Adaptation of cancellous bone to aging and immobilization in growing rats

Two-and-half-month-old female rats were subjected to right hindlimb immobilization or served as controls for 0, 1, 2, 8, 14, and 20 weeks. The right hindlimb was immobilized by bandaging it against the abdomen, thus unloading it. Cancellous bone histomorphometry was performed on microradiographs and double-fluorescent labeled 20 microns sections of the distal femoral metaphyses. Primary spongiosa bone loss occurred rapidly by 2 weeks, and secondary spongiosa bone loss occurred rapidly by 8 weeks of immobilization, and then equilibrated at 60% less bone mass than age-related controls. The negative bone balance induced by immobilization was caused by transient increase in bone resorption, decrease in bone formation, and longitudinal bone growth. The dynamic data of secondary spongiosa cancellous bone showed that percent eroded perimeter was transiently elevated by 55 to 82% between 1 and 8 weeks, percent labeled perimeter was transiently depressed by 32% to 50% between 1 and 14 weeks, mineral apposition rate was depressed by 23% and 19% at 1 and 2 weeks, and bone formation rate-bone area referent was transiently depressed by 35% and 59% at 1 and 2 weeks. All the above parameters were at age-related control levels by 20 weeks of immobilization. However, bone formation rate-tissue area referent was depressed (-65%) throughout the study. Immobilization depressed completely longitudinal bone growth by 2 weeks and remained so. Only 0.65 mm of new metaphysis was generated in the immobilized versus 2.1 mm in controls during the study period. The immobilization induced an early cancellous bone loss which equilibrated at a new steady state with less bone and a normal (age-related control) bone turnover rate. When these findings were compared to an earlier study of 9-month-old virgin females subjected to right hindlimb immobilization up to 26 weeks, we found the adaptive responses of the cancellous bone were identical except that they occurred earlier and equilibrated sooner in younger rats.

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).

Prostaglandin E2 alleviates cyclosporin A-induced bone loss in the rat

Cyclosporine A (CsA) administered to the male and female rat produces high-turnover osteopenia. Prostaglandins have both bone-resorbing and bone-forming properties, but administration of prostaglandin E2 (PGE2) to the rat in vivo produces a net increase in cancellous bone. To investigate the effects of PGE2 on CsA-induced alteration in bone mass, 43 male Sprague-Dawley rats (9 weeks old) were administered 15 mg/kg of CsA by oral gavage and/or 6 mg/kg of PGE2 by subcutaneous injection daily for 21 days according to the following protocol: group A was an age-matched control; group B received CsA only; group C received PGE2 only; and group D received CsA and PGE2. Serum was assayed on days 0, 7, 14, and 21 for bone gla protein (BGP), PTH, and 1,25-dihydroxyvitamin D [1,25-(OH)2D]. A computerized image analysis system was used for bone histomorphometry of the proximal tibial metaphysis after double tetracycline labeling. Compared to control animals (group A), treatment with CsA alone (group B) and PGE2 alone (group C) significantly elevated BGP levels. Combination therapy (group D) resulted in BGP levels that were significantly higher on days 7 and 14 than with either agent alone. 1,25-(OH)2D was significantly elevated in the CsA group only (group B). Therapy with CsA alone (group B) resulted in a significant osteopenia. The concurrent administration of PGE2 with CsA (group D) alleviated the altered bone mass induced by CsA alone by adding a significant amount of additional bone. This report confirms and extends the current knowledge of the different effects of CsA and PGE2 on bone mineral metabolism and demonstrates that PGE2 can alleviate the deleterious effects of CsA on bone.

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

The effects of daily prostaglandin E2 (PGE2) treatment (on) and PGE2 treatment followed by withdrawal (on-off) on cancellous bone in lumbar vertebral bodies were studied in 7-month-old male Sprague-Dawley rats. The first groups of rats were given daily subcutaneous injections of 0, 1, 3, and 6 mg PGE2/kg/d for 60, 120, and 180 days, and the second group of rats were given PGE2 for 60 days followed by withdrawal for 60 and 120 days. Histomorphometric analyses were performed on double-fluorescent labeled undecalcified sections of fourth lumbar vertebral bodies. Systemic PGE2 treatment elevated cancellous bone mass of lumbar vertebral bodies 26-60% above control levels within 60 days and continued treatment maintained it for another 120 days, but the excess bone was lost after the treatment was withdrawn. PGE2 treatment for 60 days increased trabecular bone area, trabecular width, and bone formation parameters, and shortened remodeling periods in a dose-response manner. These changes were sustained at the levels achieved by 60-day treatment in the rats treated for 120 and 180 days. The eroded perimeter increased at day 60 and further at day 120 and then plateaued. In the on-off treated rats, the cancellous bone area, bone formation, and resorption parameters returned to near age-related controls by 60 days after withdrawal and were maintained there after 120 days of withdrawal. Therefore we conclude that the continuous treatment is needed in order to maintain the PGE2-induced bone gain. When these findings were compared to those previously reported for the proximal tibial metaphyses, we found that the proximal tibial spongiosa was much more responsive to PGE2 treatment than the fourth lumbar vertebral body.

On the rat model of human osteopenias and osteoporoses

The idea that rats cannot model human osteopenias errs. The same mechanisms control gains in bone mass (longitudinal bone growth and modeling drifts) and losses (BMU-based remodeling), in young and aged rats and humans. Furthermore, they respond similarly in rats and man to mechanical influences, hormones, drugs and other agents.

Restoring and maintaining bone in osteopenic female rat skeleton: I. Changes in bone mass and structure

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 anabolic agents to restore bone mass and architecture (+ phase) and then switches to an agent with the established ability to maintain bone mass, to keep the new bone (+/- phase). The purpose of this study was to learn whether switching to an agent known chiefly for its ability to maintain existing bone mass preserves new bone induced by PGE2 in osteopenic, estrogen-depleted rats. The current study had three phases, the bone loss (-), restore (+), and maintain (+/-) phases. We ovariectomized (OX) or sham ovariectomized (sham-OX) 5.5-month-old female rats (- phase). The OX rats were treated 5 months postovariectomy with 1-6 mg PGE2 per kg/day for 75 days to restore lost cancellous bone mass (+ phase), and then PGE2 treatment was stopped and treatment began with 1 or 5 micrograms/kg of risedronate, a bisphosphonate, twice a week for 60 days (+/- phase). During the loss (-) phase, the cancellous bone volume of the proximal tibial metaphysis in the OX rat fell to 19% of initial and 30% of age-matched control levels. During the restore (+) phase, the cancellous bone volume in OX rats doubled. When PGE2 treatment was stopped, however, and no special maintenance efforts were made during the maintain (+/-) phase, the PGE2-induced cancellous bone disappeared. In contrast, the PGE2-induced cancellous bone persisted when the PGE2 treatment was followed by either a 1 or 5 micrograms treatment of risedronate per kg given twice a week for 60 days during the maintain (+/-) phase. The tibial shaft demonstrated very little cortical bone loss during the loss (-) phase in OX rats. The tibial shaft cortical bone fell some 8%. During the restore (+) phase, new cortical bone in OX rats increased by 22%. When PGE2 treatment was stopped and nothing was given during the maintain (+/-) phase, however, all but the PGE2-induced subperiosteal bone disappeared. In contrast, when PGE2 treatment was stopped and 1 micron risedronate per kg twice a week for 60 days was administered during the maintenance (+/-) phase, the PGE2-induced subperiosteal bone and some of the subendocortical bone and marrow trabeculae persisted.(ABSTRACT TRUNCATED AT 400 WORDS)

Hit rates and radiation doses to nuclei of bone lining cells from alpha-particle-emitting radionuclides

Factors relating the local concentration of a bone-seeking alpha-particle emitter to the mean hit rate have been determined for nuclei of bone lining cells using a Monte Carlo procedure. Cell nuclei were approximated by oblate spheroids with dimensions and location taken from a previous histomorphometric study. The Monte Carlo simulation is applicable for planar and diffuse labels at plane or cylindrical bone surfaces. Additionally, the mean nuclear dose per hit, the dose mean per hit, the mean track segment length and its second moment, the percentage of stoppers, and the frequency distribution of the dose have been determined. Some basic features of the hit statistics for bone lining cells have been outlined, and the consequences of existing standards of radiation protection with regard to the hit frequency to cell nuclei are discussed.

Local distribution and dosimetry of 226Ra in the trabecular skeleton of the beagle

Young adult beagle dogs received a single injection of 38.1 kBq/kg body wt 226Ra and were serially sacrificed at 4 to 2955 days postinjection. Samples of sites of trabecular bone in the lumbar vertebral body, proximal ulna, and distal femoral metaphysis and epiphysis were analyzed autoradiographically. The time-dependent changes in the average 226Ra concentrations in the four regions were analyzed in terms of a compartmental model. The clearance rate from the lumbar vertebral body was about four times more rapid than for the proximal ulna and distal femoral epiphysis. Ratios of hotspot to diffuse label concentrations varied from about 10 to 23. The dose rate to the endosteum ranged between 8.7 and 39.5 mGy/day initially and 4 and 10.5 mGy/day toward the end of the observation period. Mean marrow dose rates were lower by a factor of 3 to 9.5. During their residence time the nuclei of bone lining cells receive a maximum dose of 8 Gy in the proximal ulna (2955 days after injection) and a minimum dose of 0.63 Gy in the lumbar vertebra (2955 days after injection). This corresponds on the average to 17 and 1.4 alpha-particle hits to the cell nuclei, respectively.

Loss of prostaglandin E2-induced extra cortical bone after its withdrawal in rats

The object of this study was to determine the fate of PGE2-induced new cortical bone mass after withdrawal of PGE2 administration. Seven-month-old male Sprague-Dawley rats were given subcutaneous injections of 1, 3 and 6 mg PGE2/kg/day for 60 days and then withdrawn for 60 and 120 days (on/off treatment). Histomorphometric analyses were performed on double-fluorescent-labeled undecalcified tibial shaft sections (proximal to the tibiofibular junction). In a previous report we showed that after 60, 120 and 180 days of daily PGE2 (on)treatment, a new steady state was achieved marked by increased total bone area (+16%, +25% and +34% with 1, 3 and 6 mg PGE2/kg/day) when compared to age-matched controls. The continuous PGE2 treatment stimulated periosteal and endocortical lamellar bone formation, activated endocortical woven trabecular bone formation and intracortical bone resorption. These responses increased cortical bone mass since the bone formation exceeded bone resorption. The current study showed that after withdrawal of PGE2 for 60 and 120 days, the extra endocortical bone, which was induced by the first 60-days treatment, was resorbed, but the new subperiosteal bone persisted resulting in a tibial shaft with larger cross sectional and marrow areas. Despite that, there was still the same amount of bone mass in these shafts as in age-related controls. A new steady state was achieved after 60 days of withdrawal, in which the bone mass and bone formation activity approximated that of age-related controls. It was concluded that maintaining the extra PGE2-induced cortical bone mass depends on continuous daily administration of PGE2.

Effects of long-term daily administration of prostaglandin-E2 on maintaining elevated proximal tibial metaphyseal cancellous bone mass in male rats

The effects of long-term prostaglandin E2 (PGE2) on cancellous bone in proximal tibial metaphysis were studied in 7-month-old male Sprague-Dawley rats given daily subcutaneous injections of 0, 1, 3, and 6 mg PGE2/kg/day and sacrificed after 60, 120, and 180 days. Histomorphometric analyses were performed on double fluorescent-labeled undecalcified bone specimens. After 60 days of treatment, PGE2 produced diffusely labeled trabecular bone area, increased trabecular bone area, eroded and labeled trabecular perimeter, mineral apposition rate, and bone formation rate at all dose levels when compared with age-matched controls. In rats given PGE2 for longer time periods (120 and 180 days), trabecular bone area, diffusely labeled trabecular bone area, labeled perimeter, mineral apposition, and bone formation rates were sustained at the elevated levels achieved earlier at 60-day treatment. The eroded perimeter continued to increase until 120 days, then plateau. The observation that continuous systemic PGE2 administration to adult male rats elevated metaphyseal cancellous bone mass to 3.5-fold of the control level within 60 days and maintained it for another 120 days indicates that the powerful skeletal anabolic effects of PGE2 can be sustained with continuous administration.

Production of new trabecular bone in osteopenic ovariectomized rats by prostaglandin E2

Serum chemistry and bone morphometry of the proximal tibial metaphysis were performed in 3-month-old double fluorescent-labeled, female Sprague-Dawley rats subjected to bilateral ovariectomy or sham surgery for 4 months prior to treatment with 0, 0.3, 1, 3, or 6 mg of prostaglandin E2 (PGE2)/kg/day subcutaneously for 30 days. The 4-month postovariectomized rats possessed an osteopenic proximal tibial metaphysis with 7% trabecular area compared with controls (19%). PGE2 treatment elevated osteocalcin levels and augmented proximal tibial metaphyseal bone area in ovariectomized and sham-operated rats. Osteopenic, ovariectomized rats treated with 6 mg PGE2/kg/day for 30 days restored bone area to levels of age-matched sham-operated rats. Morphometric analyses showed increased woven and lamellar bone area, fluorescent-labeled perimeter (osteoblastic recruitment), mineral apposition rate (osteoblastic activity), bone formation rate (BFR/BV), and longitudinal bone growth. These dramatic bone changes were all significantly increased at the dose-response manner. This study showed that in vivo PGE2 is a powerful activator of bone remodeling, it increases both bone resorption and bone formation, and produces an anabolic effect by shifting bone balance to the positive direction. Furthermore, PGE2-induced augmentation of metaphyseal bone area in ovariectomized rats was at least two times greater than in sham-operated rats.

Prostaglandin E2 prevents bone loss and adds extra bone to immobilized distal femoral metaphysis in female rats

The object of this study was to determine whether prostaglandin E2 (PGE2) can prevent disuse (underloading)-induced cancellous bone loss. Thirteen-month-old retired female Sprague-Dawley breeders served as controls or were subjected to right hindlimb immobilization by bandaging and simultaneously treated subcutaneously daily with 0, 1, 3, or 6 mg PGE2/kg/d for two and six weeks. Histomorphometric analyses were performed on the cancellous bone using double-fluorescent labeled, 20 micron thick, undecalcified distal femoral metaphysis sections. We found that PGE2 administration not only prevented diuse-induced bone loss, but also added extra bone to disuse cancellous bone in a dose-response manner. PGE2 prevented the disuse-induced osteopenia by stimulating more bone formation than resorption and shortening the period of bone remodeling. It activated woven bone formation, stimulated lamellar bone formation, and increased the eroded bone surface above that caused by disuse alone. While underloading increased the remodeling period (sigma), PGE2 treatment of underloaded bone shortened the time for osteoclastic bone resorption and bone remodeling, and thus reduced the remodeling space. The study shows that PGE2 is a powerful anabolic agent that prevents disuse-induced osteopenia and adds extra bone to these same bones.

Prostaglandin E2 prevents disuse-induced cortical bone loss

The object of this study was to determine whether prostaglandin E2 (PGE2) can prevent disuse (underloaded)-induced cortical bone loss as well as add extra bone to underloaded bones. Thirteen-month-old retired female Sprague-Dawley breeders served as controls or were subjected to simultaneous right hindlimb immobilization by bandaging and daily subcutaneous doses of 0, 1, 3, or 6 mg PGE2/kg/d for two and six weeks. Histomorphometric analyses were performed on double-fluorescent labeled undecalcified tibial shaft sections (proximal to the tibiofibular junction). Disuse-induced cortical bone loss occurred by enlarging the marrow cavity and increasing intracortical porosity. PGE2 treatment of disuse shafts further increased intracortical porosity above that in disuse alone controls. This bone loss was counteracted by enhancement of periosteal and corticoendosteal bone formation. Stimulation of periosteal and corticoendosteal bone formation slightly enlarged the total tissue (cross-sectional) area and inhibited marrow cavity enlargement. These PGE2-induced activities netted the same percentage of cortical bone with a different distribution than the beginning and age-related controls. These findings indicate the PGE2-induced increase in bone formation compensated for the disuse and PGE2-induced bone loss, and thus prevented immobilization-induced bone loss.

Long-term anabolic effects of prostaglandin-E2 on tibial diaphyseal bone in male rats

The effects of long-term prostaglandin E2 (PGE2) on tibial diaphyseal bone were studied in 7-month-old male Sprague-Dawley rats given daily subcutaneous injections of 0, 1, 3 and 6 mg PGE2/kg/day for 60, 120 and 180 days. The tibial shaft was measured by single photon absorptiometry and dynamic histomorphometric analyses were performed on double-fluorescent labeled undecalcified tibial diaphyseal bone samples. Exogenous PGE2 administration produced the following transient changes in a dose-response manner between zero and 60 days: 1) increased bone width and mineral density; 2) increased total tissue and total bone areas; 3) decreased marrow area; 4) increased periosteal and corticoendosteal lamellar bone formation; 5) activated corticoendosteal lamellar and woven trabecular bone formation and 6) activated intracortical bone remodeling. A new steady-state of increased tibial diaphyseal bone mass and elevated bone activities were observed from day 60 onward. The elevated bone mass level attained after 60 days of PGE2 treatment was maintained at 120 and 180 days. These observations indicate that the powerful anabolic effects of PGE2 will increase both periosteal and corticoendosteal bone mass and sustain the transient increase in bone mass with continuous daily administration of PGE2.

Adaptation of diaphyseal structure with aging and increased mechanical usage in the adult rat: a histomorphometrical and biomechanical study

The experimental increase in mechanical usage or overloading of the left hindlimb was produced by immobilization of the contralateral hindlimb. The right hindlimb was placed in a flexed position against the body and was immobilized using an elastic bandage. Some control animals were sacrificed initially at time zero and increased mechanical usage and age-matched control animals were sacrificed after 2, 10, 18, and 26 weeks of treatment. All animals received double bone fluorochrome labeling prior to sacrifice. Cortical bone histomorphometry and cross-sectional moments of inertia were determined. Marrow cavity enlargement and total cross-sectional area expansion represented the age-related cortical bone changes. Increased mechanical usage enhanced periosteal bone modeling in the formation mode and dampened endocortical bone remodeling and bone modeling in the resorption mode (resorption drift) to create a slight positive bone balance. These observations are in general agreement with Frost's postulate for mechanical effects on bone modeling and remodeling (Frost, H.M. 1987b. Bone "mass" and the "mechanostat." A proposal. Anat. Rec. 219: 1-9). The maximum moment of inertia did not change significantly in either control or overloaded tibial shafts. The minimum and polar moment of inertias in overloaded bones increases over those of controls at 18 and 26 weeks of the experiment.

Adaptation of diaphyseal structure to aging and decreased mechanical loading in the adult rat: a densitometric and histomorphometric study

Nine-month-old female rats were subjected to right hindlimb immobilization or served as controls for 0, 2, 10, 18, and 26 weeks. They were double-labeled with bone markers prior to sacrifice. Experimental unloading was produced by immobilizing the right limb against the abdomen with an elastic bandage. Single-photon absorptiometry was performed on the intact femurs; static and dynamic histomorphometry were performed on 20-micron thick toluidine blue-stained, undecalcified cross sections of the tibial shafts. Changes in the continuously immobilized tibiae were compared to those in both tibiae of age-matched controls. Unloading shut off nearly all periosteal bone formation and accelerates bone marrow expansion over that which occurs in age-related controls. The effect of unloading appeared to be mediated by recruiting fewer osteoblasts which showed inhibited activity. Furthermore, unloading increased endocortical percentage eroded surface. These histological changes lowered cortical bone mass by inhibiting diaphyseal cross sectional expansion and enlarging the bone marrow cavity. The results support Frost's suggestion that decrease mechanical usage depresses bone modeling-dependent bone gain by decreasing activation of modeling in the formation mode. It also stimulates bone remodeling-dependent bone loss by increasing activation of remodeling in the resorption mode.

Partial loss of anabolic effect of prostaglandin E2 on bone after its withdrawal in rats

The object of this study was to determine the fate of PGE2-induced new bone mass after withdrawal of PGE2 administration. Seven-month-old male Sprague-Dawley rats were given subcutaneous injections of 1, 3, and 6 mg PGE2/kg/d for 60 days and then withdrawn for 60 and 120 days. Histomorphometric analyses were performed on double fluorescent labeled undecalcified proximal tibial bone specimens. After 60 days of PGE2 treatment, a new steady state of increased trabecular bone area (+67% and +81% with 3 and 6 mg PGE2/kg/d) from woven bone and stimulated lamellar bone formation, elevated bone turnover, and shortened remodeling periods were achieved compared to age-matched controls. In contrast, after 60 and 120 days withdrawal of PGE2, a new steady state characterized by less trabecular bone area (+40% to +60% of controls with 3 and 6 mg/kg/d doses), normal lamellar bone formation, no woven bone formation from controls, and eroded surface greater than those seen in controls and previously in 60-day PGE2 treated rats. The decrease in new bone mass after withdrawal of PGE2 was due to a further elevation of bone resorption above that induced by the PGE2 treatment and a reduction in PGE2 stimulated bone formation activities. Although there is more trabecular bone than in controls after 120 days' withdrawal of PGE2, we postulate that the skeletal adaptation to mechanical usage will eventually reduce the bone mass to control levels. Thus, it is conservative to conclude that the anabolic effect of PGE2 was dependent upon continuous daily administration of PGE2 in these older rats.

The role of prostaglandins in bone in vivo

Prostaglandins of the E series, primarily E2 and E1, have the greatest activity in bone. Following discovery of their potent ability to stimulate bone resorption in vitro, clinical investigations have placed prostaglandins at sites of localized bone resorption associated with inflammatory or space occupying lesions in vivo. These studies have shown that prostaglandin production at such sites may be increased by cytokines such as interleukin-1 but the mechanisms by which prostaglandins stimulate bone resorption are not yet known. Observation of periosteal bone formation in patients given, pharmacological doses of prostaglandin has led to investigation of its bone forming activity. Young, growing rats have increased metaphyseal bone formation and this is accompanied by increased periosteal and endocortical bone formation in older animals. In the mature animals there is a generalized activation of remodelling with increased formation in the remodeling cycle. This is also seen in oophorectomized rats and results in repletion of the lost bone in this model of osteoporosis. In animal models of localized disuse osteopenia, prostaglandins are found to be elevated at the site of bone loss and prostaglandin inhibitors at least partially protect against the exaggerated resorption that occurs. This is also seen in models of orthodontic tooth movement, periodontitis and osteomyelitis. Prostaglandin synthesis inhibitors have been shown to delay healing of bone and this has led to limitations on their use clinically in some situations. Exogenously administered prostaglandins have been found to enhance periosteal callus formation, but healing is not uniformly enhanced. Prostaglandins have also been associated with hypercalcemia in certain animal tumors that model human hypercalcemia of malignancy but are probably most important in this condition as mediators in the localized resorption of bone at tumor sites. These in vivo studies have shown that prostaglandins are involved with increases in both bone formation and bone resorption. In vitro studies have shown that prostaglandins stimulate osteoblasts as well as osteoclastic bone resorption but understanding these effects under in vivo conditions will require further investigation.

A model of osteon closure in cortical bone

A model of osteon closure is presented that incorporates some physiologic features of cortical bone remodeling, such as matrix synthesizing activity of osteoblasts, their burial as osteocytes, and elimination of cells. A simplified version of the model assumes a constant osteoblast activity throughout radial closure. An extended version allows for variable osteoblasts activity and is based on Lee's law of radial closure kinetics. The model calculations (extended model) show that both in humans and beagle dogs osteoblast activity steadily decreases during radial closure of the osteon. The potential of the model is also illustrated by calculating the dynamic change of the geometrical shape of the closing cone and the time dependence of the osteoid seam width, number of osteoblasts and bone formation rate in the closing cone.

Adaptation of cancellous bone to overloading in the adult rat: a single photon absorptiometry and histomorphometry study

Nine-month-old female rats were subjected to right hindlimb immobilization or served as controls for 0, 2, 10, 18, and 26 weeks and were double-labeled with bone markers. The right limb was immobilized against the abdomen and considered unloaded, while the left limb was overloaded during ambulation. Single-photon absorptiometry was performed on intact femur; static and dynamic histomorphometry were performed on 20 microns thick undecalcified frontal sections of the proximal tibial metaphysis. Changes in the continuously overloaded limb was compared to that in both limbs of age-matched control animals. Single-photon absorptiometry detected increases of bone mineral density of +6%, +6%, and +5% in the proximal and +9%, +7%, and +10% in the distal femoral metaphyses after 10, 18, and 26 weeks of continuous overloading. Morphometrically, significant changes occurred in proximal tibial metaphyses compared to age-matched controls: trabecular area increased +41% and +45%, trabecular number increased +31% and +32%, and trabecular separation decreased -30% and -31% after 18 and 26 weeks of overloading. A significant increase in mineral apposition rate (+38%) was found only at 26 weeks of overloading. Insignificant decreases in both eroded and labeled bone surfaces occurred at all time periods. The histomorphometric changes indicated that increased cancellous bone mass was caused by an increase in bone formation activity (i.e., increases in mineral apposition and bone formation rates) and a decrease in remodeling space (i.e., decrease in bone eroded surface). These findings indicate that the adult skeleton can quickly adapt to the increased biomechanical needs by increasing its cancellous bone mass with an adequate structural pattern.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of cancellous bone to aging and immobilization in the rat: a single photon absorptiometry and histomorphometry study

Nine-month-old female rats were double-labeled with bone markers and subjected to right hindlimb immobilization or served as control for 0, 2, 10, 18, or 26 weeks. The right limb was immobilized against the abdomen, thus unloading it, while the left limb was overloaded during ambulation. Single photon absorptiometry and cancellous bone histomorphometry were performed on dissected intact femur and 20-microns-thick undecalcified specimens of the proximal tibial metaphysis. In the unloaded limb, immobilization-induced muscle and cancellous bone loss occurred rapidly before 10 weeks and stabilized at 50% less bone mass after 18 weeks. Unloading caused a negative bone balance from a combination of elevated bone resorption and depressed bone formation. At 2, 10, and 18 weeks of immobilization, the ratios of bone resorption to bone formation surfaces were 1.6, 1.5, and 1.3, respectively; at 26 weeks, the ratio was 1. The bone loss was accompanied by poorer trabecular architecture (trabecular number decreased and trabecular separation increased), reaching the maximum at 18 weeks and stabilizing thereafter. These observations are in general agreement with Frost's postulate for mechanical effects on lamellar bone remodeling, and the findings on disuse osteoporosis in man. Therefore, the one-legged immobilization model can be useful in studies of the mechanisms of structural adaptation to mechanical usage.

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

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

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

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

Effects of prostaglandin E2 on production of new cancellous bone in the axial skeleton of ovariectomized rats

The effects of prostaglandin E2 (PGE2) were histomorphometrically evaluated in cancellous bone of the axial skeleton of ovariectomized, osteopenic rats. Four months following bilateral ovariectomy (OVX) and sham-ovariectomy (SHAM) at 3 months of age, rats received daily subcutaneous injections of PGE2 at 0, 0.3, 1.0, 3.0 and 6.0 mg/kg/day for 30 days. The undecalcified fourth lumbar vertebral bodies (LVB) were processed for static and dynamic bone histomorphometry. The OVX rats possessed a slightly osteopenic LVB (17% vs. 24% cancellous bone mass). In rats given PGE2 at 3 and 6 mg/kg/day for 30 days, bone turnover, lamellar bone mass, and formation of new woven bone trabeculae were increased. Observations supported the conclusion that PGE2 activates bone modeling and remodeling, and shifts bone balance in favor of formation. In OVX rats given 6 mg PGE2/kg/day, cancellous bone mass and trabecular numbers were restored to levels found in untreated SHAM rats. Cancellous bone mass in the LVB of SHAM rats given 3 and 6 mg PGE2/kg/day increased by 16% and 30% over that of control rats. In addition, PGE2 stimulated longitudinal bone growth in both OVX and SHAM rats, a response that differed from male rats.

Ultrastructural features of osteoclasts in situ

The morphology of in situ osteoclasts on endocortical surfaces of the femoral midshaft was examined by scanning electron microscopy. Mice were perfusion fixed and bone marrow plugs were flushed out of femoral diaphyseal cylinders. The bones were split longitudinally and the endocortical surfaces examined. This method left on the bone surface most of the endosteal cells in their natural, in situ shape and position. Most of the bone surface was lined by contiguous bone lining cells covering resting bone surfaces, making a clear physical barrier between the bone and marrow compartments. On resorption surfaces, which were characterized by excavation cavities, osteoclasts were very polymorphic and spread on the bone surface, extending large pseudopods. The in vivo morphology of individual osteoclasts appears somewhat similar to that described by other investigators on calvaria surfaces and for isolated osteoclasts adherent to artificial substrates. In the resorption domains, osteoclasts appeared to be connected with adjacent osteoclasts, suggesting that the cells form a functional syncytium in resorption areas.

Bone structural parameters, dosimetry, and relative radiation risk in the beagle skeleton

A variety of morphometric and histomorphometric parameters such as the mass of bone and marrow, bone surface areas, percentage of bone volume, percentage of the surface that is trabecular, and percentage of surfaces that are forming and resting are calculated for all major parts of the beagle skeleton. The total bone surface of the beagle is estimated at 2.9 m2 with 53.7% of the surface area being associated with trabecular bone. There are about 4.5 x 10(9) bone-lining cells and about 1 x 10(9) osteoblasts. From the fractional retention in each part of the skeleton, the initial surface concentration of 239Pu after a single injection of 592 Bq/kg body wt (0.016 microCi/kg) on resting surfaces and at sites of bone formation is calculated for various values of the affinity ratios of trabecular/cortical and forming/resting surfaces. These estimated concentrations then yield dose rates as well as cumulative and collective doses to bone-lining cells and osteoblasts in the different parts of the skeleton. On the assumption that the relative risk of tumor induction is proportional to the collective dose to either bone-lining cells or osteoblasts, the frequency of tumor occurrence is calculated and compared to observed frequencies. Both hypotheses yield approximate agreement with experimental data for different ratios of trabecular/cortical radiation sensitivity, although the differences between some bones are statistically significant.

Bone lining cells: structure and function

Bone lining cells (BLC's) cover inactive (nonremodeling) bone surfaces, particularly evident in the adult skeleton. BLC's are thinly extended over bone surfaces, have flat or slightly ovoid nuclei, connect to other BLC's via gap junctions, and send cell processes into surface canaliculi. BLC's can be induced to proliferate and differentiate into osteogenic cells and may represent a source of "determined" osteogenic precursors. BLC's and other cells of the endosteal tissues may be an integral part of the marrow stromal system and have important functions in hematopoiesis, perhaps by controlling the inductive microenvironment. Because activation of bone remodeling occurs on inactive bone surfaces, BLC's may be involved in the propagation of the activation signal that initiates bone resorption and bone remodeling. Evidence also suggests that BLC's are important in the maintenance of the bone fluids and the fluxes of ions between the bone fluid and interstitial fluid compartments for mineral homeostasis.

The effects of chronic vitamin A excess on bone remodeling in aged rats

This study was conducted to assess the effects of long-term ingestion of moderate excesses of vitamin A on trabecular bone remodeling in the fifth lumbar vertebral body of aged rats. Eighteen-month-old rats were fed diets with vitamin A content equal to the daily requirement (DR), 2-fold, and 5-fold the DR along with calcium content of either the DR or 0.3-fold the DR, for 14 months each. As expected, serum concentrations of 1,25-dihydroxyvitamin D were higher in the reduced than in the normal calcium intake groups (65.1 +/- 2.4 SEM vs 47.8 +/- 2.1 pg/ml, P less than 0.001). Calcium balance was more positive at the higher than the lower calcium intake (5.7 vs 0.9 mg, P less than 0.001) but was unaffected by vitamin A intake. Histomorphometric analysis of the fifth lumbar vertebral body revealed that the 2-fold but not the 5-fold excess in vitamin A intake resulted in a 15% increase in percentage of trabecular bone (P less than 0.02). The low calcium diet depressed bone growth (total bone tissue) but did not affect percentage of trabecular bone. Several effects of the vitamin A excess and low calcium diet were noted along the trabecular surface including increased mineral apposition rate and resorption surfaces and decreased formation surfaces. The net effect of vitamin A on trabecular bone of the rat varies as intake begins to exceed the DR. At a 2-fold excess, a modest favorable effect on percentage of trabecular bone was observed.

Flurbiprofen enhances growth and cancellous and cortical bone accumulation in rapidly growing long bones

The effects of flurbiprofen, a non-steroidal anti-inflammatory drug, on bone growth was studied by static and dynamic histomorphometry in immature (28 days old) male Sprague-Dawley rats. Flurbiprofen at 0, 0.02, 0.1, 0.5 or 2.5 mg/kg/d doses was given subcutaneously daily for 21 days. The 0.1 and 0.5 mg/kg/d doses were most effective in stimulating longitudinal and radial bone growth and enhancing the accumulation of cancellous and cortical bone. Proximal tibial longitudinal bone growth rate, growth plate thickness, and periosteal bone formation rate were increased 30-40%, while cortical bone (tibial shaft) and cancellous bone (proximal tibial metaphysis) accumulated 12% and 90% more bone than controls, respectively. Enhanced accumulation of cortical bone was attributed to stimulated periosteal bone formation without accompanying marrow cavity enlargement. Enhanced accumulation of cancellous hard tissue was postulated to be due to reduced trabecular bone resorption and no effect on bone formation. The cell counts support these conclusions. There was a decrease in osteoclast numbers (-62 to -70%), an insignificant decrease in osteoblast numbers (-5 to -30%) per mm of bone surface and a decrease in osteoclast to osteoblast ratio (-35 to -56%). The findings presented are compatible with the conclusion that flurbiprofen, induced changes in rapidly growing long bones by reducing osteoclast activity and recruitment, stimulating longitudinal and radial growth, increasing the cortical bone mass by stimulated periosteal bone growth and depressed endosteal resorption, and increasing cancellous bone mass by depressed trabecular bone resorption without affecting bone formation.

Microdistribution and local dosimetry of 226Ra in trabecular bone of the beagle

Sections of lumbar vertebral bodies of young adult beagle dogs have been analyzed autoradiographically to characterize and quantify the local distribution of 226Ra by means of a scanning microscope photometer. The animals received a single injection of 355 kBq/kg body weight and were serially sacrificed at 5 to 1381 days postinjection. Hotspot concentrations decreased from about 51 kBq/g bone at 5 days to 20 kBq/g at 1381 days postinjection. The diffuse concentration changed from 8.3 to 1.9 kBq/g. The mean 226Ra concentration in the trabecular areas scanned was initially higher and at the end of the observation period lower than the average calculated for the whole lumbar vertebral column. Density and area of, and fraction of bone activity in, hotspots virtually remained constant. With time hotspots tended to become translocated into bone volume. Mean dose rates to lining cells from both hotspots and diffuse labels decreased from about 210 mGy/d at early postinjection times to 105 mGy/d. This corresponds to 2.5 to 1.1 times the average skeletal dose rate. A discussion of the level of irradiation in terms of hit frequencies shows that osteoblasts in the initial phase of hotspot formation receive about 60 hits to their nucleus for the duration of bone formation. After about 6 months, however, the 226Ra concentration in new bone and the corresponding hit frequency appears to be low enough that interference with bone formation is unlikely. Morphometric measurements showed that abnormal bone accretion and thickening of trabeculae occurred. This was interpreted as an imbalance between bone formation and resorption. Both formation and resorption seem to be substantially lowered compared to control animals.

Effects of N,N,N',N'-ethylenediaminetetramethylene phosphonic acid on cortical bone remodeling in the adult dog

Phosphonic acids are of interest because of their potential usefulness in the treatment of disorders of bone and mineral metabolism, as a treatment for dental calculus, and as skeletal imaging agents. The effects of oral doses of N,N,N',N'-ethylenediaminetetramethylene phosphonic acid (EDITEMPA) on cortical bone remodeling and histology of adult dogs were determined. After treatment for 3 and 6 months, there were dose-related changes observed in cortical bone as measured using histomorphometric methods. With higher doses, EDITEMPA treatment resulted in the accumulation of osteoid due to impaired or delayed mineralization of bone. There was an increased mineralized and osseous tissue porosity suggesting an early increase in bone resorption perhaps due to an increased activation of bone remodeling.

Skeletal tissue responses to thermal injury: an experimental study

Skeletal changes occurring secondary to burn injuries were studied in an experimental animal model for thermal injury. One hindlimb of female Sprague-Dawley rats (200-250g) was subjected to a standardized thermal injury; the other hindlimb was left untreated. Control animals received no experimental treatment. Effects on skeletal architecture were studied at the proximal tibial metaphysis and tibial diaphysis using static histomorphometry. Bone formation dynamics were studied from a series of bone fluorochrome labels administered before the experiment began, early (days 8, 9) postburn treatment (PBT) and late PBT (days 17, 18). Animals were sacrificed on day 21 PBT. In proximal tibial metaphyses of burn-treated limbs, trabecular bone area (TBA) and trabecular number in all regions except the primary spongiosa, were significantly reduced. TBA was also decreased, but not significantly in nontreated limbs. Longitudinal growth rate, growth plate thickness and growth cartilage cell production rate are greater in burn-treated than in nonburned and control bones. Burn-treated diaphyses showed extensive woven bone formation at periosteal surfaces, and corresponding increases of bone areas and periosteal perimeters. Endocortical surfaces showed only typical occasional resorption areas. No intracortical changes were observed. Mineral appositional rate (MAR) and bone formation rate (BFR) at endocortical surfaces were markedly depressed after thermal injury, significant changes were noted in both limbs of treated animals. Among burned limbs, the early PBT label was absent from all specimens, indicating a virtual shutdown of osteoblast activity and recruitment. Similarly in nonburned limb bone, the label was absent from 50% of the specimens; in those bones in which the label was present, label lengths, appositional and bone formation rates were significantly reduced relative to the control specimens. Comparison of average bone formation dynamics for the total PBT interval indicates that MAR and BFR in burned treated tibiae were reduced to approximately 25% of control values. MAR and BFR from the nonburned side of treated animals were significantly reduced as well, to about 55% of control values. These data indicate that the principal metaphyseal effects of thermal injury are stimulation of growth cartilage proliferation, and depression of ossification and osteoblast activity. In diaphyses, thermal injury causes extensive local periosteal woven bone proliferation and a dramatic depression of endosteal bone formation. The latter effect, while more severe locally, is also evident systemically.

Flurbiprofen-induced stimulation of periosteal bone formation and inhibition of bone resorption in older rats

The skeletal effects of flurbiprofen (Fb), a nonsteroidal anti-inflammatory drug, was studied by histomorphometry in 9-month-old retired female breeder, Sprague-Dawley rats. Flurbiprofen was given subcutaneously at 0, 0.2, 0.1, 0.5, 2.5, or 5 mg/kg/d for 21 days. Flurbiprofen had no effect on longitudinal growth, but stimulated radial growth (+200%) over controls. In the tibial shaft, Fb stimulated the mineral apposition rate (+25%), mineral bone formation rate (+100%), and periosteal labeling length (+64%) at the 2.5 and 5.0 mg Fb/kg dose levels, and had no effect on marrow cavity size compared to controls. However, these changes were insufficient to increase cortical bone mass. In the proximal tibial metaphysis, Fb suppressed osteoclasts/mm2 of metaphyseal tissue (-47%), osteoclasts/mm of bone surface (-46%), and the osteoclast/osteoblast ratio (-50%), increased the calcified cartilage core population (+100%), and had no effect on osteoblast numbers at all dose levels. There was an insignificant increase in metaphyseal cancellous bone mass. The current study leads to the conclusion that flurbiprofen-stimulated periosteal bone growth was due to direct stimulation of osteoblast recruitment and activity independent of longitudinal bone growth. Further, it confirms early findings in young rats that flurbiprofen induced depressed bone resorption without lowering bone formation. However, because of insufficient treatment time, the older rat did not accumulate bone as the young rats did.

Bone age and remodeling: a mathematical treatise

Remodeling of an element of mineralized bone is described by specifying the probability per unit time of being replaced as a function of age of the element. The functional dependence of this probability is called the "law of remodeling." The properties of this stochastic law of remodeling are discussed in terms of random, selective, and redundant remodeling. A relationship between the law of remodeling and the mean bone tissue age is derived. It is shown how the probability density of bone age for individual elements of bone depends on the law of remodeling. The mathematical formalism is exemplified by assuming a specific parametric form of the law. A procedure for experimentally determining the law is suggested based on tracing the resorption of single fluorescent labels. Finally, the extension of the model to a real skeleton with regional differences in turnover rates is discussed.

The bone lining cell: a distinct phenotype?

It is argued that the flat cells that line nonremodeling endosteal bone surfaces are a distinct phenotype. These cells have a distinct morphology and they most likely have important functional roles in skeletal physiology, metabolism, and remodeling. For these reasons this cell seems deserving of a proper name. The name bone lining cell seems to have gained some acceptance but there will be continued confusion as long as skeletologists use this same term to generically describe cells that line bone surfaces, regardless of their actual identity.

The role of bone cells in increasing metaphyseal hard tissue in rapidly growing rats treated with prostaglandin E2

The skeletal effects of graded doses of prostaglandin E2 (PGE2) given to weanling Sprague-Dawley rats for 3 weeks were investigated to elucidate the role of bone cells in increasing hard tissue mass. Decalcified (3 micron) sections were quantified in the light microscope by point hit and intersect counting using a Merz grid. Hard tissue mass (bone and calcified cartilage) and osteoblast, osteoclast and osteoprogenitor cell numbers were counted in metaphyseal tissue bands 0.24, 0.48, 0.72, 1.20, 1.68, 2.16, 2.64, 3.12, 3.60 and 4.08 mm from the growth plate metaphyseal junction. Changes were different and more marked in the secondary spongiosa than the primary spongiosa of the proximal tibial metaphysis of treated rats. In the primary spongiosa of rats treated with 3 or 6 mg PGE2/kg/d (1) an increase in bone and hard tissue masses and (2) a decrease in osteoclasts, osteoprogenitor cell numbers and surface to volume ratio was observed. In the secondary spongiosa (lower metaphysis) of rats treated with 2 same dose levels (1) an increase in bone mass, calcified cartilage cores, and hard tissue mass and perimeter, an elevation of osteoprogenitor cell and osteoblast numbers, a depression of osteoclast, osteoclast nuclei numbers and surface to volume ratio and new sites of intramembranous ossification (woven bone formation) originating from the cortico-endosteal envelope was observed. In this growing rat skeletal model, we showed that PGE2 increases metaphyseal calcified tissue mass by depressing hard tissue resorption and stimulating the replication and differentiation of osteoblast precursors to form new foci of woven bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Humoral and ionic regulation of osteoclast acidity

Regulation of the acidity of osteoclasts was determined in situ on the endocranial surfaces of mouse calvaria using acridine orange, a fluorescent weak base. Osteoclasts could be identified by large size, multiple nuclei, relatively small numbers of cells, and the way and the extent to which they took up the dye. Nonosteoclastic cells were stained mainly in their nuclei and occasionally in a few lysosomes surrounding their nuclei, which were uniformly single in nonosteoclasts. Nuclei in osteoclasts were also stained, but the staining of the nuclei was partially masked by the intensity and completeness of the staining of the cytoplasm. In some cells the cytoplasmic staining appeared to be in discrete granules, giving the cytoplasm a bright, frothy appearance. This fluorescence was present in both treated and untreated cells and aided in identifying the osteoclasts. Acridine orange fluorescence at 624 nm intensity, and hence, osteoclast acidity, was increased by parathyroid hormone and prostaglandin E2. Parathyroid hormone-induced increases in acidity were inhibited by calcitonin, cortisol, sodium fluoride, and prostaglandin E2. Furthermore, osteoclast acidity was dependent largely or partially on maintenance of K+ and Na+ gradients, patent Na+ channels, chloride-bicarbonate exchange, and H+, K+-ATPase. These findings demonstrate that osteoclasts become acidified by mechanisms similar to those occurring in gastric parietal cells.

Effect of 16,16-dimethyl prostaglandin E2 methyl ester on weanling rat skeleton: daily and systemic administration

The effects of 0, 0.3, 1.0, and 3.0 mg of 16,16-dimethyl prostaglandin E2 methyl ester (Di-M-PGE2) per kilogram per day administered subcutaneously for 21 days to fluorescent-labeled weanling rats were studied, by single-photon absorptiometric and static and dynamic histomorphometric techniques, to determine possible alterations in growth and mineralized tissue mass and their mechanisms of response. Specimens of femurs, proximal tibia, and tibial shaft were analyzed. Di-M-PGE2 caused a reduction in bone elongation and a dramatic accumulation in metaphyseal trabecular hard tissue mass. At high doses, the growth cartilage exhibited reduced thickness and degenerative cell size and cell production rate. The increased metaphyseal trabecular hard tissue mass was restricted to the secondary spongiosa region and was observed at all dose levels. The metaphysis was further characterized by an increase in bone and calcified cartilage cores, a marked elevation in osteoblast and osteoclast numbers, in osteoblast-to-osteoclast ratios, and in ratios of differentiated cells to osteoprogenitor cells. These findings were consistent with the interpretations that Di-M-PGE2 depressed bone elongation by delaying the division and maturation of growth plate chondrocytes; stimulated the differentiation of osteoblasts and osteoclasts, thus generating more differentiated bone cells but suppressing their activities; and increased metaphyseal trabecular hard tissue by creating an imbalance in osteoblasts over osteoclasts and suppressing hard tissue resorption.

Assessment of cortical and trabecular bone distribution in the beagle skeleton by neutron activation analysis

The distribution of bone calcium between morphologically identifiable cortical and trabecular bone obtained by dissection and quantitated by neutron activation analysis (NAA) is described. The skeleton of a female beagle dog was dissected into approximately 400 pieces and assayed for 49Ca produced in the University of California, Irvine TRIGA reactor. For each of the skeletal sections, we give the initial weight of the alcohol-fixed tissue, which includes cortical bone, trabecular bone, marrow, and cartilage, and a final tissue weight after the marrow and trabecular bone have been dissected away; total section and cortical section calcium weights are reported. The level of detail is represented, for example, by the vertebrae, which were divided into three parts (body, spine, and transverse processes) and by the long bones, which were divided into 10-12 parts such that characterization of the epiphysis, metaphysis, and diaphysis was accomplished. The median percentage cortical calcium values for cervical, thoracic, and lumbar vertebrae were 82%, 56%, and 66%, respectively; however, variation within these groups and among individual vertebral sections was about a factor of 2. For long bones, the median percentage cortical calcium varied from 90-100% in the midshaft to below 50% in the proximal and distal sections. The final calculated cortical tissue-to-calcium mass ratio (TCR) varied from about 4.5 for midshafts of the long bones to about 9 for thoracic vertebral bodies and indicated that the mineral fraction of cortical bone is not constant throughout the skeleton. The ratio of cortical to trabecular calcium in the skeleton was 79.6:20.4.

Immobilization-associated osteoporosis in primates

The progressive osteopenic changes in tibial compact bone in adult male monkeys (Macaca nemestrina) were examined histologically during chronic studies of immobilization. The animals were restrained in a semirecumbent position, which reduces normally occurring stresses in the lower extremities and results in bone mass loss. The longest immobilization studies were of 7 months duration. Losses of haversian bone tended to occur predominantly in the proximal tibia and were characterized by increased activation with excessive depth of penetration of osteoclastic activity. There was no apparent regulation of the size and orientation of resorption cavities. Rapid bone loss seen during 10 weeks of immobilization appeared to be due to unrestrained osteoclastic activity without controls and regulation, which are characteristic of adaptive systems. The general pattern of loss persisted throughout 7 months of immobilization. Clear-cut evidence of a formation phase in haversian bone was seen only after 2 months of reambulation. During this period osteoblasts accumulated within resorption cavities, and there was matrix apposition. Within 6 months of recovery there was increased bone turnover, and resorption cavities with diameters of 500-1500 micron were filled partially with new bone; the mean wall thickness of new bone was 2 to 3 times larger than normal. In addition there were numerous remodeling sites that were of normal size and orientation. Trabecular bone was lost during immobilization, and it is probable that losses of large trabecular plates are not replaced, and consequently original bone volume in the cross section is not recovered. In this immobilization model we observed bone resorption occurring for long periods without apparent interruption.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of carbonic anhydrase in osteoclasts by parathyroid hormone

Changes in the acidity of osteoclasts were evaluated by direct measurement of the fluorescent intensity of osteoclasts exposed to acridine orange, a fluorescent weak base which becomes concentrated in acid-containing subcellular compartments. Parathyroid hormone (PTH) produced dose-dependent increases in fluorescent intensity; maximal increases in intensity occurred at doses between 3 and 10 micrograms PTH/ml of culture medium. Acetazolamide, a potent inhibitor of carbonic anhydrase, inhibited the increase in fluorescence induced by PTH, but this drug was less effective in reducing fluorescence in maximally than in submaximally stimulated osteoclasts, indicating that either more enzyme or more resistant enzyme was present in the PTH-stimulated cells. Because increased fluorescence of acridine orange is a sign of greater acidity, these results suggest that (1) PTH stimulates the acidity of osteoclasts, (2) carbonic anhydrase activity is necessary for maximum acidity, and (3) carbonic anhydrase is activated by PTH.

The effects of prostaglandin E2 in growing rats: increased metaphyseal hard tissue and cortico-endosteal bone formation

To assess the efficacy of PGE2 in inducing in vivo bone formation, graded doses of prostaglandins E2 were administered to 255 g rats. Histomorphometric analyses of selected sequential fluorescent-labeled bones of rats treated with 0, 0.3, 1.0, 3, or 6 mg PGE2/kg/d for 21 days showed that the doses PGE2 depressed longitudinal bone growth, increased growth cartilage thickness slightly, decreased degenerative cartilage cell size and cartilage cell production slightly, and increased proximal tibial metaphyseal hard-tissue mass markedly. Periosteal bone formation was depressed at the higher doses, and an early, slight depression in endosteal bone formation was also observed, along with a striking late increase in endosteal bone formation and in the formation of trabecular bone in the marrow cavity of the tibial shaft. The characteristics and magnitude of these responses were quite similar to those observed in our previous study of the effects of PGE2 on weanling rats except for the delayed increase in cortico-endosteal bone formation.

Effects of N,N,N',N'-ethylenediaminetetramethylene phosphonic acid and 1-hydroxyethylidene-1,1-bisphosphonic acid on calcium absorption, plasma calcium, longitudinal bone growth, and bone histology in the growing rat

Phosphonate compounds are of considerable interest because of their effects on biological mineralization and mineral metabolism. The effects of graded doses given parenterally or orally of N,N,N',N'-ethylenediamine tetramethylene phosphonic acid (EDITEMPA) on calcium absorption, plasma calcium, endochondral bone elongation, and osseous tissue histology of rats were determined. In some experiments 1-hydroxyethylidene-1,1-bisphosphonic acid (HEBP) was included for comparison. Given sc for 7 days, EDITEMPA caused dose-related decreases in net weight gain, intestinal calcium absorption, and longitudinal bone growth. There were also increases in total plasma calcium concentrations and excess osteoid accumulation in bone. When EDITEMPA was given po for 7, 28, or 90 days, few changes in osseous tissues were observed and then only at the highest dose (333 mg/kg/day). An equivalent dose of HEBP caused substantial changes in osseous tissues and calcium parameters.

The effects of prostaglandin E2 in rapidly growing rats: depressed longitudinal and radial growth and increased metaphyseal hard tissue mass

The effects of 0, 0.3, 1.0, 3.0, or 6.0 mg of prostaglandin E2 (PGE2)/kg/day administered subcutaneously for 3 weeks to triple fluorochrome-labeled weanling rats are reported. Microradiographs and undecalcified sections of proximal tibiae, tibial shafts, and seventh caudal vertebrae were evaluated by static and dynamic bone histomorphometry techniques. Significant changes were observed only at higher dose levels. Proximal tibial longitudinal growth rates were depressed in doses of 1, 3, or 6 mg PGE2/kg/day. Growth plate thickness and the size of hypertrophic cartilage cells were decreased in animals given 3 and 6 mg of PGE2/kg/day, but the calculated rate of cartilage cell production was unaffected. At doses of 6 mg PGE2/kg/day, periosteal bone apposition rates between Day -1 and Day +19 in both the tibial shafts and caudal vertebral cortices were depressed by less than 25%. Cortical bone mass and endosteal bone apposition rates in the tibial shaft and caudal vertebrae were unaffected. Hard tissue mass in the secondary spongiosa of the proximal tibial metaphysis increased dramatically (28%, 44%, and 60%, respectively) in rats treated with 1, 3, or 6 mg PGE2/kg/day. In addition, the secondary spongiosa contained numerous islands of woven trabecular bone along with an increased number of trabeculae. The study demonstrates that high doses of PGE2 stimulate new woven trabecular bone production and depress longitudinal and radial growth in rapidly growing rats.

Effects of dietary calcium, phosphorus and magnesium on intranephronic calculosis in rats

The effects of varying dietary levels of calcium, phosphorus and magnesium on the incidence and severity of intranephronic calculosis were studied. Renal calculi were induced by feeding female rats the AIN-76TM semipurified diet for 4 weeks. During this time period, dietary levels of 350, 450 or 550 mg calcium per 100 g diet did not influence the occurrence of urolithiasis. Increasing dietary magnesium levels from 50 to 350 mg was beneficial in preventing the occurrence of calculi if the diet contained 400 mg or less phosphorus. The protective effects of dietary magnesium were counteracted when dietary phosphorus levels were increased from 400 mg to 550 or 700 mg. If the dietary content of phosphorus and magnesium permitted the formation of renal calculi, the severity of the condition was also influenced by the dietary level of calcium. Some animal groups fed semipurified diets did not have microscopic or radiographic evidence of renal calculi but were found to have significantly elevated renal calcium values. It was suggested that these animals might be in a precalculus-forming state.

Effect of diet and intranephronic calculosis on bone modeling and parathyroid volume in rats

The effects of diet and intranephronic calculosis on bone modeling parameters and parathyroid volumes in the rat were studied. Bone chemical analyses and histomorphometric evaluation indicated that a modified AIN diet, containing minimal required levels of calcium, phosphorus and magnesium, was not nutritionally adequate. Although the mineral content of the modified AIN diet barely met the demands for normal bone modeling, both the AIN-76TM and modified AIN diet caused intranephronic calculosis. Nephrolithiasis was not observed in rats fed a natural-product diet. After 10 days, the medullary cavity size and the index of resorption were slightly greater in rats fed the calculogenic diets, but these effects were not evident at 9 weeks. A slight but highly significant difference in the bone magnesium content was observed between rats fed the semipurified and natural-product diets. At 9 weeks, the parathyroid volumes were slightly increased in rats fed the AIN-76 diet compared to rats fed the natural-product diet. It was concluded that differences in dietary content that promote or inhibit intranephronic calculosis do not alter intestinal absorption as much as they influence molecular and cellular events occurring after absorption.

Changes in endochondral bone elongation rates during pregnancy and lactation in rats

The rates of endochondral bone elongation during pregnancy and lactation in rats have been studied. The rate of growth at the distal femoral epiphyseal growth plate was measured using fluorescent bone markers. Endochondral growth rates were substantially increased in pregnant animals when compared with age-matched, nonmated controls. There were also increases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cell production during pregnancy. At parturition, this growth trend was reversed and during lactation there were significant decreases in endochondral growth rates. There were also corresponding decreases in growth plate thickness, hypertrophic cell lacunar height, and the calculated rate of cartilage cell production. These results indicate that significant changes occur in maternal endochondral growth rates during the reproductive cycle in rats.

Effects of cortisol and fluoride on ion-transporting ATPase activities in cultured osteoblastlike cells

Na+,K+-ATPase, HCO3(-)-ATPase, Ca2+,Mg2+,-ATPase, Ca2+-ATPase, and alkaline phosphatase activities were measured in cultures of osteoblastlike cells treated with fluoride and cortisol separately and in combinations. Low concentrations of cortisol increased HCO3- -ATPase (10(-11) to 10(-18) M cortisol) and alkaline phosphatase (10(-11) to 10(-9) M cortisol) activities, but higher cortisol concentrations reduced these activities. Na+,K+-ATPase, Ca2+,Mg2+-ATPase, and Ca2+-ATPase activities tended only to be reduced by cortisol. Fluoride (10(-6) and 5 X 10(-6) M) increased HCO3(-)-ATPase and alkaline phosphatase activities, but these activities were similar to controls in the presence of 10(-5) M fluoride. Ca2+,Mg2+-ATPase activity was decreased and Na+,K+-ATPase activity was increased as the concentration of fluoride increased (10(-6) to 10(-5) M). Preliminary experiments with fluoride indicated that lower concentrations (10(-7) M) were without effect. Cortisol concentrations of 10(-9) and 10(-8) M were chosen for studies with combinations of cortisol and fluoride because the effects of these concentrations on alkaline phosphatase activity were opposite, i.e. 10(-9) M increased whereas 10(-8) M decreased activity. Fluoride concentrations of 10(-6), 5 X 10(-6), and 10(-5) M were chosen because a peak of alkaline phosphatase activity occurred at 5 X 10(-6) M fluoride. Higher (10(-4) M) and lower (10(-7) M) fluoride concentrations were without effect. The effects of combinations of cortisol and fluoride depend on the enzyme activity measured. Fluoride (10(-6) M) combined with cortisol (10(-9) M) produced a peak of Na+,K+-ATPase activity. The increased activity obtained with all concentrations of fluoride alone was preserved when fluoride was combined with 10(-8) M cortisol, although the activity tended to be reduced at 5 X 10(-6) and 10(-5) M fluoride. HCO3(-)-ATPase activity was increased by fluoride combined with 10(-8) M cortisol and decreased by fluoride combined with 10(-9) M cortisol compared to the activities obtained with fluoride alone. The decrease in Ca2+,Mg2+-ATPase activity caused by fluoride alone was prevented by 10(-9) and enhanced by 10(-8) M cortisol, although all treatments produced the same activity at 10(-5) M fluoride. Ca2+-ATPase activity tended to be increased by combinations of fluoride and cortisol, but significantly so only at 10(-5) M fluoride in combinations with 10(-8) and 10(-9) M cortisol.(ABSTRACT TRUNCATED AT 400 WORDS)