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G A Rodan

Publications and source records attributed to G A Rodan.

At least 91 records · Page 5Linked to original sources
PubMed1994

Induction of vascular endothelial growth factor expression by prostaglandin E2 and E1 in osteoblasts.

PGE1 and PGE2 are potent stimulators of bone formation. Osteogenesis is strongly dependent on angiogenesis. Vascular endothelial growth factor (VEFG), a secreted endothelial cell-specific mitogen, has been implicated in physiological and pathological angiogenesis. The aim of this study was to examine the possible role of VEGF in PG stimulation of bone formation. We found that in rat calvaria-derived osteoblast-enriched cells and in the osteoblastic RCT-3 cell line PGE2 and E1 increased VEGF mRNA and protein levels. The increased expression of VEGF mRNA produced by PGE2 was rapid (maximal at 1 h), transient (declined by 3 h), potentiated by cycloheximide, and abolished by actinomycin D. PGE2 had no effect on VEGF mRNA stability, suggesting transcriptional regulation of VEGF expression by PGF2. Rp-cAMP, a cAMP antagonist, suppressed VEGF mRNA induced by PGE2, indicating cAMP mediation. The upregulation of VEGF expression by PGE2 in the preosteoblastic RCT-1 cells was potentiated by treatment with retinoic acid, which induces the differentiation of these cells. The upregulation of VEGF mRNA by PGE2 was inhibited by dexamethasone treatment. In addition, Northern blot analysis showed that VEGF mRNA is expressed in adult rat tibia. In summary, we documented, for the first time, the expression of VEGF in osteoblasts and in bone tissue. Stimulation of VEGF expression by PGs and its suppression by glucocorticoids, which, respectively, stimulate and suppress bone formation, strongly implicate the involvement of VEGF in bone metabolism.

Alprostadil↗
PubMed1994

Msx-2/Hox 8.1: a transcriptional regulator of the rat osteocalcin promoter.

We recently defined an element (ACTAATTGG) within the rat osteocalcin (OC) promoter at -84 to -92 which provides approximately 70% of basal promoter activity in osteoblastic cell lines and binds a specific nuclear factor found in OC-producing ROS 17/2.8 osteosarcoma cells. Since this element closely resembles the recently described Msx-1 (Hox 7.1) homeodomain DNA binding cognate, we examined rodent osteoblastic cells lines for expression of Msx homeodomain-encoding messages. We have found and cloned a cDNA for rat Msx-2 (Hox 8.1) from a ROS 17/2.8 library and detect high levels of expression in various osteoblastic cell lines (ROS 17/2.8, RCT3, RCT1) as well as in culture passage 3 neonatal rat calvarial osteoblastic cells. Little to no expression was detected in phenotypically immature MC3T3E1 osteoblastic cells or in a variety of nonosteoblastic (ROS 25/1, C2C12, TRAB 11) mesenchymal cell lines. Dexamethasone (DEX) down-regulates Msx-2 message levels in both RCT3 and ROS 17/2.8 cells. Recombinant rat Msx-2 homeodomain expressed in Escherichia coli as a glutathione-S-transferase fusion protein binds to the rat OC promoter region -74 to -100 as determined by gel shift analysis. Recognition is dependent upon the intact ACTAATTGG motif at -84 to -92. In transient cotransfection assays using MC3T3E1 cells (which expresses very little or no endogenous Msx-2), Msx-2 suppresses the rat OC promoter 2- to 3-fold via the Msx-2 binding motif at -84 to -92. However, in ROS 17/2.8 cells, where a high level of endogenous Msx-2 mRNA is present, expression of exogenous Msx-2 does not suppress the rat OC promoter; surprisingly, Msx-2 further augments basal promoter activity by approximately 50-70%, again dependent upon the ACTAATTGG motif at -84 to -92. These data directly demonstrate that the Msx-2 homeodomain binds the rat OC promoter and that Msx-2 can act as a sequence-specific transcriptional regulator of the rat OC promoter in cultured osteoblastic cell lines. This activity is dependent upon the specific osteoblastic cellular context, similar to previous observations in nonosseous systems with other homeodomain transcription factors. These data suggest that Msx-2 may play a role in the transcriptional regulation of the osteoblast phenotype during development in the morphogenetic fields where it is expressed.

Animals↗
PubMed1994

Characterization of retinoic acid- and cell-dependent sequences which regulate zif268 gene expression in osteoblastic cells.

We have previously shown that retinoic acid (RA) induces differentiation in an osteoblastic cell line derived from embryonic rat calvaria and that RA has selective effects on zif268 gene expression in these preosteoblastic cells,distinct from those in more mature osteoblasts. In this study we demonstrate that the RA-dependent transcriptional increase in zif268 gene expression is mediated by the interaction of RA receptors (RARs) with a 17 base pair sequence in the zif268 promoter containing a single half-site motif (GTTCA), identical to each of the direct repeats seen in the RAR beta 2 gene. The sequence appears relatively RA-specific, since the zif268 RA-responsive element is not activated by 1,25-dihydroxyvitamin D3 or thyroid hormone (T3). However, cotransfection of RAR expression vectors and an SV-40 promoter chloramphenicol acetyltransferase (CAT) construct containing the single zif268 RA-responsive motif into CV-1 cells demonstrates that the alpha-, beta-, and gamma-RARs transactivate through this element. Extensive mutagenesis of the zif268 promoter region containing the RA response element (RARE) motif confirms that the transactivation and nuclear protein binding activity of this region requires only the half-site motif. The direct involvement of RAR in this DNA-protein interaction has been demonstrated by competitive gel retardation analysis using consensus RAREs and super-shifting of the DNA-protein complex with mouse alpha- or gamma-RAR monoclonal antibodies. In addition, we found that cell-specific suppression of RA-stimulated zif268 gene expression can be attributed to a 29 base pair nucleotide sequence, located downstream of the RA-responsive region in the zif268 gene. This sequence appears to be bound specifically by nuclear protein(s) from several cell types, including osteoblasts. The presence of this sequence in cis to the zif268 RARE or the consensus beta RARE completely blocks the RA-responsiveness of the zif268 gene in differentiated osteoblasts. These data extend the broad spectrum of RA-responsive sequences necessary for DNA binding and transactivation to include regulation via single RARE half-site motifs and suggest that the lack of RA responsiveness in differentiated osteoblasts may be mediated by cell-specific suppression of gene expression.

Animals↗
PubMed1994

Activity of the rat osteocalcin basal promoter in osteoblastic cells is dependent upon homeodomain and CP1 binding motifs.

A detailed analysis of the transcriptional machinery responsible for osteoblast-specific gene expression should provide tools useful for understanding osteoblast commitment and differentiation. We have defined three cis-elements important for basal activity of the rat osteocalcin (OC) promoter, located at about -200 to -180, -170 to -138, and -121 to -64 relative to the transcription initiation site. A motif (TCTGATTGTGT) present in the region between -200 and -170 that binds a multisubunit CP1/NFY/CBF-like CAAT factor complex contributes significantly to high level basal activity and presumably functions as the CAAT box for the rat OC promoter. We show that the region -121 to 32 is sufficient to confer osteoblastic cell type specificity in transient transfection assays of cultured cell lines using luciferase as a reporter. The basal promoter is active in rodent osteoblastic cell lines, but not in rodent fibroblastic or muscle cell lines. Although the rat OC box (-100 to -74) contains a CAAT motif, we could not detect CP1-like CAAT factor binding to this region. In fact, we demonstrate that a Msx-1 (Hox 7.1) homeodomain binding motif (ACTAATTG; bottom strand) in the 3'-end of the rat OC box is necessary for high level activity of the rat OC basal promoter in osteoblastic cells. A nuclear factor that recognizes this motif appears to be present in osteoblastic ROS 17/2.8 cells, which produce OC, but not in fibroblastic ROS 25/1 cells, which fail to express OC. This ROS 17/2.8 nuclear factor also recognizes the A/T-rich DNA cognates of the homeodomain-containing POU family of transcription factors. Taken together, these data suggest that a ubiquitous CP1-like CAAT factor and a cell type-restricted homeodomain containing (Msx or POU family) transcription factor interact with the proximal rat OC promoter to direct appropriate basal OC transcription in osteoblastic cells.

3T3 Cells↗
PubMed1993

Pattern of gene expression following rat tibial marrow ablation.

Following injury to bone marrow there is a phase of osteogenesis in which bone trabeculae replace the initial blood clot and fill the marrow cavity. The newly formed bone is subsequently fully resorbed by osteoclasts and normal bone marrow is restored. In this study we correlated the morphologic events with the pattern of gene expression that defines this sequence. Following marrow ablation, the trabecular bone volume in the affected section of the marrow cavity increased from control to 27% at day 6, declined to 18% at day 8, and eventually returned to control levels at day 14. Osteoblast number increased up to day 6 and declined substantially by day 8, but the number of osteoclasts peaked between days 8 and 10. Histologic analysis of alkaline phosphatase (AP) and tartrate-resistant acid phosphatase (TRAP) activity correlated with the observed cellular changes. Northern blot analysis of the levels of AP, osteocalcin (OC), and osteopontin (OP) mRNA shows a specific pattern of regulated gene expression, with AP mRNA maximal at day 6, OC mRNA very low until days 6-8, and OP mRNA expressed at very high levels throughout. In addition, procollagen alpha 1(I) and alpha 1(III) mRNAs show a regulated pattern of expression, with procollagen alpha 1(I) maximally expressed between days 4 and 10 and procollagen alpha 1(III) expressed at lower levels between days 4 and 6. The mRNA encoding insulin-like growth factor I (IGF-I) was found to be highly expressed between days 5 and 12; however, transforming growth factor beta 1 (TGF-beta 1) and TGF-beta 3 mRNA were only weakly expressed between days 4 and 10. These data demonstrate a temporal pattern of gene expression consistent with the observed morphologic profile, identify changes in growth factor mRNA that may be related to this repair process, and suggest that this is a suitable model for studying in vivo a synchronized sequence of bone formation and resorption at a well-defined anatomic site.

Alkaline Phosphatase↗
PubMed1993

Expression of alpha v and beta 3 integrin subunits in rat osteoclasts in situ.

Recent findings suggest that the vitronectin receptor, a member of the integrin family, plays an important role in the attachment of osteoclasts to bone matrix. We report here the localization by in situ hybridization of the mRNA for the alpha and beta subunits of the vitronectin receptor in rat bone sections. To generate the rat-specific RNA probes used in this study, we cloned cDNA fragments of integrin chains alpha v, beta 3, and beta 5 by the polymerase chain reaction from rat cDNA. These fragments share 86-91% homology with the respective human sequences. In situ hybridization localized the alpha v and beta 3 mRNAs to regions undergoing extensive bone resorption. The histologic appearance and prestaining of bone sections for tartrate-resistant acid phosphatase (TRAP) indicated the presence of these mRNAs in osteoclasts. These observations support immunohistochemical findings that osteoclasts express high levels of the vitronectin receptor, confirm the identity of this receptor as integrin alpha v/beta 3, and suggest that osteoclasts may actively synthesize these molecules.

Amino Acid Sequence↗
PubMed1993

Effects of combined prostaglandin and alendronate treatment on the histomorphometry and biomechanical properties of bone in ovariectomized rats.

Prostaglandin E2 (PGE2) has been shown to stimulate both bone resorption and formation in experimental animals, leading to augmentation of trabecular and cortical bone. The amino bisphosphonate alendronate (ALN) is a potent inhibitor of bone resorption. The objectives of this study were to examine if PGE2 stimulation of bone formation was dependent on bone resorption and if the bone accrued as a result of PGE2 treatment contributed to bone strength. The 48 female Sprague-Dawley rats were assigned to six groups as follows: five groups (8/group) were ovariectomized at the age of 6 months. One group was sacrificed 2 months later to establish baseline conditions, and four groups were treated for 25 days with (1) vehicle, (2) PGE2 at 3 mg/kg/day, (3) ALN sc at 0.8 micrograms/kg/day, and (4) PGE2 + ALN at the respective doses. The sixth group served as nonovariectomized untreated controls. Histomorphometric analysis of 6-10 microns thick tibial sections after in vivo fluorochrome double labeling showed that treatment with PGE2 alone increased endocortical mineral apposition rate and bone formation rate, stimulated production of bone trabeculae in the marrow cavity, and increased cortical porosity. Combined ALN + PGE2 treatment prevented the resorption induced by PGE2 but not the stimulation of bone formation on endocortical and periosteal surfaces and resulted in a significant increase in cortical thickness. Consistent with these observations, the femoral midshaft tested to failure in three-point bending showed a significant increase in strength in the PGE2 + ALN group (181 +/- 15 N) compared to time 0 controls (145 +/- 23 N) or to the ovariectomized vehicle-treated group (141 +/- 28 N).(ABSTRACT TRUNCATED AT 250 WORDS)

Alendronate↗
PubMed1993

The effects of the aminobisphosphonate alendronate on thyroid hormone-induced osteopenia in rats.

Hyperthyroidism, either endogenous or iatrogenic, leads to increased bone turnover and osteopenia. This study was conducted to examine (1) whether thyroid hormone excess in rats causes bone changes similar to those seen in patients with hyperthyroidism, and (2) the effects of the aminobisphosphonate alendronate on the thyroid hormone-induced bone changes. Sprague-Dawley male rats, divided into four groups, received L-thyroxine (T4) 250 micrograms/kg/day (+T4) or vehicle (-T4) subcutaneously six times per week and alendronate 1.75 mg/kg (+ALN) or vehicle (-ALN) orally twice a week. Rats were sacrificed after 3 weeks of treatment, blood samples were analyzed for serum T4, triiodo-L-thyronine (T3), and osteocalcin, and the proximal tibiae were processed for histomorphometric analysis. Serum T4 and T3 levels measured 20-24 hours after the last injection were 2 to 2.5-fold higher in +T4 groups than in -T4 groups. Serum osteocalcin was significantly (P < 0.05) higher in +T4/-ALN group than in the other groups, which were not statistically different from each other. T4 treatment (+T4/-ALN) significantly decreased the amount of cancellous bone volume (-45%) and increased osteoid surface (+254%), osteoblast surface (+111%), and osteoclast surface (+176%) relative to control values. Alendronate increased the bone volume above control values in both T4-treated (+T4/+ALN) and untreated (-T4/+ALN) rats, and prevented the T4-induced increase in bone turnover in +T4/+ALN rats. It is concluded that (1) excess thyroid hormone induces cancellous bone loss associated with high bone turnover in the rat, and (2) this bone loss can be prevented by alendronate through the inhibition of osteoclastic activity.

Alendronate↗
PubMed1993

Preclinical pharmacology of alendronate.

This brief review summarizes some of the preclinical findings of studies aimed at assessing the efficacy and safety of the aminobisphosphonate alendronate (ALN) in preventing or restoring the bone loss caused by calcium or estrogen deficiency. Mode of action studies show that ALN localizes at sites of bone resorption and inhibits osteoclastic activity. In secondary hyperparathyroidism caused by calcium-deficient diets in the rat, ALN reduced the bone loss. For low doses, daily administration proved most efficient. In ovariectomized rats, ALN both prevented and reversed the bone changes produced by estrogen deficiency at oral doses equivalent to 0.1 mg/kg per day or higher, and also maintained the mechanical strength of vertebrae. In ovariectomized baboons, which show bone changes similar to those seen in ovariectomized women, ALN also prevented the increase in bone turnover and increased both bone volume and bone strength in vertebrae. In a comparative study between ALN and etidronate, we found that ALN was 1000-fold more potent in inhibiting bone resorption and had at least a 1000-fold higher safety margin with respect to inhibition of mineralization and osteomalacia.

Alendronate↗
PubMed1993

Differential expression of insulin-like growth factor-I (IGF-I) and IGF-II messenger ribonucleic acid in growing rat bone.

Insulin-like growth factors (IGF-I and IGF-II) are among the most abundant growth factors found in bone. Although their local production has been implicated in growth and development, localization of the cells that express these proteins is not well documented. We have studied, by in situ hybridization, the temporal and spatial expression of IGF-I and IGF-II mRNA in rat long bones at different stages of postnatal bone development. In 2-day-old rats, IGF-II was highly expressed in cartilage and in the mesodermal structures that surround the bone. At later stages of bone development, the IGF-II signal decreased in intensity, but could still be detected in the growth plate of tibial bones at 3 and 5 weeks. At this stage, the IGF-II signal in the epiphyseal growth plate was unevenly distributed and was stronger in the periphery than in the center, where it was mainly concentrated in the germinal layer and in some, but not all, cartilage columns. IGF-I, on the other hand, was only faintly detected in the periosteum at the early cartilaginous stage of bone development. At later stages, IGF-I was strongly associated with regions of ossification in the trabecular bone of the metaphysis and epiphysis and along the endosteal and periosteal surfaces. Surprisingly, we did not detect at any time IGF-I mRNA in chondrocytes of the epiphyseal growth plate. These results suggest that in the rat, IGF-II plays a role in early development of bone and in the longitudinal growth of the epiphyseal plate. IGF-I is more closely associated with the osteogenic regions and does not replace the declining levels of IGF-II in the growth plate.

Aging↗
PubMed1993

Inhibition of osteoclastic bone resorption in vivo by echistatin, an "arginyl-glycyl-aspartyl" (RGD)-containing protein.

Osteoclastic bone resorption requires the formation of a tightly sealed compartment between the osteoclast and the mineralized bone matrix. This compartment functions as an extracellular "lysosome" which contains proteolytic enzymes and acids. Vitronectin receptors (VnR, integrin alpha v beta 3) displayed on the osteoclast cell surface may play a role in the attachment of osteoclasts to the resorption surface. VnR are known to bind to arginyl-glycyl-aspartyl (RGD)-containing matrix proteins and it has recently been reported that soluble peptides containing RGD sequences can block osteoclast attachment to bone and inhibit bone resorption in vitro. In this study echistatin, a naturally-occurring protein containing an RGD-sequence motif, was shown to completely inhibit osteoclast-mediated bone resorption in vivo. Echistatin or smaller derivative peptides may prove useful in the treatment of disorders characterized by excess bone resorption, such as osteoporosis and metastatic bone disease.

Animals↗
PubMed1993

Bisphosphonates in the treatment of metabolic bone diseases.

Bisphosphonates (BP) are pyrophosphate analogs, P-C-P with various carbon side-chains. The phosphate groups are responsible for the low gastrointestinal absorption (about 1%), limited penetration into cells, adsorption to bone mineral and rapid excretion in the urine. Based on the C side-chains, BPs can inhibit osteoclastic bone resorption with potencies which differ by as much as 10,000-fold among compounds. The most potent inhibitors are aminobisphosphonates. Studies of the amino-BP alendronate show preferential uptake at sites of bone resorption where they block osteoclastic activity by inhibiting ruffled border formation. BPs are the treatment of choice for hypercalcaemia of malignancy, where a single infusion with a potent BP will normalize serum calcium in 80% of the patients. Paget's disease also shows an excellent long-term response to BPs. In addition, BPs are being studied for the treatment of osteoporosis and other bone disorders which could be helped by inhibition of bone resorption.

Alendronate↗
PubMed1992

Impaired bone activity in aged rats: alterations at the cellular and molecular levels.

We have used a model of rapid bone induction and resorption in rats initiated by the removal of bone marrow to define age-associated deficits. Here we report the sequential expression of various genes implicated in the formation and removal of bone following marrow ablation. Significant increases in alkaline phosphatase and procollagen alpha 1(I) mRNA were observed by day 5, and of osteocalcin and osteopontin by day 6. At their peak, these mRNA levels were elevated three- to eight-fold and correlated with histological evidence of bone formation. No change in collagen II mRNA was observed, indicating that there was no cartilage phase. Collagenase activity increased 10-fold at day 9 and coincided with the beginning of bone resorption. Actin mRNA, a reference gene marker, remained at constant levels. Comparison of the response between adult (6 mo.) and old (24 mo.) rats showed the same temporal pattern, but a lower expression of bone-related genes in older rats. Histological examination also showed that the bone volume and osteoblast number at day 6 were significantly lower in old rats. Furthermore, the percentage of mineralized bone was greatly reduced in the aged rat. This model system is currently being used to evaluate the effectiveness of interventions to up-regulate the bone activity in senescent rats.

Aging↗
PubMed1992

Introduction to bone biology.

Bone at the tissue level undergoes remodeling: it is continuously being resorbed and rebuilt (or formed). A negative balance between bone resorption and formation, frequently due to excessive resorption, is the basis of many bone diseases. Resorption is carried out by osteoclasts, which are specialized multinucleated cells of hemopoietic origin. Bone resorption takes place at a specialized area of the osteoclast cell membrane called "ruffled border," which comprises a sealed lysosomal compartment where the acidic pH solubilizes the mineral and the proteolytic enzymes digest the matrix. Among the agents that inhibit bone resorption, only calcitonin and bisphosphonate have been shown to act directly on osteoclasts. Other hormones and agents, which modulate bone turnover, probably act on the osteoblasts or cells of the osteoblast lineage. Osteoblasts are bone-forming cells, originating from cells resident in bone committed to the osteoblastic lineage. They synthesize and secrete most of the proteins of the bone matrix, including type I collagen and noncollagenous proteins. They possess high levels of alkaline phosphatase, which participates in mineralization. Proteins, produced by osteoblasts, spill over into the blood and are used as indicators of bone formation. In addition to the matrix-forming ability, cells of the osteoblastic family (osteocytes, lining cells, and maybe other cells) participate in the regulation of bone turnover. They respond to parathyroid hormone, glucocorticoids, vitamin D, sex steroids, insulin, prostaglandins, growth factors, and so on. There are a significant number of cytokines, that are locally produced and may control bone resorption. These include prostaglandins, IL1, TNF alpha, possibily IL6.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Identification of a new member of the steroid hormone receptor superfamily that is activated by a peroxisome proliferator and fatty acids.

We have identified a novel member of the steroid hormone receptor superfamily by cDNA cloning from a human osteosarcoma SAOS-2/B10 cell library. Sequence analysis predicts a protein of 441 amino acids, which includes the conserved amino acid residues characteristic of the DNA- and ligand-binding domains of nuclear receptors. Amino acid sequence alignment and transcriptional activation experiments revealed that the new protein is closely related to the mouse peroxisome proliferator activated receptor. The overall homology is 62%, and the highest similarity is seen in the DNA- and ligand-binding domains, 86% and 71%, respectively. Northern blot analysis showed that in mature rats, the receptor is highly expressed in heart, kidney, and lung as a transcript of approximately 3500 nucleotides. In human cells, the size of the mRNA is approximately 4000 nucleotides. Transcription assays using hybrid receptors consisting of the ligand-binding domain of the new protein and the DNA-binding domain of the glucocorticoid receptor showed weak stimulation by the peroxisome proliferator activator WY14643, suggesting a relationship to that receptor. Similar stimulation was observed with arachidonic and oleic acid (100-250 microM).

Amino Acid Sequence↗
PubMed1992

Retinoic acid stimulates transcriptional activity from the alkaline phosphatase promoter in the immortalized rat calvarial cell line, RCT-1.

The immortalized rat calvarial bone cell line RCT-1 responds to treatment with retinoic acid (RA) by increased expression of osteoblast phenotype-related features, including the induction of liver/bone/kidney alkaline phosphatase (ALP) activity. ALP mRNA could not be demonstrated in unstimulated cells, but was first detected in cells treated for 6 h with 1 microM RA. Cycloheximide failed to block the RA induction of ALP mRNA, indicating that de novo protein synthesis was not a requirement for the RA effect and that the ALP gene may be a direct target for RA action. This was confirmed by nuclear run-on assays, which demonstrated a 2.5-fold increase in the abundance of ALP transcripts after 6 h of RA treatment. To determine whether the RA responsiveness was mediated by a specific segment of the ALP promoter, RCT-1 cells were transfected with a series of plasmids containing deletions of the 5'-flanking sequence of the human ALP gene fused to the chloramphenicol acetyl transferase (CAT) gene. CAT activity was measured in cells cultured in the presence of RA or vehicle. All but the smallest construct, which contained 44 basepairs up-stream of the initiation of transcription, were found to mediate a 2- to 3-fold increase in the expression of CAT activity in response to RA. Furthermore, when the region -108 to -45 of the human ALP gene was inserted into the expression vector pBLcat2, in a position immediately up-stream of the herpes simplex virus thymidine kinase promoter, the construct was found to mediate a 2-fold enhancement of CAT activity in response to RA. In gel retardation assays, a major band was present corresponding to the formation of a complex between the 32P-labeled probe containing the -108 to -45 sequence and proteins present in nuclear extracts of RCT-1 cells stimulated for 3 h with RA. These data suggest that the sequence of 64 basepairs (-108 to -45) 5' to the transcription start site is involved in the RA inducibility of the human ALP gene.

Alkaline Phosphatase↗
PubMed1991

Depression of osteoblastic activity in immobilized limbs of suckling rats.

Recently we characterized the immobilization-related osteopenia in adult rats and showed that it is caused by increased bone resorption and decreased bone formation (Weinreb et al. 1989 Bone 10:187). To assess the effect of age on disuse osteopenia, this study investigated the effects of immobilization on bone turnover in very young, suckling rats. The 15-day-old rats underwent unilateral hind limb immobilization by sciatic neurectomy; the contralateral limb was left intact and served as control. Experimental or sham-operated animals were killed after 0, 2, 4, or 12 days postsurgery. Dry, fat-free weight and ash weight were determined in both femora, and both tibiae were subjected to static and dynamic histomorphometry. Immobilization caused a progressive deficit in bone mass in the immobilized limb compared to the contralateral intact limb but did not affect femoral longitudinal growth. The total mineral content in the immobilized femora was 13.6% less than that in the intact limb by day 12. Concomitantly, tibial cancellous bone area and perimeter declined in the immobilized limb by 37.3 and 32.2%, respectively. This reduction in bone mass in the tibiae of immobilized limbs was associated with increased bone resorption, expressed as osteoclast perimeter, number of osteoclasts per mm surface, and number of osteoclasts per mm2 tissue area. Bone formation was reduced as a result of impaired osteoblast activity as evidenced by (1) decreased endocortical and trabecular mineral apposition rate; (2) reduced trabecular mineral formation rate; (3) decreased percentage of ash of the femoral dry weight; and (4) increased volume of unmineralized osteoid in the tibial metaphysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗