Bone Strength In Rats Research Articles

Exogenous growth hormone (GH) increases dimension and reduces strength of vertebrae in Atlantic salmon (Salmo salar)

Growth hormone (GH) administration increases bone strength in rats, mainly through alterations of the dimensional proportions of vertebrae. However, similar knowledge is lacking in other clades. In order to pursue this, Atlantic salmon were fed diets with high or low vitamin C and E and administered a slow-release implant of bovine GH.While neither the diets or the GH treatment affected growth, GH reduced condition factor and carcass-somatic index, and increased the hepatosomatic and visceral-somatic indexes. GH had an impact on the dimensional proportions of the vertebrae and increased the relative dorso-ventral and lateral vertebra diameters. Moreover, GH decreased the mechanical strength of the vertebra, both with (modulus of elasticity, failure point) and without (stiffness) normalizing the mechanical data for the dimensions of the vertebrae. Diet did not impact on the vertebra morphometry or mechanical strength. RNA-seq of vertebra centra show that several genes associated with bone growth and remodelling were affected by GH, while there was an interactive effect between diet vitamin level and GH on several genes associated with lipid metabolism.The data may suggest that exogenous GH affects the bone cellular mechanisms of Atlantic salmon postsmolts in such a way that their vertebrae increase in diameter but not length, not disturbing growth in body length. The GH and diet effects on lipid metabolism associated genes indicate a complex regulation of the lipids stored inside the vertebral column by endocrine and nutritional factors.

Puerarin specifically disrupts osteoclast activation via blocking integrin-β3 Pyk2/Src/Cbl signaling pathway.

ObjectiveGiven the limitations of current anti-resorption agents for postmenopausal osteoporosis, there is a need for alternatives without impairing coupling crosstalk between bone resorption and bone formation ie. osteoclastogenesis. Puerarin, a unique C-glycoside isoflavonoid, was found to be able to prevent bone loss by inhibiting bone resorption, but the underlying mechanism was controversial. In this study, we investigated the effects of puerarin on osteoclastic differentiation, activation and bone resorption and its underlying molecular mechanism in vitro, and then evaluated the effects of puerarin on bone metabolism using an ovariectomized (OVX) rat model.MethodsIn vitro, the effect of puerarin on osteoclastic cytotoxicity, differentiation, apoptosis, activation and function were studied in raw 264.7 ​cells and mouse BMMs. Mechanistically, osteoclast-related makers were determined by RT-PCR, western blot, immunofluorescence, and kinase activity assay. In vivo, Micro-CT, histology, serum bone biomarker, and mechanical testing were used to evaluate the effects of puerarin on preventing osteoporosis.ResultsPuerarin significantly inhibited osteoclast activation and bone resorption, without affecting osteoclastogenesis or apoptosis. In terms of mechanism, the expressions of protein of integrin-β3 and phosphorylations of Src, Pyk2 and Cbl were lower in puerarin group than those in the control group. Oral administration of puerarin prevented OVX-induced trabecular bone loss and significantly improved bone strength in rats. Moreover, puerarin significantly decreased trap positive osteoclast numbers and serum TRAP-5b, CTx1, without affecting bone formation rate.ConclusionsCollectively, puerarin prevented the bone loss in OVX rat through suppression of osteoclast activation and bone resorption, by inhibiting integrin-β3-Pyk2/Cbl/Src signaling pathway, without affecting osteoclasts formation or apoptosis.Translational potential of this articleThese results demonstrate the unique mechanism of puerarin on bone metabolism and provide a novel agent for prevention of postmenopausal osteoporosis.

Elevated Levels of Peripheral Kynurenine Decrease Bone Strength in Rats with Chronic Kidney Disease.

The diagnosis and treatment of bone disorders in patients with chronic kidney disease (CKD) represent a clinical challenge. CKD leads to mineral and bone complications starting early in the course of renal failure. Recently, we have observed the positive relationship between intensified central kynurenine turnover and bone strength in rats with subtotal 5/6 nephrectomy (5/6 Nx)-induced CKD. The aim of the present study was to determine the association between peripheral kynurenine pathway metabolites and bone strength in rats with 5/6 Nx-induced CKD. The animals were sacrificed 1 and 3 months after 5/6 Nx or sham operation. Nephrectomized rats presented higher concentrations of serum creatinine, urea nitrogen, and parathyroid hormone both 1 and 3 months after nephrectomy. These animals revealed higher concentrations of kynurenine and 3-hydroxykynurenine in the serum and higher gene expression of aryl hydrocarbon receptor (AhR) as a physiological receptor for kynurenine and AhR-dependent cytochrome in the bone tissue. Furthermore, nephrectomy significantly increased the number of osteoclasts in the bone without affecting their resorptive activity measured in serum. These changes were particularly evident in rats 1 month after 5/6 Nx. The main bone biomechanical parameters of the tibia were unchanged between nephrectomized and sham-operated rats but were significantly increased in older compared to younger animals. A similar trend was observed for geometrical parameters measured with calipers, bone mineral density based on Archimedes' method and image of bone microarchitecture obtained from micro-computed tomography analyses of tibial cortical bone. In nephrectomized animals, peripheral kynurenine levels correlated negatively with the main parameters of bone biomechanics, bone geometry, and bone mineral density values. In conclusion, our data suggest that CKD-induced elevated levels of peripheral kynurenine cause pathological changes in bone structure via AhR pathway. This finding opens new opportunities for the treatment/prevention of osteoporosis in CKD.

A link between central kynurenine metabolism and bone strength in rats with chronic kidney disease.

BackgroundDisturbances in mineral and bone metabolism represent one of the most complex complications of chronic kidney disease (CKD). Serotonin, a monoamine synthesized from tryptophan, may play a potential role in bone metabolism. Brain-derived serotonin exerts a positive effect on the bone structure by limiting bone resorption and enhancing bone formation. Tryptophan is the precursor not only to the serotonin but also and primarily to kynurenine metabolites. The ultimate aim of the present study was to determine the association between central kynurenine metabolism and biomechanical as well as geometrical properties of bone in the experimental model of the early stage of CKD.MethodsThirty-three Wistar rats were randomly divided into two groups (sham-operated and subtotal nephrectomized animals). Three months after surgery, serum samples were obtained for the determination of biochemical parameters, bone turnover biomarkers, and kynurenine pathway metabolites; tibias were collected for bone biomechanical, bone geometrical, and bone mass density analysis; brains were removed and divided into five regions for the determination of kynurenine pathway metabolites.ResultsSubtotal nephrectomized rats presented higher serum concentrations of creatinine, urea nitrogen, and parathyroid hormone, and developed hypocalcemia. Several biomechanical and geometrical parameters were significantly elevated in rats with experimentally induced CKD. Subtotal nephrectomized rats presented significantly higher kynurenine concentrations and kynurenine/tryptophan ratio and significantly lower tryptophan levels in all studied parts of the brain. Kynurenine in the frontal cortex and tryptophan in the hypothalamus and striatum correlated positively with the main parameters of bone biomechanics and bone geometry.DiscussionIn addition to the complex mineral, hormone, and metabolite changes, intensified central kynurenine turnover may play an important role in the development of bone changes in the course of CKD.

An evaluation of the effect of pulsed wave low-level laser therapy on the biomechanical properties of the vertebral body in two experimental osteoporosis rat models.

Osteoporosis (OP) increases vertebral fragility as a result of the biomechanical effects of diminished bone structure and composition. This study has aimed to assess the effects of pulsed wave low-level laser therapy (PW LLLT) on cancellous bone strength of an ovariectomized (OVX-d) experimental rat model and a glucocorticoid-induced OP (GIOP) experimental rat model. There were four OVX-d groups and four dexamethasone-treated groups. A group of healthy rats was used for baseline evaluations. The OVX-d rats were further subdivided into the following groups: control rats with OP, OVX-d rats that received alendronate, OVX-d rats treated with PW LLLT, and OVX-d rats treated with alendronate and PW LLLT. The remaining rats received dexamethasone and were divided into four groups: control, alendronate-treated rats, laser-treated rats, and laser-treated rats with concomitant administration of alendronate. PW LLLT (890 nm, 80 Hz, 0.972 J/cm(2)) was performed on the spinal processes of the T12, L1, L2, and L3 vertebras. We extracted the L1 vertebrae and submitted them to a mechanical compression test. Biomechanical test findings showed positive effects of the PW LLLT and alendronate administration on increasing bending stiffness and maximum force of the osteoporotic bones compared to the healthy group. However, laser treatment of OVA-d rats significantly increased stress high load compared to OVA-d control rats. PW LLLT preserved the cancellous (trabecular) bone of vertebra against the detrimental effects of OV-induced OP on bone strength in rats compared to control OV rats.

Lycopene Effects on Serum Mineral Elements and Bone Strength in Rats

This study investigated the beneficial effect of lycopene on bone biomarkers in ovariectomized (OVX) rats. Female Wistar rats were either sham operated or surgically ovariectomized and then fed with lycopene for 8 weeks. Serum Ca, P, alkaline phosphatase (ALP), interleukin 6 (IL-6) and bone gla protein (BGP) concentration was significantly higher in the untreated OVX group compared with that of the sham group, whereas serum estrogen levels were lower. Bone mineral density (BMD), BMD/wt, bone mineral content (BMC), BMC/wt values, maximum load, stiffness, energy and maximum stress were significantly lower in the untreated OVX group compared with that of the sham group. Administration of lycopene (20, 30 and 40 mg/kg b.w.) for 8 weeks significantly decreased serum Ca, P, ALP, and IL-6 concentration, and enhanced serum estrogen level, BMD, BMD/wt, BMC, BMC/wt values, maximum load, stiffness, energy and maximum stress in lycopene-treated OVX groups. In conclusion, the consumption of lycopene may have the most protective effect on bone in OVX rats.

Cortical bone is more sensitive to alcohol dose effects than trabecular bone in the rat

ObjectiveWhile chronic alcohol consumption is known to decrease bone mineral content (BMC), bone mineral density (BMD), and negatively modify trabecular bone microarchitecture, the impact of alcohol on cortical microarchitecture is still unclear. The aim of this study was to investigate the effects of various doses of alcohol on bone density, trabecular and cortical parameters and bone strength in rats. MethodsForty-eight male Wistar rats were divided into four groups: control (C), alcohol 25% v/v (A25), alcohol 30% v/v (A30) and alcohol 35% v/v (A35). Rats in the alcohol groups were fed a solution composed of ethanol and water for 17weeks while the control group drank only water. Bone quality and quantity were evaluated through the analysis of density, trabecular and cortical bone microarchitectural parameters, osteocalcin and N-Telopeptide concentrations and a 3-point bending test. ResultsBone density along with trabecular and cortical thickness were lower in alcohol groups compared to C. BMD was lower in A35 vs. A30 and cortical thickness was lower in A35 vs. A25 and A30. Pore number was increased by alcohol and the porosity was greater in A35 compared to C. N-Telopeptide concentration was decreased in alcohol groups compared to control whereas no differences were observed in osteocalcin concentrations. Maximal energy to failure was lower in A25 and A35 compared to C. ConclusionChronic ethanol consumption increases cortical bone damage in rats and may have detrimental effects on bone strength. These effects were dose-dependent, with greater negative effects proportionate to greater alcohol doses.

Vitamin K2 Improves Renal Function and Increases Femoral Bone Strength in Rats with Renal Insufficiency

Renal insufficiency induces cortical bone loss in rats. The present study examined the influence of vitamin K(2) on renal function, cortical bone mass, and bone strength in rats with renal insufficiency. Thirty male Sprague-Dawley rats (8weeks old) were randomized by the stratified weight method to the following three groups of 10 animals each: sham operation (control), 5/6 nephrectomy, and 5/6 nephrectomy+oral vitamin K(2) (menaquinone-4, menatetrenone, 30mg/kg, 5days/week). Treatment was initiated 10days after surgery. After 6weeks of treatment, samples of serum, urine, and bone (femur and tibia) were obtained. Renal function was evaluated, bone histomorphometric analysis was performed on the tibial diaphysis, and the bone mineral density (BMD) and mechanical strength of the femoral diaphysis were determined by peripheral quantitative computed tomography and a three-point bending test, respectively. Nephrectomy induced renal dysfunction, as indicated by increased levels of serum creatinine and urea nitrogen along with a decrease of creatinine clearance; and it also decreased BMD without significantly affecting bone strength at the femoral diaphysis. Vitamin K(2) improved renal function parameters but did not significantly influence BMD at the femoral diaphysis. However, vitamin K(2) decreased the bone marrow area of the tibial diaphysis and increased the stiffness of the femoral diaphysis. These findings suggest that administration of vitamin K(2) improves renal function and increases cortical bone strength without altering BMD in rats with renal insufficiency.

Administration of growth hormone in selectively protein-deprived rats decreases BMD and bone strength

Introduction Isocaloric protein undernutrition is associated with decreased bone mass and decreased bone strength, together with lower IGF-I levels. It remains unclear whether administration of growth hormone (GH) corrects these alterations in bone metabolism. Materials and methods Six-month-old female rats were fed isocaloric diets containing either 2.5% or 15% casein for 2 weeks. Bovine growth hormone (bGH, 0.5 or 2.5 mg/kg of body weight) or vehicle was then administered as subcutaneous injections, twice daily, to rats on either diet for 4 weeks. At the proximal tibia, analysis of bone mineral density (BMD), maximal load and histomorphometry were performed. In addition, urinary deoxypyridinoline, plasma osteocalcin and IGF-I concentrations were measured. Weight was monitored weekly. Results bGH caused a dose-dependent increase in plasma IGF-I regardless of the dietary protein content. However, bGH dose-dependently decreased BMD and bone strength in rats fed the low-protein diet. There was no significant effect of bGH on BMD in rats fed the normal protein diet within this short-term treatment period, however bone formation as detected by histomorphometry was improved in this group but not the low-protein group. Osteoclast surface was increased in the low-protein bGH-treated animals only. Changes in bone turnover markers were detectable under both normal and low-protein diets. Conclusion These results emphasize the major importance of dietary protein intake in the bone response to short-term GH administration, and highlight the need for further investigation into the effects of GH treatment in patients with reduced protein intake.

Ovariectomy-Induced Hyperphagia Does Not Modulate Bone Mineral Density or Bone Strength in Rats

The ovariectomized (OVX) rat is a widely used animal model for the development of prevention and treatment strategies for postmenopausal osteoporosis. However, ovariectomy-induced hyperphagia results in weight gain and adiposity. To prevent potential protective effects of increased body weight on bone from confounding outcomes of preclinical studies, pair-feeding is used in some but not all studies to control food intake, but its importance is not well elucidated. We investigated if the type of feeding, pair-feeding vs. consumption of diet ad libitum, modulates bone mineral and bone strength in OVX rats. Three-month-old female Sprague-Dawley rats (n = 12/group) were randomized to 1) sham-operated control (SHAM); 2) OVX pair-fed (OVX-PF); and 3) OVX ad libitum (OVX-AL). For 14 wk, OVX-PF rats were pair-fed with the SHAM group and daily food intakes and weekly body weights were obtained. At necropsy, regional body composition was measured by dual energy X-ray absorptiometry. Bone mineral density (BMD) and biomechanical bone strength of femurs and lumbar vertebrae (LV) were also measured. OVX-AL rats had higher overall food intake (P < 0.01), final body weight (P < 0.01), weight gain (P < 0.01), and fat mass (P < 0.05) than either SHAM and OVX-PF rats. Conversely, SHAM rats had higher femur (P < 0.001) and LV1–3 BMD (P < 0.001) as well as LV4 peak load (P < 0.01) than both the OVX groups, whereas bone outcomes did not differ between the OVX-PF and OVX-AL groups. In summary, ovariectomy-induced hyperphagia and weight gain do not modulate BMD or biomechanical strength at 14 wk postovariectomy, suggesting that pair-feeding is not essential.

Short- to Mid-Term Effects of Ovariectomy on Bone Turnover, Bone Mass and Bone Strength in Rats

To determine the short- to mid-term effects of ovariectomy on bone turnover, bone mass and bone strength in rats. SD rats aged 12 weeks were randomly divided into No-treatment, Sham and OVX groups. The rats were sacrificed for sample collection at week 0, week 4 and week 18 after surgical operation. Chemistries in serum and urine were measured by standard colorimetric methods and bone turnover markers were measured by ELISA kits. Bone mass and bone strength were determined using pQCT system and three-point bending tests, respectively. At week 4, OVX rats showed drastic increase in the level of urine Ca, P and DPD. At week 18, in OVX rats the levels of serum ALP, urine DPD and Ca were much higher and the level of serum Ca was much lower when comparing with Sham rats. Ovariectomy produced significant reduction in cancellous BMD, total BMD and SSI of proximal tibial metaphysis rapidly at week 4 and continuously at week 18 after surgical operation. However, no marked changes of bone mass and bone strength were found in the diaphysis of tibia and femur, respectively. The current study concluded that ovariectomy induced the uncoupling of bone turnover, and the proximal metaphysis of long bone was the sensitive site for the short- to mid-term effect of ovariectomy, demonstrated as the markers of bone mass and stress strain index.

Prevention of osteoporosis by foods and dietary supplements. Magnesium and bone metabolism

About half the total magnesium (Mg) of the body is existed in bone. Bone is one of the main Mg pools in the body. Epidemiologic studies have demonstrated a positive correlation between Mg intake and bone mineral density. It is also reported that Mg deficiency induced a decrease in osteoblasts number, an increase in osteoclasts number and a decrease in bone strength in rats. In contrast, dietary Mg supplementation improved bone formation, bone resorption and bone strength in ovariectomized rats. Mg deficiency is known as a risk factor for osteoporosis, since Mg is essential mineral for normal bone growth. However, the detail of effects of Mg on bone metabolism remains unclear. Further studies should be developed to clarify the details.

A single intravenous administration of zoledronic acid prevents the bone loss and mechanical compromise induced by aromatase inhibition in rats

Recent evidence has demonstrated that long-term estrogen deprivation using aromatase inhibitor therapy in postmenopausal women with breast cancer results in bone loss and increased fracture risk. Bisphosphonates are potent inhibitors of bone resorption and have demonstrated efficacy in preventing bone loss in postmenopausal women with low bone mineral density (BMD) and in patients with breast cancer receiving estrogen deprivation therapy. Therefore, this study investigated the effects of the bisphosphonate zoledronic acid on BMD and bone strength in rats treated with the aromatase inhibitor, letrozole. Peripheral quantitative computed tomography demonstrated that treatment of rats with daily oral letrozole (1 mg/kg) induced significant bone loss and cortical thinning compared with control animals ( P < 0.01). A single prior intravenous dose of zoledronic acid dose dependently protected against letrozole-induced bone loss and cortical thinning, with the highest evaluated dose (20 μg/kg) resulting in BMD values that were not significantly different from controls over the 24 weeks of letrozole treatment. Furthermore, biomechanical testing of the distal femoral metaphysis demonstrated that zoledronic acid (20 μg/kg) significantly prevented the decrease in stiffness and elastic modulus induced by letrozole treatment. Taken together, these data support the use of zoledronic acid for the prevention of bone loss in women with breast cancer receiving aromatase inhibitor therapy.

Effects of risedronate on femoral bone mineral density and bone strength in sciatic neurectomized young rats

Immobilization induces a rapid loss of bone density and bone strength in rats. The purpose of the present study was to examine the effects of risedronate (Ris) on the femoral bone density and bone strength of sciatic neurectomized young rats. Forty male Sprague-Dawley rats, 6 weeks of age, were randomized by the stratified weight method into the following four treatment groups of 10 rats each: sham-operation, bilateral sciatic neurectomy (NX), NX + low-dose Ris (0.25 mg/kg/day, orally), and NX + high-dose Ris (0.5 mg/kg/day, orally). After 8 weeks of feeding, the volumetric bone mineral density (vBMD) and stress strain index (SSI) of the femoral distal metaphysis and middiaphysis of the rats were measured by peripheral quantitative computed tomography. The mechanical properties of the femoral distal metaphysis and middiaphysis were measured by the compression and three-point bending tests, respectively. The femoral length was also measured. As compared with the findings in the sham-operated controls, NX resulted in a loss of femoral length, cancellous vBMD, SSI, maximum load, stiffness, and breaking energy of the femoral distal metaphysis; there was also loss of cortical thickness, SSI, maximum load, and stiffness of the femoral middiaphysis, with no significant effects on the cortical vBMD or breaking energy of the femoral middiaphysis. High-dose Ris increased the vBMD to values higher than those in the sham-operated controls, and prevented the loss of SSI, maximum load, and stiffness of the femoral distal metaphysis, while low-dose Ris prevented the loss of cancellous vBMD of the femoral distal metaphysis. Neither high- nor low-dose Ris affected any of the cortical bone parameters of the femoral middiaphysis, except for cortical thickness, or the femoral length. These findings suggest that Ris may prevent immobilization-induced loss of cancellous bone density and bone strength in a dose-dependent manner without interfering with bone growth, but has no apparent effects on the cortical bone in sciatic neurectomized young rats. The results of the present preclinical study should be taken into consideration prior to the commencement of Ris treatment for disabled children.

Bazedoxifene Acetate: A Selective Estrogen Receptor Modulator with Improved Selectivity

We assessed the preclinical characteristics of a novel, stringently screened selective estrogen receptor modulator, bazedoxifene acetate, including its ability to bind to and activate estrogen receptors and promote increased bone mineral density and bone strength in rats, and the effects impacting the uterine endometrium, breast cancer cell proliferation, and central nervous system-associated vasomotor responses in an animal model. Bazedoxifene bound to estrogen receptor-alpha with an IC50 of 26 nm, an affinity similar to that of raloxifene. Bazedoxifene did not stimulate proliferation of MCF-7 cells but did inhibit 17beta-estradiol-induced proliferation with an IC50 of 0.19 nm. In an immature rat uterine model, bazedoxifene (0.5 and 5.0 mg/kg) was associated with less increase in uterine wet weight than either ethinyl estradiol (10 microg/kg) or raloxifene (0.5 and 5.0 mg/kg). Histological analysis revealed that coadministration of bazedoxifene also appeared to reduce raloxifene-stimulated endometrial luminal epithelial cell and myometrial cell hypertrophy. In ovariectomized rats, bazedoxifene was associated with significant increases in bone mineral density at 6 wk, compared with control, and better compressive strength of bone samples from the L4 vertebrae, compared with samples from ovariectomized animals. In the morphine-addicted rat model of vasomotor activity, bone-sparing doses of bazedoxifene alone were not associated with 17beta-estradiol inhibition of increased vasomotor activity. Bazedoxifene acetate represents a promising new treatment for osteoporosis, with a potential for less uterine and vasomotor effects than selective estrogen receptor modulators currently used in clinical practice. Controlled clinical trial data will be needed to confirm these effects.

Effects of vitamin K 2 (menatetrenone) and alendronate on bone mineral density and bone strength in rats fed a low-magnesium diet

In this study, we examined changes in bone parameters and bone strength in rats fed low-Mg diets (experiment 1) and the effects of vitamin K 2 (MK-4, experiment 3) and alendronate (ALN, experiment 2) in this model. In experiment 1, 5-week-old male Wistar rats were fed three low-Mg diets (Mg 9, 6, 3 mg/100 g diet) for 4 weeks. Although the cortical bone mineral content (CtBMC) and cortical thickness (CtTh) of the femoral diaphysis in all low-Mg-diet groups were the same as or greater than those in the intact group (Mg: 90 mg/100 g diet), the maximum load and elastic modulus were significantly reduced in the 3-mg-Mg group. In experiment 2, 4-week-old Wistar rats were fed a 6-mg-Mg diet for 8 weeks, and the effect of ALN (2, 20, and 200 μg/kg twice a week) was evaluated. The administration of ALN at 200 μg/kg increased the cortical bone mineral content (CtBMC), CtTh, and maximum load, but had no effect on the elastic modulus, as compared with the low-Mg-control group. In experiment 3, the effect of MK-4 was evaluated under the same conditions as in experiment 2. The administration of MK-4 had no effect on CtBMC, CtTh, or bone components of the femoral diaphysis. However, MK-4 inhibited the decreases in maximum load and elastic modulus due to the low-Mg diet. Since there is no other experimental model in which there is a decrease in bone mechanical properties without a decrease in bone mineral content, the low-Mg diet model is considered to be an excellent model for examining bone quality. Our results from this model suggest that MK-4 and ALN affect bone mechanical properties by different mechanisms.

Development of tensile strength during distraction osteogenesis in a rat model

These studies were designed to determine the reliability of in vitro tensile testing to measure the temporal development of regenerate bone strength in rats during limb lengthening (distraction osteogenesis, DO). External fixators were placed on the right tibiae of 36 virus-free, 400–450 g male Sprague Dawley rats, and osteotomies ( n=33) were performed. Distraction was initiated the following morning (0 day latency) at 0.4 mm/day and continued to day 20. The 8 mm gap was allowed to consolidate for up to 50 days (day 70 postop). Contralateral unoperated and operated (fixator only) controls were included. On days 20, 30, 50 and 70 postop, the rats were anesthetized, and their tibiae were radiographed prior to undergoing sacrifice for histological or tensile analysis. On day 70, an additional group was tested by three-point bending. Radiodensity measurements demonstrated progressive mineralization of the DO gap, and histology confirmed typical intramembranous ossification of collagen bundles oriented parallel to the distraction force. Tensile stiffness increased significantly between days 20 and 30 postop, this increase correlated with initial radiographic and histologic bridging of the DO gap. Energy to failure and ultimate tensile strength increased progressively to day 70. At day 70, the force to failure for three-point bending was 65% of control tibiae. In conclusion, in vitro tensile testing provides a reliable method to test the development of structural integrity during the early stages of DO. Therefore, the biomechanical effects of postulated modulators of bone repair can be measured during early stages (bone formation, bridging, early consolidation) of DO in a rat model.

GH and bone--experimental and clinical studies.

GH increases bone formation both via a direct interaction with GH receptors on osteoblasts and via locally produced IGF-I (autocrine/paracrine action). GH deficiency results in decreased bone mass in both man and laboratory animals and treatment of GHD patients with GH for several months results in increased bone mass. GH treatment also increases bone mass and the total mechanical strength of bones in rats with normal GH secretion. Because of the short duration of GH-treatment in man with normal GH secretion, the effect on bone mass is still inconclusive. The action of GH on bone metabolism in GHD adults is twofold: It stimulates both bone resorption and bone formation. A "Biphasic model" of GH action in bone remodeling has recently been proposed [1] (Fig. 2). According to this model the net effect of GH first results in a loss of bone mass, followed by a net increase in bone mass. The transition point occurs when bone formation proceeds at a higher rate than bone resorption. Taking all clinical studies of GH-treatment of GHD adults into account, it appears that the "transition point" occurs after approximately six months and that a net increase in bone mass usually is seen after 12-18 months of GH treatment. It should be emphasized that the "Biphasic model" of GH action in bone remodeling is proposed based on findings in GHD adults, and it remains to be clarified whether or not it is valid for subjects with normal GH secretion.

The effects of dietary boric acid on bone strength in rats.

The effects of dietary boron (B) (from boric acid [BA]) on bone strength were evaluated using male F344 rats. B was administered by dietary admixture of BA to NIH-07 feed at concentrations of 200, 1000, 3000, and 9000 ppm. The latter two levels were found in previous studies to be reproductively toxic to both males and the developing fetus. The first two levels are below and just at, respectively, the levels for producing fetal malformations, and are below the dose required to produce male reproductive toxicity. Resistance to destructive testing was measured on femora, tibiae, and lumbar vertebrae. Although femur and tibia resistance to bending force were not affected by any amount of dietary B, vertebral resistance to a crushing force was increased by approximately 10%, at all dose levels (200-9000 ppm). These data show that even levels of BA that are not reproductively toxic can affect the strength of the axial skeleton in rats.

Growth hormone and bone.

It is well known that GH is important in the regulation of longitudinal bone growth. Its role in the regulation of bone metabolism in man has not been understood until recently. Several in vivo and in vitro studies have demonstrated that GH is important in the regulation of both bone formation and bone resorption. In Figure 9 a simplified model for the cellular effects of GH in the regulation of bone remodeling is presented (Fig. 9). GH increases bone formation in two ways: via a direct interaction with GHRs on osteoblasts and via an induction of endocrine and autocrine/paracrine IGF-I. It is difficult to say how much of the GH effect is mediated by IGFs and how much is IGF-independent. GH treatment also results in increased bone resorption. It is still unknown whether osteoclasts express functional GHRs, but recent in vitro studies indicate that GH regulates osteoclast formation in bone marrow cultures. Possible modulations of the GH/IGF axis by glucocorticoids and estrogens are also included in Fig. 9. GH deficiency results in a decreased bone mass in both man and experimental animals. Long-term treatment (> 18 months) of GHD patients with GH results in an increased bone mass. GH treatment also increases bone mass and the total mechanical strength of bones in rats with a normal GH secretion. Recent clinical studies demonstrate that GH treatment of patients with normal GH secretion increases biochemical markers for both bone formation and bone resorption. Because of the short duration of GH treatment in man with normal GH secretion, the effect on bone mass is still inconclusive. Interestingly, GH treatment to GHD adults initially results in increased bone resorption with an increased number of bone-remodeling units and more newly produced unmineralized bone, resulting in an apparent low or unchanged bone mass. However, GH treatment for more than 18 months gives increased bone formation and bone mineralization of newly produced bone and a concomitant increase in bone mass as determined with DEXA. Thus, the action of GH on bone metabolism in GHD adults is 2-fold: it stimulates both bone resorption and bone formation. We therefore propose "the biphasic model" of GH action in bone remodeling (Fig. 10). According to this model, GH initially increases bone resorption with a concomitant bone loss that is followed by a phase of increased bone formation. After the moment when bone formation is stimulated more than bone resorption (transition point), bone mass is increased. However, a net gain of bone mass caused by GH may take some time as the initial decrease in bone mass must first be replaced (Fig. 10). When all clinical studies of GH treatment of GHD adults are taken into account, it appears that the "transition point" occurs after approximately 6 months and that a net increase of bone mass will be seen after 12-18 months of GH treatment. It should be emphasized that the biphasic model of GH action in bone remodeling is based on findings in GHD adults. It remains to be clarified whether or not it is valid for subjects with normal GH secretion. A treatment intended to increase the effects of GH/IGF-I axis on bone metabolism might include: 1) GH, 2) IGF, 3) other hormones/factors increasing the local IGF-I production in bone, and 4) GH-releasing factors. Other hormones/growth factors increasing local IGF may be important but are not discussed in this article. IGF-I has been shown to increase bone mass in animal models and biochemical markers in humans. However, no effect on bone mass has yet been presented in humans. Because the financial cost for GH treatment is high it has been suggested that GH-releasing factors might be used to stimulate the GH/IGF-I axis. The advantage of GH-releasing factors over GH is that some of them can be administered orally and that they may induce a more physiological GH secretion. (ABSTRACT TRUNCATED)