radionuclide metabolism and effects in various mammalian species revealed important similarities between animals and humans and between some animal species. These include skeletal deposition of radium and radiostrontium in bone volume; deposition on bone surfaces of plutonium and other actinides; liver deposition of actinides; induction of skeletal or liver malignancies by these radionuclides; induction of tooth and jaw abnormalities; mammary cancer induction by radium in humans and in the beagle; depression of circulating cells in blood; and induction of bone fractures. There are also inter-species differences that may not have been noted if multiple species (including humans) had not been studied. Some of these are more rapid excretion of radium in humans compared with most other mammals; induction by radium of eye melanomas in animals but not humans; rapid loss of deposited plutonium from liver in many species of mice and rats but not in humans and dog; substantial sex-related differences in skeletal plutonium retention and bone sarcoma induction in mice but not in humans or dog; and induction of head sinus carcinomas by 226Ra in humans but not the beagle. Leukemia and other related neoplasms were not induced in radionuclide-injected lifespan dogs in excess of the occurrence in control animals. Much of our current understanding of skeletal biology and radionuclide behavior in mammals was derived from this and related projects. The primary goal of the Utah experiment of estimating toxicities of bone-seeking radionuclides relative to radium has been accomplished. For 226Ra = 1.0, comparative toxicities (ratios) of a single injection for bone tumor induction in beagles were about 16 +/- 5 for monomeric 239Pu (32 +/- 10 for chronic exposure), 6 +/- 0.8 for 241Am, 8.5 +/- 2.3 for 228Th, 6 +/- 3 for 249Cf, 4 +/- 2 for 252Cf, 6 +/- 2 for 224Ra (16 +/- 5 for 50 weekly injections), 2 +/- 0.5 for 228Ra, and between 0.01 +/- 0.01 and 1.0 +/- 0.5 for 90Sr, depending on the dose-rate, with the lowest dose-rates approaching a ratio of zero. Corresponding ratios in mice for 226Ra = 1.0 were 16 +/- 4 for monomeric 239Pu, 5.4 +/- 2.0 for 224Ra (16 for 50 weekly injections), 4.9 +/- 1.4 for 241Am, 5.0 +/- 1.4 for 249Cf, 2.6 +/- 0.8 for 252Cf, 4.4 +/- 1.8 for 243,244Cm and about 1.0 for 90Sr at high doses, decreasing to near zero for low doses.

Comparative effects of droloxifene, tamoxifen, and estrogen on bone, serum cholesterol, and uterine histology in the ovariectomized rat model

The purpose of this study was to compare the effects of droloxifene (DRO), tamoxifen (TAM), and 17 alpha-ethynyl estradiol (EE) on bone mineral density, bone histomorphometry, total serum cholesterol, and uterine histology in the ovariectomized (ovx) rat model. Sprague-Dawley female rats at five months of age were sham-operated and treated orally with vehicle (n = 8), or ovx (n = 56) and treated (p.o.) with either vehicle, DRO at 0.1 or 1.0 mg/kg daily, TAM at 0.1 or 1 mg/kg daily, or EE at 3 or 30 micrograms/kg daily for 4 weeks. The uterine wet weight and uterine histologic parameters (cross-sectional tissue area, stromal thickness, and luminal epithelial thickness) were determined. Femoral and lumbar vertebral bone mineral density was determined ex vivo using dual energy x-ray absorptiometry. Static and dynamic cancellous bone histomorphometry was performed on double-labeled, undecalcified longitudinal sections from proximal tibial metaphyses. Furthermore, the changes in total serum cholesterol and body weight gain were also determined. Compared to sham controls, ovx for four weeks significantly decreased uterine weight (-72%), uterine cross-sectional tissue area (-74%), stromal thickness (-52%), and luminal epithelial thickness (-53%). ovx rats treated with EE at 30 micrograms/kg/day maintained these parameters at the levels of sham controls. Uterine weight and uterine cross-sectional tissue area in 3 micrograms/kg/day of EE treated ovx rats were higher than that of vehicle-treated ovx rats. In ovx rats treated with TAM at both 0.1 and 1 mg/kg/day, these parameters were significantly less than sham controls but significantly higher than ovx controls. DRO at 0.1 mg/kg/day had no effects on all above parameters. Uterine weight and cross-sectional tissue area in 1 mg/kg/day of DRO treated ovx rats was slightly but significantly higher than that in ovx controls. However, DRO at 1 mg/kg/day had no effects on uterine stromal thickness and luminal epithelial thickness compared to ovx controls. The ovx-induced decrease in femoral and lumbar vertebral bone mineral density was prevented by treatment with EE at 30 micrograms/kg/day, TAM at both 0.1 and 1 mg/kg/day, or DRO at 1 mg/kg/day. Similarly, the decrease in bone mass and the increase in bone resorption and bone turnover in proximal tibial metaphyses were prevented by treatment with EE at 30 micrograms/kg/day or TAM at both 0.1 and 1 mg/kg/day, or DRO at 1 mg/kg/day. Total serum cholesterol decreased significantly in ovx rats treated with either EE, DRO, or TAM at all dose levels compared to vehicle treated ovx controls (-32% to -56%). The ovx-induced body weight gain was completely prevented by EE at 30 micrograms/kg/day, and partially prevented by DRO at 1 mg/kg/day. TAM at both 0.1 and 1 mg doses caused a significant decrease in body weight compared to both sham and ovx controls. Our results indicated that DRO prevented ovx-induced bone loss and lowered total serum cholesterol with an ED50 less than 1 mg/kg/day. The bone protective and cholesterol lowering effects of DRO were comparable to those observed with TAM and EE. However, DRO differed from TAM and EE in its lack of significant estrogenic effects on uterine tissue at doses which were bone protective. These data suggest that DRO may be a significant alternative to EE and TAM for prevention and treatment of postmenopausal osteoporosis.

Combined flurbiprofen and cyclosporin-A does not attenuate bone loss and exaggerates renal impairment

Cyclosporine (CsA) is a potent immunosuppressant that has revolutionized the success of organ transplantation. Flurbiprofen (FB), a propionic acid derivative NSAID, has been demonstrated in vivo to reduce osteoclast numbers in normal rats. The aim of this experiment was to determine whether addition of FB to CsA-treated rats could prevent the bone changes associated with CsA therapy. Forty-eight 10-12-week-old male Sprague-Dawley rats were randomized to receive, daily for 28 days: (1) CsA vehicle p.o. plus FB vehicle sc; (2) CsA (15 mg/kg) p.o. plus FB vehicle sc, (3) CsA vehicle p.o. plus FB (1.5 mg/kg) sc; and (4) CsA (15 mg/kg) p.o. plus FB (1.5 mg/kg) sc. Rats were weighed and venous blood sampled at baseline, 14 days, and 28 days for determination of glucose, Ca+2, BUN, creatinine, PTH, osteocalcin, and 1,25(OH)2 vitamin D. Tibiae were removed following killing, after double labeling for histomorphometry. Body mass was significantly lower than control in all rats receiving CsA on days 14 and 28 while blood glucose was only elevated in the CsA alone group. Day 28 BUN and creatinine were significantly elevated in the CsA group and the combination of CsA and FB revealed an exacerbation of this trend. Vitamin D and osteocalcin were consistently increased in the CsA and CsA/FB groups. Bone histomorphometry showed evidence of trabecular osteopenia in CsA and CsA/FB groups. CsA alone resulted in elevated bone turnover. FB was unable to prevent the trabecular bone loss induced by CsA therapy. This experiment indicates no role for FB as a therapeutic option in CsA-induced bone disease at the given doses and duration of treatment by virtue of its lack of bone sparing ability and adverse renal effects when the two drugs are administered concurrently.

Raloxifene analog (LY117018 HCL) ameliorates cyclosporin A-induced osteopenia in oophorectomized rats

Cyclosporin A (CsA) administered to the oophorectomized (Ox) rat exacerbates the high turnover osteopenia associated with estrogen deficiency. 17 beta-estradiol replacement therapy prevent this bone loss. The aim of this study was to see whether an estrogen-like compound, Raloxifene analog (LY117018 HCL, Ral) could likewise ameliorate CsA-induced osteopenia in the Ox rat. Sixty 6-month-old Sprague-Dawley rats, divided into five groups, underwent oophorectomy. One group acted as a basal group and the others received either vehicle (group B), CsA 15 mg/kg/day (group C), Ral 3 mg/kg/day (group D), or CsA 15 mg/kg/day and Ral 3 mg/kg/day (group E) for 28 days by gavage. A sixth sham operated group of 12 rats received vehicle only (group A). Rats were weighed and bled on days 0, 14, and 28 for measurement of ionized calcium, glucose, osteocalcin (BGP), 17 beta-estradiol, and 1,25-dihydroxyvitamin D3 (1,25[OH]2D3). Tibiae were removed on day 28 for bone histomorphometry after double tetracycline and calcein labeling. Oophorectomy caused a significant gain in weight in groups B and C which was prevented by Ral in groups D and E. Randomized blood glucose levels and 1,25(OH)2D3 levels were elevated in both CsA-treated groups. Blood ionized calcium levels were lower in vehicle (group B) compared with sham (group A) on day 28. Ox (group B) had significantly higher serum BGP levels compared with sham-operated rats. Serum BGP levels were further elevated in group C compared with vehicle and were lowered in both Ral-treated groups to vehicle levels by day 28. Bone histomorphometry revealed a high turnover osteopenia with increased parameters of bone formation and resorption and loss of cancellous bone volume postoophorectomy (group B). CsA (group C) exacerbated the effects of oophorectomy. Ral (group D) completely prevented the high turnover osteopenia caused by oophorectomy and was able to attenuate substantially the effects of CsA in the Ox rat (group E). Ral therapy ameliorated CsA-induced osteopenia in the Ox rat and might prove a useful agent in preventing bone loss in postmenopausal women receiving CsA.

Time responses of cancellous and cortical bones to sciatic neurectomy in growing female rats

Effects of unilateral sciatic neurectomy on the responses of both cancellous and cortical bones were studied in growing female rats at 0, 1, 4, 8, and 12 weeks after operation. Using double-fluorescent labeling techniques, histomorphometric analyses were performed on longitudinal sections of proximal tibial metaphyseal secondary spongiosa (PTM) and on cross sections of tibial shaft (TX). In PTM, sciatic neurectomy not only inhibited the age-related bone gain, but also reduced the trabecular bone mass by 46%, which was accompanied by decreases in trabecular number, thickness, and node to node density, and an increase in trabecular separation and free end to free end density. The bone loss occurred mainly between 1 and 4 weeks after operation. A sharp increase in bone formation indices was observed during the first week after nerve section. However, these endpoints quickly dropped to levels lower than those of sham-operated controls at 4 weeks, and were not different from the control levels at 8 weeks after operation. Eroded surface increased progressively after sciatic neurectomy during the 12 weeks experimental period. In TX, sciatic neurectomy inhibited the age-related increase in total tissue area that maintained it at the basal control level. However, the cortical bone area in neurectomized legs was lower than that in sham-operated controls. Sciatic neurectomy also stimulated the bone formation indices on both periosteal and endocortical surfaces during the first week after operation. These endpoints declined sharply between 1 and 4 weeks and then maintained at control levels between 8 and 12 weeks post surgery. Endocortical eroded surface increased 1 week after neurectomy, reached the peak at 8 weeks, and then decreased thereafter. These findings suggest that (1) sciatic neurectomy not only inhibited age-related bone gain but also induced marked bone loss in cancellous bone site and inhibited age-related bone gain in cortical bone site, which mainly resulted from the decrease in bone formation and the increase in bone resorption; (2) the changes in both cancellous and cortical bones responded to sciatic neurectomy occurred mostly within the first 4 weeks and stabilized between 8 and 12 weeks after surgical intervention. In conclusion, the unilateral sciatic neurectomized rat is a complex model in which to study osteopenia. Despite sciatic neurectomy being a simple operation, the interactions of skeletal responses to postsurgical regional acceleratory phenomenon (RAP) and disuse and adaptation changes cannot be clearly differentiated. Furthermore, the complications from growth and aging should be avoided.

Short-term systemic insulin-like growth factor-1 is unable to prevent cyclosporin A-induced osteopenia in the rat

Immunosuppression with cyclosporin A (CsA) is effective in a number of immune-mediated diseases and in preventing rejection following organ transplantation. We have repeatedly demonstrated that CsA in the rat model produces accelerated bone remodelling with net bone loss, best characterized in trabecular bone. IGF-I holds promise as a treatment for various osteopenic conditions. Although currently a subject of much controversy, various studies have suggested that in vivo it is anabolic to cortical as well as trabecular bone. The purpose of this study was, in part, to further characterize the effects of CsA and IGF-I on trabecular and cortical bone, and to see whether systemic IGF-I is able to modulate CsA's deleterious skeletal effects. Sixty 10 week-old, male, Sprague-Dawley rats were randomized to receive the following daily for 3 weeks: (1) CsA vehicle (veh) per os (po) + recombinant human (rh) IGF-1 veh subcutaneously (sc); (2) CsA 15 mg/kg po + rhIGF-I-veh; (3) CsA-veh + rhIGF-I 200 microg/kg sc; (4) CsA-veh + rhIGF-I 600 microg/kg sc; (5) CsA 15 mg/kg + rhIGF-I 200 microg/kg, and (6) CsA 15 mg/kg + rhIGF-I 600 microg/kg. Rats were weighed and venous blood was sampled serially for determination of glucose, ionized calcium (Ca2+), PTH, vitamin D, and osteocalcin. Following sacrifice on day 20, histomorphometry was performed on double calcein-labeled tibial metaphysis and diaphysis. All rats receiving CsA had elevated levels of blood glucose and osteocalcin by day 9 and vitamin D at day 20. PTH was similar in all groups, and Ca2+ was only raised in the CsA and CsA + IGF-I 200 microg/kg groups. Rats receiving IGF-I 200 microg/kg and IGF-I 600 microg/kg gained more weight than either vehicle- or CsA-treated animals, attesting to IGF-1's anabolic properties. CsA caused severe trabecular bone loss, not prevented by IGF-I; it even further increased the eroded surface. CsA and IGF-I had little effect on cortical bone volume or marrow area. IGF-I increased endocortical matrix synthesis, as evidenced by the increases in the percent endocortical osteoid perimeter, an effect negated by the addition of CsA. This experiment demonstrates that trabecular bone is more susceptible than cortical bone to the deleterious effects of CsA and indicates little role for IGF-1 in the pathophysiology or treatment of CsA-induced bone disease at the given doses and duration of treatment.

T lymphocytes play a critical role in the development of cyclosporin A-induced osteopenia

The T lymphocyte suppressor, cyclosporin A, has been shown to cause high turnover osteoporosis. We postulated that cyclosporin A may exert its effects via the T cell rather than direct activity on bone. In this study we administered cyclosporin A (15 mg/kg.day by gavage) to 11 10-week-old Rowett athymic nude rats and to 12 age-matched immunocompetent Sprague-Dawley rats. Placebo was administered to control groups (n = 12 for both). After 28 days of treatment, the Sprague-Dawley rats displayed high turnover bone loss, but the nude rats were largely unaffected by the drug. Sprague-Dawley treated rats had less than half the percent trabecular area of their controls as measured at the secondary spongiosa of the proximal tibial metaphysis (P < 0.001; strain by treatment, P = 0.007). The same pattern was evident for trabecular number, separation, and thickness (strain by treatment, P = 0.034, P = 0.001, and P = 0.021, respectively). Only the Sprague-Dawley rats had an elevated percent eroded perimeter and an elevated bone area referent bone formation rate (strain by treatment, P = 0.002 and P = 0.0003, respectively). Mass, glucose, ionized calcium, PTH, osteocalcin, 1,25-dihydroxyvitamin D, and creatinine all responded similarly to cyclosporin A regardless of strain. T Lymphocytes thus appear to be a prerequisite for the development of cyclosporin A-induced osteopenia.

Functional characterization of prostaglandin E2 inducible osteogenic colony forming units in cultures of cells isolated from the neonatal rat calvarium

Prostaglandin E2 (PGE2) increases the number of mineralized nodules that form in cultures of rat calvarial (RC) cells. The purpose of our study was to characterize PGE2-inducible osteogenic colony forming units (CFU-Os) by determining their number, the cell populations from which they were released, their specific responsive period to PGE2, and their proliferating and differentiating characteristics under the stimulation of PGE2. Limiting dilution analysis was used to determine the number of PGE2-inducible CFU-Os. Sequential digestion of intact rat parietal bones with collagenase isolated 5 subpopulations of RC cells that were used to estimate the cell populations where PGE2-inducible CFU-Os resided. The responsive period of PGE2-inducible CFU-Os to PGE2 was evaluated by treating cultures of mixed RC cells for all possible combinations of days 1-10, 11-20, and 21-30. PGE2 effects on proliferation and differentiation of CFU-Os were evaluated by comparing the DNA synthesis and AP activity in subpopulations I and IV on days 3, 6, and 9. Results showed: (1) PGE2-inducible CFU-Os represent 0.27% of cells in the mixed RC population, (2) the majority of determined and PGE2-inducible CFU-Os were found in the subpopulations released during the 60-100 min digestion periods, (3) the response of PGE2-inducible CFU-Os is limited to the first 10 days of culture, and (4) PGE2-stimulated nodule formation is associated with an early increase in DNA synthesis and a sustained increase in alkaline phosphatase activity. We conclude that, functionally, PGE2-inducible CFU-Os are slowly proliferating AP negative cells primarily found in the subpopulations III-V. PGE2 stimulates them to proliferate and become AP+, and function as determined CFU-Os to form mineralized nodules in vitro.