Skeletal calcium homeostasis and countermeasures to prevent disuse osteoporosis

Exploring the Implications of Golgi Apparatus Dysfunction in Bone Diseases

The Golgi apparatus is an organelle responsible for protein processing, sorting, and transport in cells. Recent research has shed light on its possible role in the pathogenesis of various bone diseases. This review seeks to explore its significance in osteoporosis, osteogenesis imperfecta, and other bone conditions such as dysplasias. Numerous lines of evidence demonstrate that perturbations to Golgi apparatus function can disrupt post-translational protein modification, folding and trafficking functions crucial for bone formation, mineralization, and remodeling. Abnormalities related to glycosylation, protein sorting, or vesicular transport in Golgi have been associated with altered osteoblast and osteoclast function, compromised extracellular matrix composition, as well as disrupted signaling pathways involved with homeostasis of bones. Mutations or dysregulation of Golgi-associated proteins, including golgins and coat protein complex I and coat protein complex II coat components, have also been implicated in bone diseases. Such genetic alterations may disrupt Golgi structure, membrane dynamics, and protein transport, leading to bone phenotype abnormalities. Understanding the links between Golgi apparatus dysfunction and bone diseases could provide novel insights into disease pathogenesis and potential therapeutic targets. Future research should focus on unraveling specific molecular mechanisms underlying Golgi dysfunction associated with bone diseases to develop targeted interventions for restoring normal bone homeostasis while decreasing clinical manifestations associated with these issues.

On the effect of antiresorptive drugs on the bone remodeling of the mandible after dental implantation: a mathematical model

Bone remodeling identifies the process of permanent bone change with new bone formation and old bone resorption. Understanding this process is essential in many applications, such as optimizing the treatment of diseases like osteoporosis, maintaining bone density in long-term periods of disuse, or assessing the long-term evolution of the bone surrounding prostheses after implantation. A particular case of study is the bone remodeling process after dental implantation. Despite the overall success of this type of implants, the increasing life expectancy in developed countries has boosted the demand for dental implants in patients with osteoporosis. Although several studies demonstrate a high success rate of dental implants in osteoporotic patients, it is also known that the healing time and the failure rate increase, necessitating the adoption of pharmacological measures to improve bone quality in those patients. However, the general efficacy of these antiresorptive drugs for osteoporotic patients is still controversial, requiring more experimental and clinical studies. In this work, we investigate the effect of different doses of several drugs, used nowadays in osteoporotic patients, on the evolution of bone density after dental implantation. With this aim, we use a pharmacokinetic-pharmacodynamic (PK/PD) mathematical model that includes the effect of antiresorptive drugs on the RANK/RANK-L/OPG pathway, as well as the mechano-chemical coupling with external mechanical loads. This mechano-PK/PD model is then used to analyze the evolution of bone in normal and osteoporotic mandibles after dental implantation with different drug dosages. We show that using antiresorptive agents such as bisphosphonates or denosumab increases bone density and the associated mechanical properties, but at the same time, it also increases bone brittleness. We conclude that, despite the many limitations of these very complex models, the one presented here is capable of predicting qualitatively the evolution of some of the main biological and chemical variables associated with the process of bone remodeling in patients receiving drugs for osteoporosis, so it could be used to optimize dental implant design and coating for osteoporotic patients, as well as the drug dosage protocol for patient-specific treatments.

Baseline Dietary Intake of Individuals with Spinal Cord Injury Who Are Overweight or Obese

BACKGROUND

Individuals with spinal cord injury (SCI) experience significant secondary health conditions including excess adiposity. Dietary guidelines for individuals with chronic SCI do not exist.

OBJECTIVE

To describe baseline dietary intake and quality based on conformance with dietary recommendations in participants enrolled in GoHealthySCI, a weight loss intervention for individuals with SCI, which promotes lifestyle change.

DESIGN

Cross-sectional analyses were conducted on data collected from April through August 2017 in a randomized pilot study.

PARTICIPANTS

Thirty-seven participants enrolled in the study in Houston, TX. All participants were at least 1 year post injury with a self-reported body mass index (calculated as kg/m²) ≥23. The racially/ethnically diverse sample was predominantly male (n=23), average age was 41.8±13.5 years, and average number of years since injury was 18.1±14.9. Participants varied in terms of level of injury; 19 participants identified as having tetraplegia and 19 identified as having paraplegia.

MAIN OUTCOME MEASURES

The Automated Self-Administered 24-Hour Recall dietary assessment was used to obtain baseline dietary intake data. Participants reported food intake on 3 nonconsecutive days.

STATISTICAL ANALYSIS

Descriptive statistics were conducted for the primary research objectives. Mean macronutrient and micronutrient intake and Healthy Eating Index-2015 total and component scores are described.

RESULTS

Average daily energy intake was 1618±434 kcal. Daily intakes of whole fruits (0.6±0.7 cups), vegetables (1.6±0.9 cups), and whole grains (15%) of total grains were lower than recommendations from the 2015-2020 Dietary Guidelines for Americans. Average daily fiber (15.0g±6.0) met the Academy of Nutrition and Dietetics Evidence Analysis Library minimum target range for individuals with SCI. All percentages of calories from macronutrients were within the acceptable macronutrient distribution ranges: total fat (34.3%±6.2%), protein (16.7%±4.2%), and carbohydrate (49.3%±8.4%). Mean Healthy Eating Index-2015 score was 54.4.

CONCLUSIONS

This study provides a description of dietary intake by individuals with SCI who are overweight or obese. Although macronutrients are within the acceptable distribution range, calories from fat are at the high end and those from protein are at the low end of those ranges. In addition, on average, individuals reported inadequate intake of fruits, vegetables, whole grains, fiber, seafood and plant protein, and healthy fats and excess intake of added sugars and saturated fat. Results provide preliminary evidence of dietary inadequacies and suggest that larger studies examining dietary intake are warranted.

Long-duration bed rest as an analog to microgravity

Long-duration bed rest is widely employed to simulate the effects of microgravity on various physiological systems, especially for studies of bone, muscle, and the cardiovascular system. This microgravity analog is also extensively used to develop and test countermeasures to microgravity-altered adaptations to Earth gravity. Initial investigations of bone loss used horizontal bed rest with the view that this model represented the closest approximation to inactivity and minimization of hydrostatic effects, but all Earth-based analogs must contend with the constant force of gravity by adjustment of the G vector. Later concerns about the lack of similarity between headward fluid shifts in space and those with horizontal bed rest encouraged the use of 6 degree head-down tilt (HDT) bed rest as pioneered by Russian investigators. Headward fluid shifts in space may redistribute bone from the legs to the head. At present, HDT bed rest with normal volunteers is the most common analog for microgravity simulation and to test countermeasures for bone loss, muscle and cardiac atrophy, orthostatic intolerance, and reduced muscle strength/exercise capacity. Also, current physiologic countermeasures are focused on long-duration missions such as Mars, so in this review we emphasize HDT bed rest studies with durations of 30 days and longer. However, recent results suggest that the HDT bed rest analog is less representative as an analog for other important physiological problems of long-duration space flight such as fluid shifts, spinal dysfunction and radiation hazards.

Bone marrow MR imaging findings in disuse osteoporosis

OBJECTIVE

To demonstrate MR imaging findings in the cortical and trabecular bone as well as marrow changes in patients with disuse osteoporosis (DO).

MATERIALS AND METHODS

Sixteen patients (14 men, 2 women, aged 27-86 years) with clinical and radiographic evidence of DO of a lower limb joint (10 knees, 6 ankles) with MR examination of the same joint performed within a 1-month period were selected, as well as 16 healthy volunteers (7 men, 9 women, aged 25-75 years, 10 knees and 6 ankles). MR imaging findings of the bone marrow were analyzed by 2 musculoskeletal radiologists in consensus regarding: diffuse or focal signal alteration, reinforcement of vertical or longitudinal trabecular lines, and presence of abnormal vascularization.

RESULTS

All patients (100%,16/16) with DO presented MR imaging abnormalities of the bone marrow, such as: accentuation of vertical trabecular lines (50%, 8/16), presence of subchondral lobules of fat (37.5%, 6/16), presence of horizontal trabecular lines (31%, 5/16), prominence of bone vessels (25%, 4/16), and presence of dotted areas of high signal intensity on T2-weighted fat-suppressed sequences (12.5%, 2/16). Such MR findings did not appear in the control individuals.

CONCLUSION

There are several MR imaging findings in bones with DO that range from accentuation of vertical and horizontal marrow lines, presence of subchondral lobules of fat, prominent bone vascularization and the presence of dotted foci of high signal intensity on T2-weighted fat-suppressed sequences. Recognition of these signs may prove helpful in the identification of DO as well as distinguishing these findings from other entities.

Adipose-derived stem cells in functional bone tissue engineering: lessons from bone mechanobiology

This review aims to highlight the current and significant work in the use of adipose-derived stem cells (ASC) in functional bone tissue engineering framed through the bone mechanobiology perspective. Over a century of work on the principles of bone mechanosensitivity is now being applied to our understanding of bone development. We are just beginning to harness that potential using stem cells in bone tissue engineering. ASC are the primary focus of this review due to their abundance and relative ease of accessibility for autologous procedures. This article outlines the current knowledge base in bone mechanobiology to investigate how the knowledge from this area has been applied to the various stem cell-based approaches to engineering bone tissue constructs. Specific emphasis is placed on the use of human ASC for this application.

Prolonged unilateral disuse osteopenia 14 years post external fixator removal: a case history and critical review

Disuse osteopenia is a complication of immobilisation, with reversal generally noted upon remobilisation. This case report focuses on a patient who was seen 18 years following a road traffic collision when multiple fractures were sustained. The patient had an external fixator fitted for a tibia and fibula fracture, which remained in situ for a period of 4 years. Following removal, the patient was mobilised but, still required a single crutch to aid walking. Fourteen years post removal of the fixator, the patient had a DXA scan which, demonstrated a T-score 2.5 SD lower on the affected hip. This places the patient at an increased risk of hip fracture on this side, which requires monitoring. There appear to be no current studies investigating prolonged disuse-osteopenia in patients following removal of long-term external fixators. Further research is required to quantify unilateral long-term effects to bone health and fracture risk in this population.

Triiodothyronine increases calcium loss in a bed rest antigravity model for space flight

Bed rest has been used as a model to simulate the effects of space flight on bone metabolism. Thyroid hormones accelerate bone metabolism. Thus, supraphysiologic doses of this hormone might be used as a model to accelerate bone metabolism during bed rest and potentially simulate space flight. The objective of the study was to quantitate the changes in bone turnover after low doses of triiodothyronine (T(3)) added to short-term bed rest. Nine men and 5 women were restricted to bed rest for 28 days with their heads positioned 6 degrees below their feet. Subjects were randomly assigned to receive either placebo or oral T(3) at doses of 50 to 75 microg/d in a single-blind fashion. Calcium balance was measured over 5-day periods; and T(3), thyroxine, thyroid-stimulating hormone, immunoreactive parathyroid hormone, osteocalcin, bone alkaline phosphatase, and urinary deoxypyridinoline were measured weekly. Triiodothyronine increased 2-fold in the men and 5-fold in the women during treatment, suppressing both thyroxine and thyroid-stimulating hormone. Calcium balance was negative by 300 to 400 mg/d in the T(3)-treated volunteers, primarily because of the increased fecal loss that was not present in the placebo group. Urinary deoxypyridinoline to creatinine ratio, a marker of bone resorption, increased 60% in the placebo group during bed rest, but more than doubled in the T(3)-treated subjects (P < .01), suggesting that bone resorption was enhanced by treatment with T(3). Changes in serum osteocalcin and bone-specific alkaline phosphatase, markers of bone formation, were similar in T(3)- and placebo-treated subjects. Triiodothyronine increases bone resorption and fecal calcium loss in subjects at bed rest.

Renal stone risk in a simulated microgravity environment: impact of treadmill exercise with lower body negative pressure

PURPOSE

Prolonged exposure to microgravity during spaceflight causes metabolic changes that increase the risk of renal stone formation. Studies during the Gemini, Apollo, Skylab and Shuttle missions demonstrated alterations in renal function, fluid homeostasis and bone resorption that result in increased urinary supersaturation of calcium oxalate, brushite, sodium urate and uric acid. Developing countermeasures to increased urinary supersaturation is an important priority as the duration of space missions increases.

MATERIALS AND METHODS

A total of 11 sets of identical twins remained on 6-degree head down, tilt bed rest for 30 days to simulate prolonged microgravity. One twin per pair was randomly selected to exercise while supine in a lower body negative pressure chamber 6 days weekly for 40 minutes, followed by 5 minutes of resting lower body negative pressure at 50 mm Hg. The other twin served as a nonexercise control. Pressure in the exercise lower body negative pressure chamber (52 to 63 mm Hg) was adjusted to produce footward forces equivalent to those for upright running on Earth at 1.0 to 1.2 x body weight. Pre-bed rest urinary stone risk profiles were done elsewhere after 5 days of a standardized diet, consisting of 170 mEq sodium, 1,000 mg calcium, 0.8 gm/kg animal protein and 2,500 kcal, and then throughout the bed rest and recovery phases of the protocol.

RESULTS

A significant increase in urinary calcium after just 1 week of bed rest was noted in the nonexercise control group (p = 0.001). However, no such increase was noted in the exercise group. Brushite supersaturation increased significantly from bed rest in each group, although the increase was significantly higher in the nonexercise control group than in the exercise group (p = 0.006). Calcium oxalate supersaturation increased during bed rest in the exercise group (p = 0.004). It trended toward a higher level in the nonexercise control group, although this did not achieve significance (p = 0.055) Mean urine volume +/- SD was significantly higher in the nonexercise control group than in the exercise group at bed rest week 2 and at week 3 (2.01 +/- 0.21 vs 1.63 0.18 l and 2.03 +/- 0.22 vs 1.81 +/- 0.20, respectively). Urinary pH was significantly higher in the nonexercise control group than in the exercise group at week 1 and week 3 (6.62 +/- 0.7 vs 6.49 +/- 0.5 and 6.58 +/- 0.6 vs 6.49 +/- 0.8, respectively, p = 0.01).

CONCLUSIONS

Bed rest significantly alters the urinary environment to favor calculous formation. Lower body negative pressure chamber treadmill exercise offers some protection against increases in stone risk during simulated microgravity, particularly with regard to the risks of hypercalciuria and brushite stone formation. The use of lower body negative pressure to augment aerobic exercise in space may decrease the risk of stone formation in astronauts. Adjunct measures, including aggressive hydration and alkalinization therapy, should be considered.

Effects of breath-hold diving on bone mineral density of women divers

OBJECTIVES

The relationship between bone mineral density (BMD) and swimming has been thoroughly researched. The aim of this study was to determine the effects of breath-hold diving on the BMD in the proximal femurs of women divers.

METHODS

A case-control observational study was carried out using health-checks of divers and control subjects at a hospital in Jeju City, South Korea. Women divers (N=61) were matched individually with non-diver controls (N=61) by age, weight, and postmenopausal year. The bone mineral densities of their proximal femurs (total hip, femoral neck) were assessed by dual-energy X-ray absorptiometry.

RESULTS

The average diving year of women divers was 34+/-13 years. The BMD of divers was higher than that of controls in the total hip and femur neck area (P<0.05). On multiple linear regression analysis, age and body weight were predictors of proximal femur bone mineral densities in divers. On linear regression analysis of the proximal femur BMD according to age in divers and controls, the bone mineral densities of divers tend to decrease more rapidly than those of controls in all two areas of the proximal femurs.

CONCLUSIONS

Our study results may suggest that diving in a high-pressure environment is an osteogenic stimulus. However, the weight-supported environment in diving exerts an effect that reduces BMD proportionately to the time spent in the water.

Osteoporosis after spinal cord injury

Osteoporosis is a known consequence of spinal cord injury (SCI) and occurs in almost every SCI patient. It manifests itself as an increase in the incidence of lower extremity fractures. The pattern of bone loss seen in SCI patients is different from that usually encountered with endocrine disorders and disuse osteoporosis. In general, there is no demineralization in supralesional areas following SCI. Several factors appear to have a major influence on bone mass in SCI individuals, such as the degree of the injury, muscle spasticity, age, sex and duration after injury. At the lumbar spine, bone demineralization remains relatively low compared to that of the long bones in the sublesional area. A new steady state level between bone resorption and formation is reestablished about 2 years after SCI. SCI may not only cause bone loss, but also alter bone structure and microstructure. Trabecular bone is more affected than cortical bone in the SCI population. Numerous clinical series have reported a high incidence ranging from 1 to 34% of lower extremity fractures in SCI patients. The pathogenesis of osteoporosis after SCI remains complex and perplexing. Disuse may play an important role in the pathogenesis of osteoporosis, but neural factors also appear to be important. SCI also leads to impaired calcium and phosphate metabolism and the parathyroid hormone (PTH)-vitamin D axis. Pharmacologic intervention for osteoporosis after SCI includes calcium, phosphate, vitamin D, calcitonin and biphosphonates. However, the concomitant prescription of bone-active drugs for the prevention and treatment of osteoporosis remains low, despite the availability of effective therapies. Functional stimulated exercises may contribute to the prevention of bone loss to some extent. In addition, many unanswered questions remain about the pathogenesis of osteoporosis and its clinical management.

Immobilization decreases duodenal calcium absorption through a 1,25-dihydroxyvitamin D-dependent pathway

Immobilization induces significant and progressive bone loss, with an increase in urinary excretion and a decrease in intestinal absorption of calcium. These actions lead to negative calcium balance and the development of disuse osteoporosis. The aims of this study were to evaluate the molecular mechanisms of decreased intestinal calcium absorption and to determine the effect of dietary 1,25-dihydroxyvitamin D [1,25(OH)2D] and a high-calcium diet on bone loss due to immobilization. The immobilized rat model was developed in the Bollman cage III to induce systemic disuse osteoporosis in the animals. There was a significant decrease in lumbar bone mineral density (BMD) and intestinal calcium absorption in the immobilized group compared with the controls. Serum 25-hydroxyvitamin D concentration did not change, but 1,25(OH)2D concentration decreased significantly. The mRNA levels of renal 25-hydoxyvitamin D 24-hydroxylase (24OHase) increased, whereas those of renal 25-hydroxyvitamin D 1-alpha hydroxylase (1alpha-hydroxylase), duodenal transient receptor potential cation channel, subfamily V, member 6 (TRPV6), TRPV5, and calbindin-D9k were all decreased. A high-calcium diet did not prevent the reduction in lumbar BMD or affect the mRNA expression of proteins related to calcium transport. Dietary administration of 1,25(OH)2D increased the intestinal calcium absorption that had been downregulated by immobilization. TRPV6, TRPV5, and calbindin-D9k mRNA levels were also upregulated, resulting in prevention of the reduction in lumbar BMD. Therefore, it is concluded that dietary 1,25(OH)2D prevented decreases in intestinal calcium absorption and simultaneously prevented bone loss in immobilized rats. However, it remains unclear that calcium absorption and expression of calcium transport proteins are essential for the regulation of lumbar BMD.

Immobilization induces a very rapid increase in osteoclast activity

We studied in a randomized, strictly controlled cross-over design, the effects of 6 days 6 degrees head-down tilt bed rest (HDT) in eight male healthy subjects in our metabolic ward. The study consisted of two periods (phases) of 11 days each in order to allow for the test subjects being their own controls. Both study phases were identical with respect to environmental conditions, study protocol and diet. Two days before arriving in the metabolic ward the subjects started with a diet. The diet was continued in the metabolic ward. The metabolic ward period (1l days) was divided into three parts: 4 ambulatory days, 6 days either HDT or control and 1 recovery day. Continuous urine collection started on the first day in the metabolic ward to analyze calcium excretion and bone resorption markers. On the 2nd ambulatory day in the metabolic ward and on the 5th day in HDT or control blood was drawn to analyze serum calcium, parathyroid hormone, and bone formation markers. Urinary calcium excretion was, as early as the first day in immobilization, increased (p<0.01). CTX- and NTX-excretion stayed unchanged in the first 24 h in HDT compared to the control. But already on the 2nd day of immobilization, both bone resorption markers significantly increased. We conclude from these results--pronounced rise of bone resorption markers--that already 24 h of immobilization induce a significant rise in osteoclast activity in healthy subjects. Thus, it appears possible to use short-term bed rest studies as a first step for the development of countermeasures to immobilization.

Intravenous pamidronate prevents femoral bone loss and renal stone formation during 90-day bed rest

Long-term bed rest has potential risks of bone loss and renal stone formation. We examined the effects of resistive exercise and intravenous pamidronate on BMD, bone turnover, urinary calcium, and renal stone formation in 25 healthy males during 90-day bed rest. Pamidronate prevented femoral bone loss and renal stone formation, but resistive exercise showed little effects.

INTRODUCTION

Long-term bed rest increases the risks of bone loss and urinary stone formation. Resistive exercise increases bone formation, and bisphosphonates reduce bone resorption. However, the effects of muscle exercise and bisphosphonates have not been examined side-by-side. The objectives of this study are to compare the effects of pamidronate with resistive exercise on BMD and renal stone formation during prolonged bed rest.

MATERIALS AND METHODS

Twenty-five male white volunteers, 26-45 years of age, were randomly assigned to the control (n = 9), exercise (n = 9), and pamidronate (n = 7) groups and underwent 90-day 6 degrees head-down tilt bed rest. Exercise group performed squats and heel raises on a flywheel device for 30 minutes every 3 days. Pamidronate (60 mg) was administered intravenously 14 days before bed rest. BMD of the head, forearm, lumbar spine, and proximal femur; biochemical bone markers; calcium (Ca) metabolism; and abdominal radiographs were examined during 90 days of bed rest and 360 days of reloading.

RESULTS

In controls, proximal femoral BMD decreased, and bone resorption markers and urinary Ca increased during bed rest, along with development of renal stones in two of nine subjects. Resistive exercise increased bone formation but was unable to prevent femoral BMD decrease and increases in bone resorption and urinary Ca during bed rest, with formation of renal stones in four of nine subjects. Pamidronate maintained femoral BMD, reduced bone resorption and urinary Ca, and completely prevented renal stone formation.

CONCLUSIONS

Resistive exercise increased bone formation but could not reduce bone resorption and the risk of renal stones. In contrast, inhibition of bone resorption by pamidronate could preserve bone mineral and reduce the risk of renal stone formation during prolonged bed rest.

Nitric oxide production by bone cells is fluid shear stress rate dependent

Shear stress due to mechanical loading-induced flow of interstitial fluid through the lacuno-canalicular network is a likely signal for bone cell adaptive responses. Moreover, the rate (determined by frequency and magnitude) of mechanical loading determines the amount of bone formation. Whether the bone cells' response to fluid shear stress is rate dependent is unknown. Here we investigated whether bone cell activation by fluid shear stress is rate dependent. MC3T3-E1 osteoblastic cells were subjected for 15 min to fluid shear stress of varying frequencies and amplitudes, resulting in peak fluid shear stress rates ranging from 0 to 39.6 Pa-Hz. Nitric oxide production, a parameter for bone cell activation, was found to be linearly dependent on the fluid shear stress rate; the slope was steepest at 5 min (0.11 Pa-Hz(-1)) and decreased to 0.03 Pa-Hz(-1) at 15 min. We conclude that the fluid shear stress rate is an important parameter for bone cell activation.

Osteoporosis and exercise

Osteoporosis is a common medical problem. Lifestyle measures to prevent or help treat existing osteoporosis often only receive lip service. The evidence for the role of exercise in the prevention and treatment of osteoporosis is reviewed.

Calcium and bone metabolism during space flight

Weightlessness induces bone loss. Understanding the nature of this loss and developing means to counteract it are significant challenges to potential human exploration missions. This article reviews the existing information from studies of bone and calcium metabolism conducted during space flight. It also highlights areas where nutrition may play a specific role in this bone loss, and where countermeasures may be developed to mitigate that loss.

The effect of in vivo mechanical loading on estrogen receptor alpha expression in rat ulnar osteocytes

The presence of estrogen receptor alpha (ER alpha) in osteocytes was identified immunocytochemically in transverse sections from 560 to 860 microm distal to the midshaft of normal neonatal and adult male and female rat ulnas (n = 3 of each) and from adult male rat ulnas that had been exposed to 10 days of in vivo daily 10-minute periods of cyclic loading producing peak strains of either -3000 (n = 3) or -4000 microstrain (n = 5). Each animal ambulated normally between loading periods, and its contralateral ulna was used as a control. In animals in which limbs were subject to normal locomotor loading alone, 14 +/- 1.2% SEM of all osteocytes in each bone section were ER alpha positive. There was no influence of either gender (p = 0.725) or age (p = 0.577) and no interaction between them (p = 0.658). In bones in which normal locomotion was supplemented by short periods of artificial loading, fewer osteocytes expressed ER alpha (7.5 +/- 0.91% SEM) than in contralateral control limbs, which received locomotor loading alone (14 +/- 1.68% SEM; p = 0.01; median difference, 6.43; 95% CI, 2.60, 10.25). The distribution of osteocytes expressing ER alpha was uniform across all sections and thus did not reflect local peak strain magnitude. This suggests that osteocytes respond to strain as a population, rather than as individual strain-responsive cells. These data are consistent with the hypothesis that ER alpha is involved in bone cells' responses to mechanical strain. High strains appear to decrease ER alpha expression. In osteoporotic bone, the high strains assumed to accompany postmenopausal bone loss may reduce ER alpha levels and therefore impair the capacity for appropriate adaptive remodeling.

A cysteine protease inhibitor prevents suspension-induced declines in bone weight and strength in rats

In this study, we examined the effects of a potent cysteine protease inhibitor, N-(L-3-trans-carboxyoxirane-2-cabonyl)-L-leucine-4-aminobutylamide (E-64a), on bone weight and strength in tail-suspended rats. We first administered a vehicle or 4 or 8 mg/rat of E-64a to rats fed with a low calcium diet for 7 wks to determine effective doses of E-64a on bone resorption in vivo. Femoral cathepsin K-like activity and serum hydroxyproline level in rats fed with a low calcium diet were significantly higher than those in rats fed with a standard diet. The intraperitoneal injection of 8 mg/rat of E-64a to rats decreased their serum calcium and hydroxyproline concentrations after 3 to 6 hrs in parallel with changes in femoral cathepsin K-like activity, while 4 mg/rat of E-64a had weaker effects on these parameters. Based on these results, we injected 8 mg/rat of E-64a to tail-suspended rats twice a day for 2 wks and compared the results with those of treatment with 1 mg/rat of etidronate, a bisphosphonate, twice a week. In tail-suspended rats, femoral weight and strength, assessed by three-point bending test, significantly decreased from Day 5 to 21, while femoral cathepsin K-like activity and serum calcium and hydroxyproline concentrations did not change. E-64a inhibited femoral cathepsin K-like activity in tail-suspended rats, but etidronate did not. E-64a as well as etidronate significantly prevented the suspension-induced declines in bone weight and strength. However, more frequent injection and higher doses were required for E-64a to exhibit significant efficacy of antiresorption, compared with those of etidronate. Our results suggest that a cysteine protease inhibitor could improve suspension-induced osteopenia by inhibiting cathepsin K-like activity in bone; however, it needs several improvements in the effect as a clinical drug.

In vitro modeling of human tibial strains during exercise in micro-gravity

Prolonged exposure to micro-gravity causes substantial bone loss (Leblanc et al., Journal of Bone Mineral Research 11 (1996) S323) and treadmill exercise under gravity replacement loads (GRLs) has been advocated as a countermeasure. To date, the magnitudes of GRLs employed for locomotion in space have been substantially less than the loads imposed in the earthbound 1G environment, which may account for the poor performance of locomotion as an intervention. The success of future treadmill interventions will likely require GRLs of greater magnitude. It is widely held that mechanical tissue strain is an important intermediary signal in the transduction pathway linking the external loading environment to bone maintenance and functional adaptation; yet, to our knowledge, no data exist linking alterations in external skeletal loading to alterations in bone strain. In this preliminary study, we used unique cadaver simulations of micro-gravity locomotion to determine relationships between localized tibial bone strains and external loading as a means to better predict the efficacy of future exercise interventions proposed for bone maintenance on orbit. Bone strain magnitudes in the distal tibia were found to be linearly related to ground reaction force magnitude (R(2)>0.7). Strain distributions indicated that the primary mode of tibial loading was in bending, with little variation in the neutral axis over the stance phase of gait. The greatest strains, as well as the greatest strain sensitivity to altered external loading, occurred within the anterior crest and posterior aspect of the tibia, the sites furthest removed from the neutral axis of bending. We established a technique for estimating local strain magnitudes from external loads, and equations for predicting strain during simulated micro-gravity walking are presented.

Osteobiology, strain, and microgravity. Part II: studies at the tissue level

Loading microgravity, and/or defective mechanical strain-forces have important effects on bone cells and bone quality and quantity. The complex mechanisms induced by strain and microgravity on bone cells have been reviewed in Part I of this paper. In Part II, we have considered the data on the alterations induced by unloading and microgravity on the skeleton and the mechanisms that are involved at the tissue level in animals and humans.

Artificial gravity as a countermeasure in long-duration space flight

Long-duration exposure to weightlessness results in bone demineralization, muscle atrophy, cardiovascular deconditioning, altered sensory-motor control, and central nervous system reorganizations. Exercise countermeasures and body loading methods so far employed have failed to prevent these changes. A human mission to Mars might last 2 or 3 years and without effective countermeasures could result in dangerous levels of bone and muscle loss. Artificial gravity generated by rotation of an entire space vehicle or of an inner chamber could be used to prevent structural changes. Some of the physical characteristics of rotating environments are outlined along with their implications for human performance. Artificial gravity is the centripetal force generated in a rotating vehicle and is proportional to the product of the square of angular velocity and the radius of rotation. Thus, for a particular g-level, there is a tradeoff between velocity of rotation and radius. Increased radius is vastly more expensive to achieve than velocity, so it is important to know the highest rotation rates to which humans can adapt. Early studies suggested that 3 rpm might be the upper limit because movement control and orientation were disrupted at higher velocities and motion sickness and chronic fatigue were persistent problems. Recent studies, however, are showing that, if the terminal velocity is achieved over a series of gradual steps and many body movements are made at each dwell velocity, then full adaptation of head, arm, and leg movements is possible. Rotation rates as high as 7.5-10 rpm are likely feasible. An important feature of the new studies is that they provide compelling evidence that equilibrium point theories of movement control are inadequate. The central principles of equilibrium point theories lead to the equifinality prediction, which is violated by movements made in rotating reference frames.

Simulated weightlessness-induced attenuation of testosterone production may be responsible for bone loss

This study examined the effects of simulated weightlessness on serum hormone levels and their relationship to bone mineral density (BMD). The tail-suspended (i.e., hindlimb suspended, HLS) rat model was used to simulate weightless conditions through hindlimb unloading to assess changes in hormonal profile and the associated bone loss. In the first study, 24 adult male rats were assigned to two groups with 12 rats being HLS for 12 d, and the remaining 12 rats serving as ground controls. On d 0, 6, and 12, blood samples were taken to estimate circulating hormone levels. HLS rats had significant reductions in testosterone, 1,25 (OH)2 vitamin D, and thyroxine levels by d 6 (p<0.01); their testosterone levels were almost undetectable by d 12 (p<0.001). Serum cortisol levels in these rats were elevated on d 6 (p<0.02), but returned to normal levels by d 12. No changes were observed with serum ionized calcium and other hormones examined, as well as the body weights, and weights of thymus, heart, and brain. In the second study, eight rats were ground controls, while an additional eight rats were HLS for 12 d before being removed from tail-suspension and maintained for a further 30 d. Blood samples were collected every 6th d for 42 d. This study showed that both serum thyroxine and 1,25(OH)2 vitamin D levels returned to normal levels soon after hind limb unweighting, while serum testosterone levels matched normal levels only after a further 3-4 wk. These studies showed a significant decrease of femur weights, but not weights of humeri in HLS rats suggesting that this is a specific effect on unloaded bones. On d 12 in both studies, a significant reduction in the lumbar spine (p<0.05) and the femoral neck (p<0.01) BMD appeared in HLS rats. This was confirmed in the second study, where HLS led to a significant decrease in BMD even extending to d 42. Previous studies have shown that space flight and tail-suspension lead to marked reductions in bone formation with little effect on bone resorption. Recently, we reported that androgen replacement can indeed prevent bone losses in this animal model. Therefore, it seems logical to propose that the significant decreases of serum testosterone observed in these tail-suspended animals are, at least in part, responsible for the losses of BMD seen in their affected weight-bearing bones (i.e., lumbar spine and the femur). Considering that 1. testosterone is anabolic to osteoblasts and also decreases the rate of bone turnover 2. serum testosterone levels are markedly suppressed in simulated weightlessness, and 3. testosterone replacement therapy prevented the bone loss in HLS rats, we propose that the testosterone deficiency in this animal model is related to their bone loss.

Reversal of weightlessness-induced musculoskeletal losses with androgens: quantification by MRI

Microgravity causes rapid decrement in musculoskeletal mass is associated with a marked decrease in circulatory testosterone levels, as we reported in hindlimb-suspended (HLS) rats. In this model which simulates microgravity, we hypothesized that testosterone supplementation should prevent these losses, and we tested this in two studies. Muscle volumes and bone masses were quantitated by using magnetic resonance imaging (MRI) on day 12. In the first study, 12-wk-old Sprague-Dawley rats that were HLS for 12 days lost 28.5% of muscle volume (53.3 +/- 4.8 vs. 74.5 +/- 3.6 cm3 in the ground control rats; P < 0.001) and had a 5% decrease in bone mineral density (BMD) (P < 0.05). In the second study, 30 male 12-wk-old Wistar rats were HLS and were administered either a vehicle (control), testosterone, or nandrolone decanoate (ND). An additional 20 rats were used as ground controls, one-half of which received testosterone. HLS rats had a significant reduction in muscle volume (42.9 +/- 3.0 vs. 56 +/- 1.8 cm3 in ground control rats; P < 0.01). Both testosterone and ND treatments prevented this muscle loss (51.5 +/- 2 and 51.6 +/- 1.2 cm3, respectively; a 63% improvement; P < 0. 05). There were no statistical differences between the two active treatment groups nor with the ground controls. Similarly, there was an 85% improvement in BMD in the testosterone group (1.15 +/- 0.04 vs. 1.04 +/- 0.04 density units in vehicle controls; P < 0.05) and a 76% improvement in the ND group (1.13 +/- 0.07 density units), whereas ground control rats had a BMD of 1.17 +/- 0.03 density units. Because serum testosterone levels are markedly reduced in this model of simulated microgravity, androgen replacement seems to be a rational countermeasure to prevent microgravity-induced musculoskeletal losses.

T2 vertebral bone marrow changes after space flight

Bone biopsies indicate that during immobilization bone marrow adipose tissue increases while the functional cellular fraction decreases. One objective of our Spacelab flight experiment was to determine, using in vivo volume-localized magnetic resonance spectroscopy (VLMRS), whether bone marrow composition was altered by space flight. Four crew members of a 17 day Spacelab mission participated in the experiment. The apparent cellular fraction and transverse relaxation time (T2) were determined twice before launch and at several times after flight. Immediately after flight, no significant change in the cellular fraction was found. However, the T2 of the cellular, but not the fat component increased following flight, although to a variable extent, in all crew members with a time course for return to baseline lasting several months. The T2 of seven control subjects showed no significant change. Although these observations may have several explanations, it is speculated that the observed T2 changes might reflect increased marrow osteoblastic activity during recovery from space flight.

Collagen cross-link excretion during space flight and bed rest

Extended exposure to weightlessness results in bone loss. However, little information exists as to the precise nature or time course of this bone loss. Bone resorption results in the release of collagen breakdown products, including N-telopeptide and the pyridinium (PYD) cross-links, pyridinoline and deoxypyridinoline. Urinary pyridinoline and deoxypyridinoline are known to increase during bed rest. We assessed excretion of PYD cross-links and N-telopeptide before, during, and after long (28-day, 59-day, and 84-day) Skylab missions, as well as during short (14-day) and long (119-day) bed-rest studies. During space flight, the urinary cross-link excretion level was twice those observed before flight. Urinary excretion levels of the collagen breakdown products were also 40-50% higher, during short and long bed rest, than before. These results clearly show that the changes in bone metabolism associated with space flight involve increased resorption. The rate of response (i.e. within days to weeks) suggests that alterations in bone metabolism are an early effect of weightlessness. These studies are important for a better understanding of bone metabolism in space crews and in those who are bedridden.

The effects of twelve weeks of bed rest on bone histology, biochemical markers of bone turnover, and calcium homeostasis in eleven normal subjects

This study was undertaken to examine the effects of 12 weeks of skeletal unloading on parameters of calcium homeostasis, calcitropic hormones, bone histology, and biochemical markers of bone turnover in 11 normal subjects (9 men, 2 women; 34 +/- 11 years of age). Following an ambulatory control evaluation, all subjects underwent 12 weeks of bed rest. An additional metabolic evaluation was performed after 12 days of reambulation. Bone mineral density declined at the spine (-2.9%, p = 0.092) and at the hip (-3.8%, p = 0.002 for the trochanter). Bed rest prompted a rapid, sustained, significant increase in urinary calcium and phosphorus as well as a significant increase in serum calcium. Urinary calcium increased from a pre-bed rest value of 5.3 mmol/day to values as high as 73 mmol/day during bed rest. Immunoreactive parathyroid hormone and serum 1,25-dihydroxyvitamin D declined significantly during bed rest, although the mean values remained within normal limits. Significant changes in bone histology included a suppression of osteoblastic surface for cancellous bone (3.1 +/- 1.3% to 1.9 +/- 1.5%, p = 0.0142) and increased bone resorption for both cancellous and cortical bone. Cortical eroded surface increased from 3.5 +/- 1.1% to 7.3 +/- 4.0% (p = 0.018) as did active osteoclastic surface (0.2 +/- 0.3% to 0.7 +/- 0.7%, p = 0.021). Cancellous eroded surface increased from 2.1 +/- 1.1% to 4.7 +/- 2.2% (p = 0.002), while mean active osteoclastic surface doubled (0.2 +/- 0.2% to 0.4 +/- 0.3%, p = 0.020). Serum biochemical markers of bone formation (osteocalcin, bone-specific alkaline phosphatase, and type I procollagen extension peptide) did not change significantly during bed rest. Urinary biochemical markers of bone resorption (hydroxyproline, deoxypyridinoline, and N-telopeptide of type I collagen) as well as a serum marker of bone resorption (type I collagen carboxytelopeptide) all demonstrated significant increases during bed rest which declined toward normal during reambulation. Thus, under the conditions of this study, the human skeleton appears to respond to unloading by a rapid and sustained increase in bone resorption and a more subtle decrease in bone formation.

Future human bone research in space

Skylab crewmembers demonstrated negative calcium (Ca) balance reaching about -300 mg/day by flight day 84. Limited bone density (BMD) measurements documented that bone was not lost equally from all parts of the skeleton. Subsequent BMD studies during long duration Russian flights documented the regional extent of bone loss. These studies demonstrated mean losses in the spine, femur neck, trochanter, and pelvis of about 1%-1.6% with large differences between individuals as well as between bone sites in a given individual. Limited available data indicate postflight bone recovery occurred in some individuals, but may require several years for complete restoration. Long duration bedrest studies showed a similar pattern of bone loss and calcium balance (-180 mg/day) as spaceflight. During long duration bedrest, resorption markers were elevated, formation markers were unchanged, 1,25 vitamin D (VitD) and calcium absorption were decreased, and serum ionized Ca was increased. Although this information is a good beginning, additional spaceflight research is needed to assess architectural and subregional bone changes, elucidate mechanisms, and develop efficient as well as effective countermeasures. Space research poses a number of unique problems not encountered in ground-based laboratory research. Therefore, researchers contemplating human spaceflight research need to consider a number of unique problems related to spaceflight in their experimental design.

Effects of gravitational changes on the bone system in vitro and in vivo

Spaceflight data obtained on bone cells, rodents, and humans are beginning to shed light on the importance of gravitational loading on the skeletal system. The space environment is a relevant model to explore the bone cell response to minimal strains. However, whether there is a direct effect of gravity on the cell rather than changes related to lack of convection forces in cell cultures performed in microgravity is unknown. In vitro studies carried out using osteoblastic cell cultures in space show changes in cell shape, suggesting that cell attachment structures as well as cytoskeleton reorganization might be involved. Valuable information is expected from in vitro models of an increase or decrease in mechanical stress in order to identify the different pathways of mechanoreception and mechanotransduction in the osteoblastic lineage. Results obtained from both humans and rodents after spaceflights indicated that bone mass changes are site specific rather than evenly distributed throughout the skeleton, thus emphasizing the need to perform measurements at different bone sites: weight- and non-weight-bearing bones, and cancellous and cortical envelopes. Bone mass measurements and biochemical parameters of bone remodeling are currently under evaluation in cosmonauts. Histomorphometric studies of bones from rats after space missions of various periods provided the time course of the cancellous bone cellular events: transient increase in resorption and sustained decrease in bone formation. The underlying bone loss occurred first in weight-bearing bones and later in less weight-bearing bones. During the postflight period, time required to recover the lost bone was greater than the mission length. Thus, the postflight period deserves more attention than it is currently receiving. On earth, the rat tail-suspension model is currently used to mimic spaceflight-induced bone loss. Data from the model confirmed the impairment of osteoblastic activity and showed an alteration in osteoblast recruitment with skeletal unloading. However, this model needs to be further validated.

Perspective on the impact of weightlessness on calcium and bone metabolism

As humans venture into space to colonize the moon and travel to distant planets in the 21st century, they will be confronted with a bone disease that could potentially limit their space exploration activities or put them at risk for fracture when they return to earth. It is now recognized that an unloading of the skeleton, either due to strict bed rest or in zero gravity, leads on average to a 1%-2% reduction in bone mineral density at selected skeletal sites each month. The mechanism by which unloading of the skeleton results in rapid mobilization of calcium stores from the skeleton is not fully understood, but it is thought to be related to down regulation in PTH and 1,25-dihydroxyvitamin D3 production. Bone modeling and mineralization in chick embryos is not affected by microgravity, suggesting that bone cells adapt and ultimately become addicted to gravity in order to maintain a structurally sound skeleton. Strategies need to be developed to decrease microgravity-induced bone resorption by either mimicking gravity's effect on bone metabolism, or enhancing physically or pharmacologically bone formation in order to preserve astronauts' bone health.

Changes in musculoskeletal structure and function with prolonged bed rest

Prolonged bed rest produces profound changes in muscle and bone, particularly of the lower limb. This review first addresses the various models used by researchers to study disuse-induced changes in muscle and bone as observed during prolonged bed rest in humans. Dramatic change in muscle mass occurs within 4-6 wk of bed rest, accompanied by decreases of 6 to 40% in muscle strength. Immobilization studies in humans suggest that most of this lost muscle mass and strength can be regained with appropriate resistance training within several weeks after a period of disuse. Significant decrements in bone mineral density of the lumbar spine, femoral neck, and calcaneus observed in able-bodied men after bed rest are not fully reversed after 6 months of normal weightbearing activity. Importantly, the lost bone mass is not regained for some weeks or months after muscle mass and strength have returned to normal, further contributing to the risk of fracture. Those who enter a period of bed rest with subnormal muscle and bone mass, especially the elderly, are likely to incur additional risk of injury upon reambulation. Practical implications for exercise professionals working with individuals confined to bed rest are discussed.

Alkaline phosphatase in osteoblasts is down-regulated by pulsatile fluid flow

It is our hypothesis that interstitial fluid flow plays a role in the bone remodeling response to mechanical loading. The fluid flow-induced expression of three proteins (collagen, osteopontin, and alkaline phosphatase) involved in bone remodeling was investigated. Rat calvarial osteoblasts subjected to pulsatile fluid flow at an average shear stress of 5 dyne/cm2 showed decreased alkaline phosphatase (AP) mRNA expression after only 1 hour of flow. After 3 hours of flow, AP mRNA levels had decreased to 30% of stationary control levels and remained at this level for an additional 5 hours of flow. Steady flow (4 dyne/cm2 fluid shear stress), in contrast, resulted in a delayed and less dramatic decrease in AP mRNA expression to 63% of control levels after 8 hours of flow. The reduced AP mRNA expression under pulsatile flow conditions was followed by reduced AP enzyme activity after 24 hours. No changes in collagen or osteopontin mRNA expression were detected over 8 hours of pulsatile flow. This is the first time fluid flow has been shown to affect gene expression in osteoblasts.

Changes in the immune system during and after spaceflight

The results of immunological analyses before, during and after spaceflight, have established the fact that spaceflight can result in a blunting of the immune mechanisms of human crew members and animal test species. There is some evidence that the immune function changes in short-term flights resemble those occurring after acute stress, while the changes during long-term flights resemble those caused by chronic stress. In addition, this blunting of the immune function occurs concomitant with a relative increase in potentially infectious microorganisms in the space cabin environment. This combination of events results in an increased probability of inflight infectious events. The realization of this probability has been shown to be partially negated by the judicious use of a preflight health stabilization program and other operational countermeasures. The continuation of these countermeasures, as well as microbial and immunological monitoring, are recommended for continued spaceflight safety.

Pharmacology in space: pharmacotherapy

This chapter summarized the information available on the pharmacological kits onboard spacecraft and on the use of drugs in space, while the next chapter is dedicated to the impacts of weightlessness on drug pharmacokinetics. The need of a selected group of drugs for the use of astronauts during short-term and long-term spaceflights has been discussed. Recommendations are made for a Space Pharmacopoeia as well as for the areas of research needed to adapt medication to the weightlessness of the space environment. Although the usefulness of these drugs has been clearly demonstrated, their use also raises several problems. Physiological changes due to weightlessness may induce changes in pharmacokinetic behavior of drugs and influence their dosage regimen. Inflight data obtained by salivary drug monitoring have shown changes in the distribution of scopolamine and a significant change in the disposition of the common pain-relief agent acetaminophen taken inflight, in both drug concentration and time course. The authors of this study emphasize, however, that their data are preliminary and as yet incomplete. Further simulation studies and, if possible, inflight experiments are required. In vitro studies of the antibacterial activity of antibiotics under space conditions have shown an increased resistance of Escherichia Coli to colistin and kanamycin, and a lowered resistance of Staphylococcus Aureus to oxacillin, chloramphenicol, and erythromycin. The possible consequences of these findings for the treatment of infections contracted by astronauts are yet to be elucidated. There is still a lack of pharmacological countermeasures, particularly for preventing the progressive bone demineralization occurring in weightlessness. The treatment of space motion sickness with drugs carries with it the problem of undesirable side-effects on psychomotor performance. In order to arrive at the most appropriate medical kit for a particular mission, the best trade-off of risk versus benefit for the individual and the mission must always be attempted for any pharmacological agent.

Humeral bone density losses after shoulder surgery and immobilization

This study evaluates disuse osteoporosis of the proximal humerus after shoulder surgery and immobilization. This was accomplished by quantifying bone mineral density changes in 22 patients' proximal humeri after 6 weeks of immobilization for soft-tissue shoulder surgery. The bone mineral density of the humeral head, neck, and proximal diaphyseal subregions was determined 1 to 2 weeks before surgery and at 3, 6, and 12 weeks after surgery with dual-energy x-ray absorptiometry. By the sixth postoperative week statistically significant bone mineral density decreases of 6% to 14% were observed in the humeral neck and head subregions, respectively. The changes in these three regions diminished slightly after 6 weeks of remobilization, but the differences were still statistically significant. No significant bone mineral density changes occurred in any subregion or during any time interval in the nonoperated humerus. Our study represents the first report with dual-energy x-ray absorptiometry to quantify bone loss of the proximal humerus of patients after shoulder immobilization. Further long-term study is warranted to determine the clinical significance of this bone loss and to determine whether these losses are partially or fully recoverable.

Bone metabolism in spinal cord injured individuals and in others who have prolonged immobilisation. A review

Immobilisation or disuse is a condition known to be associated with a decrease in bone mass, osteopenia and in some people leading to osteoporosis with an increased risk of fractures. In this condition, previous histomorphometric and biochemical reports have shown an uncoupling between bone formation and resorption, but the exact sequence of the events resulting in bone loss is still not fully understood. In spinal cord injury for instance, the main finding soon after the onset is decreased osteoblastic activity associated with a dramatic increase in bone degradation. The overall consequence of these metabolic events is the development of a rapid and severe osteoporosis only observed in the paralysed part of the body associated with the loss of biomechanical strength and the biosynthesis of a structurally modified matrix which is unable to sustain normal mechanical stress. This situation dramatically increases the risk of fractures. The same uncoupling phenomenon has been described in healthy individuals who have been submitted to long duration bedrest and also in astronauts during spaceflight; but the timing, intensity and the metabolic subset may be different as these people do recover after cessation of the disuse period, which does not occur in paralysed patients. As new accurate and sensitive non-invasive techniques have become available recently to assess bone and connective tissue metabolism, more information is now available regarding bone loss in paralysed and/or immobilised individuals. These techniques should be definitely helpful in orientating new therapeutic trials with drugs and/or procedures intended to correct the musculoskeletal deleterious effects of disuse. This paper is therefore aimed at a review of bone metabolism in those with a severe spinal cord injury, or with a long duration of bedrest, or with loss of biomechanical function, or with actual or simulated spaceflight, in all instances using non-invasive techniques.

Uniformity of resorptive bone loss induced by disuse

Strains induced in the skeleton by functional activity are critical to the homeostasis of bone tissue. An in vivo model of disuse osteopenia was used to examine whether the removal of these regulatory stimuli induces a uniform loss of cortical bone through the whole organ or whether the loss of bone is focused at specific sites of the cortex. The right radii of five adult male turkeys were isolated from their normal functional loading for 8 weeks. The corresponding left radius from each animal served as an intact contralateral control. An additional group of five turkeys was used as time-zero controls to assess the initial areal symmetry of the left and right radii. Areal properties were assessed at three sites at equal intervals spanning the middle 3 cm of the diaphysis. Adaptation was determined for each cross section as a whole, as well as specifically by site by division of each cross section into 12 equal angle sectors. The average across all experimental sections after 8 weeks of disuse was 12.1 +/- 1.9% (+/- SE) loss of bone mass. The change in mean cross-sectional area varied little between the three diaphyseal sites (-10.2 +/- 3.3%, -13.5 +/- 3.8%, and -12.6 +/- 4.0%) and occurred primarily (84%) by uniform expansion of the endosteal envelope. However, elevated intracortical porosity following 8 weeks of disuse was highly nonuniform, with 58% of the increased porosity preferentially located in the ventral/caudal cortex (representing only 25% of the cortical area).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the carboxyl-terminal propeptide of type I procollagen and other markers of bone formation upon five days of bed rest

This study was performed in order to investigate the influence of skeletal unloading on the serum concentration of the carboxyl-terminal propeptide of type I procollagen (sPICP) and other markers of bone formation. Blood samples were taken every third hour from nine healthy premenopausal women (22-29 years) in two 24 h studies, before and at the end of five days of bed rest. Furthermore, a set of samples were taken 12 h apart after three days of bed rest. We measured sPICP, the serum concentration of intact and N-terminal-Mid fragment osteocalcin (sOC), and the serum concentration of alkaline phosphatase (sAP). During the five days of bed rest a gradual increase in sOC was observed, while sPICP gradually decreased. sAP was unchanged. Five days of best rest resulted in the following overall changes in the 24 h mean values: sPICP: -14% (p = 0.002); sOC: +9% (p = 0.009); sAP: -1% (not significant). The circadian patterns did not change significantly after bed rest. It is puzzling that the changes in the bone formation markers are of different magnitude, and for sPICP and sOC even in opposite directions. The increase in sOC may be caused by an increase in OC secretion by the osteoblasts or a release of bone-incorporated OC from resorbing sites; the accompanying decrease in sPICP may indicate that bone formation is actually transiently decreased after short term bed rest.

Stress protection due to external fixation

Bone is sensitive to mechanical influences. The presence of an orthopaedic device will impose constraints on the mechanical environment that may influence subsequent remodelling and repair. An Oxford External Fixator was applied to six intact ovine tibiae. The strains engendered during normal walking were then recorded from strain gauges applied to the mid-shaft of the bone. The fixator configuration was then altered such that the intact fixator bar connecting the pins was replaced with a sectioned version that did not permit load transfer through the fixator, and the strain environment re-recorded. The peak strains recorded with the intact bar were significantly lower than those recorded with the sectioned bar. This showed that the use of a fixator with an intact bar resulted in significant stress protection of the underlying bone. A fixator was then applied to both the right and left tibiae of a further six animals and the resulting strain environment and corresponding remodelling response was observed over 16 weeks. In each case one fixator was configured with an intact bar (the stress protected limb), whilst the other utilised a sectioned bar (the unprotected limb). The results showed that over this period the bone mineral content fell by 9% in the stress protected limb compared to the unprotected limp. Quantitative assessment of the bones showed that this bone loss occurred as a direct consequence of resorption on the periosteal and endosteal surfaces. In addition, strain recordings at week 16 showed that the fixator was still stress protecting the tibia.