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Biomedical subjects

C H Turner

Publications and source records attributed to C H Turner.

At least 73 records · Page 4Linked to original sources
PubMed1999

Viscoelastic response of the rat loading model: implications for studies of strain-adaptive bone formation.

Studies of the adaptive skeletal response to mechanical loading require appropriate animal models. Two new approaches involve the nonsurgical application of loads to either the ulna or tibia of rats. Both of these approaches require the loading of bone through adjacent soft tissues, and thus the tissue viscoelasticity might affect the way load is transferred to the bone. The objective of this study was to characterize the mechanical strain in the rat tibia or ulna during applied loading at different frequencies. For the rat ulna model, loading was applied to the ulnae of four adult, female rats as a haversine waveform at frequencies of 1, 2, 5, 10, and 20 Hz and peak loads of 5, 10, 15, and 20 N. Mechanical strain was measured on the medial and lateral ulnar surfaces using single element strain gauges. For the rat tibia model, four-point bending loads were applied to the right tibiae of seven rats at frequencies of 0.5, 1, 2, 5, 10, and 20 Hz and peak loads of 30, 40, 50, and 60 N. Mechanical strain was measured on the lateral tibial surface at 5 mm proximal to the tibiofibular junction. We found that peak strains were linearly proportional to applied load, but decreased logarithmically as loading frequency was increased, indicating a significant viscoelastic effect in the soft tissues surrounding the ulnocarpal joint and in the soft tissues surrounding the tibia shaft. The viscoelastic response of the ulna and tibia tends to "filter out" high-frequency loading components and, as a result, the rat loading systems act as a low-pass filter. Consequently, any experiment designed to test the effect of loading frequency on bone formation in the rat ulna and tibia should employ progressively larger loads at higher loading frequencies to guarantee a consistent peak strain magnitude in the bone. The filtering effect of the ulna loading system is illustrated by an analysis of the strain waveforms from the recent study by Mosley and Lanyon (Bone 23:313-318; 1998) that was designed to evaluate the effect of strain rate on bone formation.

Adaptation, Physiological↗
PubMed1999

Anabolic effects of human biosynthetic parathyroid hormone fragment (1-34), LY333334, on remodeling and mechanical properties of cortical bone in rabbits.

Intermittent administration of parathyroid hormone (PTH) has an anabolic effect in cancellous bone of osteoporotic humans. However, the effect of PTH on cortical bone with Haversian remodeling remains controversial. The aim of this study was to determine the effects of biosynthetic human PTH(1-34) on the histology and mechanical properties of cortical bone in rabbits, which exhibit Haversian remodeling. Mature New Zealand white rabbits were treated with once daily injections of vehicle, or PTH(1-34), LY333334, at 10 micrograms/kg/day or 40 micrograms/kg/day for 140 days. Body weight in rabbits treated with PTH did not change significantly over the experimental period. Serum calcium and phosphate were within the normal range, but a 1 mg/ml increase in serum calcium was observed in rabbits given the higher dose of PTH. Histomorphometry of cortical bone in the midshaft of the tibia showed significant increases in periosteal and endocortical bone formation in these rabbits. Intracortical bone remodeling in the tibia was activated and cortical porosity increased by PTH. Cross-sectional bone area and bone mass of the midshaft of the femur increased significantly after PTH treatment. Ultimate force, stiffness, and work to failure of the midshaft of the femur of rabbits given the 40 micrograms dose of PTH were significantly greater than those in the control group, whereas elastic modulus was significantly lower than that in the rabbits given the 10 micrograms dose of PTH, but not different from controls. In the third lumbar vertebra, PTH increased both formation and resorption without increasing cancellous bone volume. The increases in bone turnover and cortical porosity were accompanied by concurrent increases in bone at the periosteal and endocortical surfaces. The combination of these phenomena resulted in an enhancement of the ultimate stress, stiffness, and work to failure of the femur.

Animals↗
PubMed1998

Analysis of differential gene expression in rat tibia after an osteogenic stimulus in vivo: mechanical loading regulates osteopontin and myeloperoxidase.

The skeleton has the ability to alter its mass, geometry, and strength in response to mechanical stress. In order to elucidate the molecular mechanisms underlying this phenomenon, differential display reverse transcriptase-polymerase chain reaction (DDRT-PCR) was used to analyze gene expression in endocortical bone of mature female rats. Female Sprague-Dawley rats, approximately 8 months old, received either a sham or bending load using a four-point loading apparatus on the right tibia. RNA was collected at 1 h and 24 h after load was applied, reverse-transcribed into cDNA, and used in DDRT-PCR. Parallel display of samples from sham and loaded bones on a sequencing gel showed several regulated bands. Further analysis of seven of these bands allowed us to isolate two genes that are regulated in response to a loading stimulus. Nucleotide analysis showed that one of the differentially expressed bands shares 99% sequence identity with rat osteopontin (OPN), a noncollagenous bone matrix protein. Northern blot analysis confirms that OPN mRNA expression is increased by nearly 4-fold, at 6 h and 24 h after loading. The second band shares 90% homology with mouse myeloperoxidase (MPO), a bactericidal enzyme found primarily in neutrophils and monocytes. Semiquantitative PCR confirms that MPO expression is decreased 4- to 10-fold, at 1 h and 24 h after loading. Tissue distribution analysis confirmed MPO expression in bone but not in other tissues examined. In vitro analysis showed that MPO expression was not detectable in total RNA from UMR 106 osteoblastic cells or in confluent primary cultures of osteoblasts derived from either rat primary spongiosa or diaphyseal marrow. Database analysis suggests that MPO is expressed by osteocytes. These findings reinforce the association of OPN expression to bone turnover and describes for the first time, decreased expression of MPO during load-induced bone formation. These results suggest a role for both OPN and MPO expression in bone cell function.

Animals↗
PubMed1998

Bone mass and structure at the hip in men and women over the age of 60 years.

Bone mass and structure at the proximal femur are important predictors of hip fracture. The aims of this study were to compare in a large sample of elderly men and women the precision of measurements of bone mass and structure at multiple sites at the proximal femur, to examine their interrelationships, to establish their relationships with age and body size, and to examine criteria for defining geometric and architectural variables in bone structure. Women (n = 336) and men (n = 141) over the age of 60 years were studied cross-sectionally. Bone mineral density (BMD) and content (BMC) at the proximal femur were measured in duplicate by dual-energy X-ray absorptiometry (DXA). Shaft and total upper femur (hip) sites in addition to femoral neck, Ward's triangle and trochanter were measured. Structural variables, measured from radiographs and from DXA images, including cortical thickness at calcar femorale, lateral cotex and midfemur, width of the femur and medulla, Singh grade, hip and femoral axis length, femoral head and neck width and the center of mass of the femoral neck. BMD and BMC had high reproducibility and there were significant differences in reproducibility across sites. Among sites, total upper femur and shaft had the highest reproducibility. Duplicate measurements substantially improved reliability of the measurement and are recommended when the value is close to a diagnostic level or when it will be used to establish rates of change. Reproducibility of structural variables was also high except for the lateral cortex, center of mass and Singh grade. Variance due to measurement error did not change with either age or gender. Women were significantly different from men, after controlling for differences in body size, in all variables except Singh grade and medulla width. BMD and BMC were negatively related to age and positively to body size. Structural variables examined in relation to age and body size fell into two categories. The first comprised variables that were not age-related but were body-size-related suggesting that they could be classified as geometric variables. The second comprised variables that were both body-size-related and age-related, suggesting that they could be classified as architectural variables. Using these criteria, calcar and lateral cortex were architectural variables, whereas shaft width, hip and femoral axis length, femoral head and neck width, and center of mass were geometric in both men and women. In women, shaft cortex width and medulla width were age-related, whereas in men they were not. Singh grade showed no consistent pattern with age or body size in women and men.

Absorptiometry, Photon↗
PubMed1998

Mechanotransduction and functional response of the skeleton to physical stress: the mechanisms and mechanics of bone adaptation.

The skeleton's primary mechanical function is to provide rigid levers for muscles to act against as they hold the body upright in defiance of gravity. Many bones are exposed to thousands of repetitive loads each day. During growth and development, the skeleton optimizes its architecture by subtle adaptations to these mechanical loads. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction including: mechanocoupling - conversion of mechanical forces into local mechanical signals, such as fluid shear stresses, that initiate a response by bone cells; biochemical coupling - transduction of a mechanical signal to a biochemical response involving pathways within the cell membrane and cytoskeleton; cell-to-cell signaling from the sensor cells (probably osteocytes and bone lining cells) to effector cells (osteoblasts or osteoclasts) using prostaglandins and nitric oxide as signaling molecules; and effector response - either bone formation or resorption to cause appropriate architectural changes. These architectural changes tend to adjust and improve the bone structure to its prevailing mechanical environment. Structural changes can be predicted, to some extent, by mathematical formulas derived from three fundamental rules: (1) bone adaptation is driven by dynamic, rather than static, loading; (2) extending the loading duration has a diminishing effect on further bone adaptation; (3) bone cells accommodate to a mechanical loading environment, making them less responsive to routine or customary loading signals.

Adaptation, Physiological↗
PubMed1998

En bloc staining of bone under load does not improve dye diffusion into microcracks.

Microdamage accumulation in bone has been implicated in the pathogenesis of some bone fractures, and in implant loosening. Standard techniques for staining microcracks may not allow all cracks to be stained. We tested the hypothesis that crack closure in bone cortices after removal of a bending load may prevent diffusion of stain to sites of microcrack nucleation. Following cyclic loading, 26 canine femurs were divided into a group stained en bloc while applying a four point bending load, and another group stained without an applied load. No differences in number or length of microcracks were observed, indicating that crack closure does not prevent diffusion of stain to the crack location. Staining under load is unnecessary.

Animals↗
PubMed1998

Does microdamage accumulation affect the mechanical properties of bone?

It has never been demonstrated that microcrack accumulation in bone leads to impaired mechanical properties. We hypothesized that microdamage accumulation is positively and linearly correlated with a reduction in bone's elastic modulus. We also tested the hypothesis that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices. Canine femurs (n = 26) were tested in four-point cyclic bending under load control until they had lost between 5 and 43% of their stiffness. Ten femurs were used as nonloaded controls. The loaded portion of the bone was stained en bloc with basic fuchsin to detect the presence of microdamage. The number of stained microcracks, their lengths and the area of damaged bone were measured under the microscope. Crack numerical density, surface density, mean crack length, and the percentage of damaged area were calculated. Significant microdamage accumulation was not detected until the bone had lost 15% of its elastic modulus. The relationship between crack density and stiffness loss was approximately quadratic, but the relationship between damaged area and stiffness loss was linear. There were significantly more microcracks in tensile cortices, but on average cracks were significantly longer in compressive cortices. We conclude that microcrack accumulation impairs the mechanical properties of bone by reducing its elastic modulus. We also conclude that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices.

Animals↗
PubMed1998

Recruitment and proliferative responses of osteoblasts after mechanical loading in vivo determined using sustained-release bromodeoxyuridine.

Mechanical bending of a rat's tibia in vivo can increase endocortical bone formation by over sixfold. It has been proposed that mechanical loading increases bone formation by driving osteoprogenitor cells in the marrow stroma to progress through the cell cycle and subsequently differentiate into osteoblasts at the cortical bone surfaces. We used a sustained-release preparation of 5-bromo-2'-deoxyuridine (SR-BrdUrd) to determine the origin of endocortical osteoblasts in rat tibiae after mechanical loading. SR-BrdUrd was bioavailable for the entire 96 h duration of the experiments, so all cells that progressed through a cell cycle were labeled with BrdUrd. Although the endocortical osteoblast surface was significantly increased (p < 0.05) at 48 h after loading, the percentage of BrdUrd-labeled osteoblasts did not increase, suggesting that the newly differentiated osteoblasts on the endocortical surface did not originate from proliferating cells. At 96 h after loading, 30-40% of the endocortical osteoblasts were BrdUrd labeled. The majority of BrdUrd-labeled osteoblasts appeared on the endocortical bone surface within the third day after loading, indicating that proliferation and differentiation of precursors into endocortical osteoblasts required 72 h after the loading stimulus. These results indicate that mechanical loading can cause two distinct osteoblastic responses: an immediate response within 48 h in which osteoblasts are recruited from nondividing preosteoblasts and/or bone-lining cells, and a delayed response involving proliferation and differentiation of preosteoblasts that requires > or =3 days.

Animals↗
PubMed1998

Decreased bone mass and bone elasticity in mice lacking the transforming growth factor-beta1 gene.

Transforming growth factor-beta1 (TGF-beta1) knockout (TGF-beta1(-/-)) mice were used to investigate the role of TGF-beta1 in postnatal bone development. Volumetric bone mineral density (BMD) and mineral content (BMC) in these mice and in their normal (TGF-beta1(+/+)) and heterozygous (TGF-beta1(+/-)) littermates were analyzed by quantitative computed tomography (pQCT). Analysis of the proximal tibial metaphysis showed a significant decrease in the BMC of the TGF-beta1(-/-) mice compared to TGF-beta1(+/+) or TGF-beta1(+/-) mice; however, no significant difference was observed in BMD between the groups of mice. pQCT analysis of the tibial midshaft diaphysis showed no difference in the BMD or BMC of cortical bone between the groups. Histomorphometry revealed no significant difference in trabecular connectivity or in trabecular bone volume, number, or thickness. However, the width of the tibial growth plate and the longitudinal growth rate were significantly decreased in the TGF-beta1(-/-) mice, resulting in shorter tibia. Acoustic velocity measurements showed significant differences between the groups of mice with an apparent dosage effect of TGF-beta1 expression on the anisotropic properties of the bone. These data show that longitudinal growth and total mineral content are affected in mice lacking TGF-beta1, as well as the elastic properties of the bone, consistent with an important role for TGF-beta1 in bone modeling and bone quality.

Animals↗
PubMed1998

Three rules for bone adaptation to mechanical stimuli.

The primary mechanical function of bones is to provide rigid levers for muscles to pull against, and to remain as light as possible to allow efficient locomotion. To accomplish this bones must adapt their shape and architecture to make efficient use of material. Bone adaptation during skeletal growth and development continuously adjusts skeletal mass and architecture to changing mechanical environments. There are three fundamental rules that govern bone adaptation: (1) It is driven by dynamic, rather than static, loading. (2) Only a short duration of mechanical loading is necessary to initiate an adaptive response. (3) Bone cells accommodate to a customary mechanical loading environment, making them less responsive to routine loading signals. From these rules, several mathematical equations can be derived that provide simple parametric models for bone adaptation.

Adaptation, Physiological↗
PubMed1998

LY353381 x HCl: an improved benzothiophene analog with bone efficacy complementary to parathyroid hormone-(1-34).

LY353381 x HCl is a benzothiophene analog that is structurally related to raloxifene with potent selective estrogen receptor modulator activity in the ovariectomized rat model of postmenopausal osteoporosis. The effects of LY353381 x HCl on bones, body weight, and uterine weight were evaluated in 7-month-old rats with osteopenia that was induced by ovariectomizing animals for 1 month before initiation of treatment with several agents individually, in combination, or in sequence. LY353381 x HCl was administered daily by itself for 90 days, in combination with the amino-terminal fragment of PTH-(1-34) (PTH) for 90 days, or sequentially after PTH when PTH was discontinued after 45 days of treatment. Additionally, comparisons were made of animals treated with PTH alone, 17alpha-ethynyl estradiol alone, equine estrogens (Premarin) alone, raloxifene alone, or combinations of PTH and equine estrogens or raloxifene. Ovariectomy induced increases in the rate of bone turnover and body weight while decreasing bone mineral density, bone mineral content, bone strength, trabecular bone volume, trabecular thickness, trabecular number, and uterine weight. LY353381 x HCl at 0.01-1 mg/kg had marginal effects on body weight and no effect on uterine weight compared with those in ovariectomized controls, in contrast to 17alpha-ethynyl estradiol or equine estrogens. LY353381 x HCl prevented further bone loss due to ovariectomy in tibia, femora, and lumbar vertebra, like 17alpha-ethynyl estradiol but unlike equine estrogens. LY353381 x HCl prevented the resorption of trabecular bone spicules, like 17alpha-ethynyl estradiol, but inhibited bone formation activity to a lesser extent than 17alpha-ethynyl estradiol. In this model, 17alpha-ethynyl estradiol appeared to be more efficacious after 3 months of treatment than equine estrogens in the proximal tibia metaphysis, suggesting efficacy differences between metabolites of 17beta-estradiol in bone. PTH at 10 microg/kg had no effect on body weight or uterine weight, but significantly increased bone mass to beyond those in sham-operated controls, baseline controls, and groups receiving other individual treatments at both axial and appendicular sites. The combination of LY353381 x HCl and PTH increased bone mass at a faster rate and to a greater extent than PTH alone or the combinations of equine estrogens/PTH and raloxifene/PTH at trabecular bone sites. The LY353381 x HCl/PTH combination improved bone mass and quality beyond any agent alone in regions enriched for cancellous bone, but was not significantly better than PTH alone on cortical bone. Additionally, when PTH was discontinued at 45 days, LY353381 x HCl prevented the rapid loss of bone observed in controls. Therefore, LY353381 x HCl appears to be useful by itself, in combination, or in sequence with PTH to replace lost bone in postmenopausal women.

Animals↗
PubMed1998

Differential responses of estrogen target tissues in rats including bone to clomiphene, enclomiphene, and zuclomiphene.

The substituted triphenylethylene antiestrogen clomiphene (CLO) prevents cancellous bone loss in ovariectomized (OVX'd) rats. However, CLO is a mixture of two stereoisomers, enclomiphene (ENC) and zuclomiphene (ZUC), which have distinctly different activities on reproductive tissues and tumor cells. The purpose of the present dose response study was to determine the effects of ENC and ZUC on nonreproductive estrogen target tissues. These studies were performed in 7-month-old female rats with moderate cancellous osteopenia that was established by ovariectomizing rats 1 month before initiating treatment. OVX resulted in increases in body weight, serum cholesterol, endocortical resorption, and indices of cancellous bone turnover, as well as decreases in uterine weight, uterine epithelial cell height, bone mineral density, bone strength, and cancellous bone area. Estrogen treatment for 3 months restored body weight, uterine histology, dynamic bone measurements, and osteoblast and osteoclast surfaces in OVX'd rats to the levels found in the age-matched sham-operated rats. In contrast, estrogen only partially restored cancellous bone volume and uterine weight, and it reduced serum cholesterol to subnormal values. CLO was a weak estrogen agonist on uterine measurements and a much more potent agonist on body weight, serum cholesterol, and dynamic bone measurements. CLO increased trabecular thickness in osteopenic rats and was the most effective treatment in improving cancellous bone volume and architecture. ZUC was a potent estrogen agonist on all tissues investigated and had dose-dependent effects. In contrast, ENC had dose-dependent effects on most measurements similar to CLO and decreased the uterotrophic effects of ZUC. It is concluded that ENC antagonizes the estrogenic effects of ZUC on the uterus but that the beneficial effects of CLO on nonreproductive tissues in OVX'd rats is conferred by both isomers. Furthermore, the combined actions of the two isomers on bone volume and architecture were more beneficial than either isomer given alone.

Animals↗
PubMed1998

LY353381.HCl: a novel raloxifene analog with improved SERM potency and efficacy in vivo.

Body weight, uteri, serum cholesterol and bones were shown previously in vivo to be sensitive to circulating levels of estrogen, as well as to synthetic, nonsteroidal ligands termed selective estrogen receptor modulators (SERM). In this study, we examined the in vivo effects of a new potent SERM on these tissues in 6-month-old, ovariectomized rats that were orally dosed with 0.0001-10 mg/kg/day LY353381.HCl for 5 weeks. LY353381.HCl prevented the ovariectomy-induced increase in body weight and serum cholesterol levels of treated rats and lowered them to below sham levels in a dose dependent manner, with maximum efficacy similar to estrogen or raloxifene. However, LY353381.HCl was consistently more potent than raloxifene, with a half maximal efficacious dose of 0.001 mg/kg for the reduction of body weight and cholesterol. In the uterus, LY353381.HCl had marginal effects on uterine weight compared to ovariectomized controls (OVX) like raloxifene, but unlike estrogen. Histological examination of uterine epithelial cell height showed little to no stimulatory effect of LY353381.HCl on the endometrium. Quantitative computed tomographic analyses (pQCT) of tibiae showed that LY353381.HCl prevented loss of bone due to ovariectomy with an ED50 of about 0.01 mg/kg with maximal efficacy observed at 0.1-1 mg/kg/day. Maximally attainable bone mineral density and content with LY353381.HCl were not significantly different from Sham or ovariectomized rats treated with estrogen or raloxifene. Interestingly, assessment of bone quality by biomechanical analyses showed that LY353381.HCl preserved the strength of the femora neck and midshaft, while improving the Young's modulus of cortical bone to beyond estrogen, raloxifene or sham levels. In uteri of immature rats treated with estrogen, LY353381.HCl antagonized the estrogen-induced elevation in uterine weight down to vehicle-dosed control levels with ED50 of 0.03 mg/kg/day. Therefore, LY353381.HCl was 30-100 times more potent than raloxifene in preventing ovariectomy effects on body weight, serum cholesterol and bone, while maintaining estrogen antagonist effects on the uterus. These animal data suggest that LY353381.HCl may have advantages over estrogen or raloxifene in the prevention of bone loss and treatment of other tissues in postmenopausal women.

Animals↗
PubMed1998

Fluid shear-induced mechanical signaling in MC3T3-E1 osteoblasts requires cytoskeleton-integrin interactions.

Mechanical stimulation of bone induces new bone formation in vivo and increases the metabolic activity and gene expression of osteoblasts in culture. We investigated the role of the actin cytoskeleton and actin-membrane interactions in the transmission of mechanical signals leading to altered gene expression in cultured MC3T3-E1 osteoblasts. Application of fluid shear to osteoblasts caused reorganization of actin filaments into contractile stress fibers and involved recruitment of beta1-integrins and alpha-actinin to focal adhesions. Fluid shear also increased expression of two proteins linked to mechanotransduction in vivo, cyclooxygenase-2 (COX-2) and the early response gene product c-fos. Inhibition of actin stress fiber development by treatment of cells with cytochalasin D, by expression of a dominant negative form of the small GTPase Rho, or by microinjection into cells of a proteolytic fragment of alpha-actinin that inhibits alpha-actinin-mediated anchoring of actin filaments to integrins at the plasma membrane each blocked fluid-shear-induced gene expression in osteoblasts. We conclude that fluid shear-induced mechanical signaling in osteoblasts leads to increased expression of COX-2 and c-Fos through a mechanism that involves reorganization of the actin cytoskeleton. Thus Rho-mediated stress fiber formation and the alpha-actinin-dependent anchorage of stress fibers to integrins in focal adhesions may promote fluid shear-induced metabolic changes in bone cells.

Actin Cytoskeleton↗
PubMed1997

Decreased bone mass and strength in ovariectomized cynomolgus monkeys (Macaca fascicularis).

Agents for prevention or treatment of osteoporosis must now be tested in a large animal species that exhibits bone remodeling. Ovariectomized, nonhuman primates provide one such model, and they consistently develop osteopenia accompanied by high bone turnover rates. The goal of this study was to further characterize this model, and particularly to determine the effect of ovariectomy on bone strength in vertebrae and femoral necks. Longitudinal evaluations of spinal bone mass and serum markers of bone turnover were performed in 19 sham-ovariectomized (SHAM) and 18 ovariectomized (OVX), domestically reared cynomolgus monkeys, aged > 9 years. OVX monkeys lost bone relative to both baseline values and SHAM controls. Serum markers of bone turnover were increased by OVX. After 72 weeks, both vertebral bone compressive strength and femoral neck breaking strength were significantly decreased in OVX animals compared with SHAM. Ovariectomized cynomolgus monkeys, like postmenopausal women, develop accelerated bone loss, increased bone turnover, and reduced bone strength, and provide a suitable large animal model for efficacy studies with agents for prevention or treatment of osteoporosis.

Alkaline Phosphatase↗