Systemic administration of transforming growth factor-beta 2 prevents the impaired bone formation and osteopenia induced by unloading in rats.

We investigated the effect of recombinant human transforming growth factor beta 2 (rhTGF-beta 2) administration on trabecular bone loss induced by unloading in rats. Hind limb suspension for 14 d inhibited bone formation and induced osteopenia as shown by decreased bone volume, calcium and protein contents in long bone metaphysis. Systemic infusion of rhTFG-beta 2 (2 micrograms/kg per day) maintained normal bone formation rate, and prevented the decrease in bone volume, bone mineral content, trabecular thickness and number induced by unloading. In vitro analysis of tibial marrow stromal cells showed that rhTGF-beta 2 infusion in unloaded rats increased the proliferation of osteoblast precursor cells, but did not affect alkaline phosphatase activity or osteocalcin production. Northern blot analysis of RNA extracted from the femoral metaphysis showed that rhTGF-beta 2 infusion in unloaded rats increased steady-state levels of type I collagen mRNA but not alkaline phosphatase mRNA levels. rhTGF-beta 2 infusion at the dose used had no effect on metaphyseal bone volume and formation, osteoblast proliferation or collagen expression in control rats. The results show that systemic administration of rhTGF-beta 2 enhances osteoblast precursor cell proliferation and type I collagen expression by osteoblasts, and prevents the impaired bone formation and osteopenia induced by unloading.

Spaceflight inhibits bone formation independent of corticosteroid status in growing rats.

Bone formation and structure have been shown repeatedly to be altered after spaceflight. However, it is not known whether these changes are related to a stress-related altered status of the corticosteroid axis. We investigated the role of corticosteroids on spaceflight-induced effects in rat pelvis and thoracic vertebrae. Thirty-six male Sprague-Dawley rats were assigned to a flight, flight control, or vivarium group (n = 12/group). Bilateral adrenalectomy was performed in six rats per group, the additional six rats undergoing sham surgery. Adrenalectomized (ADX) rats were implanted with corticosteroid pellets. On recovery from spaceflight, thoracic vertebrae and the whole pelvis were removed and processed for biochemistry, histomorphometry, or bone cell culture studies. The 17-day spaceflight resulted in decreased bone volume (BV) in the cotyle area of pelvic bones (-12%; p < 0.05) associated with approximately 50% inhibition of bone formation in the cancellous area of pelvic metaphyses and in thoracic vertebral bodies. The latter effect was associated with a decreased number of endosteal bone cells isolated from the bone surface (BS) in these samples (-42%; p < 0.05). This also was associated with a decreased number of alkaline phosphatase positive (ALP+) endosteal bone cells at 2 days and 4 days of culture, indicating decreased osteoblast precursor cell recruitment. Maintaining basal serum corticosterone levels in flight-ADX rats did not counteract the impaired bone formation in vertebral or pelvic bones. Moreover, the decreased ex vivo number of total and ALP+ endosteal bone cells induced by spaceflight occurred independent of endogenous corticosteroid hormone levels. These results indicate that the microgravity-induced inhibition of bone formation and resulting decreased trabecular bone mass in specific areas of weight-bearing skeleton in growing rats occur independently of endogenous glucocorticoid secretion.

Bone morphometric changes in adjuvant-induced polyarthritic osteopenia in rats: evidence for an early bone formation defect.

Adjuvant polyarthritis (AP) in rats is known to result in extensive bone loss. This study investigates the mechanisms responsible for the early trabecular osteopenia evaluated at a single point in time--2 weeks after adjuvant injection--in the hindpaw of female Lewis rats using biochemical and histomorphometric methods. At this early point in time, the inflammation was generalized (inflammatory score, 20; albumin/globulin, -80% versus control). Histomorphometric analysis of the noninjected femur showed that the trabecular bone volume was significantly decreased (-28% versus control) in both proximal and distal parts, and the femur growth rate was unaffected. The trabecular osteopenia was associated with a 90% decrease in osteoid surface and a concomitant thinning (-19%) of the trabeculae. Both the double-fluorescence-labeled surface and the osteoblast surface were also markedly decreased (-75%). In addition, the mineral apposition rate was reduced (-50%) and the bone formation rate was decreased by as much as 90%. The trabecular bone volume was decreased in relation with the extent of double-fluorescence labeling (r = 0.38, p = 0.03) and bone formation rate (r = 0.42, p = 0.01), suggesting that the generalized osteopenia resulted from the reduced bone formation. This was associated with a 26% reduction in plasma osteocalcin. Neither the osteoclast surface nor the number of osteoclasts was consistently affected. However, urinary hydroxyproline was increased by 100-200%, which likely reflected the cartilage and bone destruction at the site of injection. The present data show that the early extensive osteopenia observed 2 weeks after AP induction in rats results from defective bone formation with unchanged bone resorption. The role of cytokines in such an inhibitory effect on bone formation remains to be determined.

Insulin-like growth factor-I increases trabecular bone formation and osteoblastic cell proliferation in unloaded rats.

We previously found that the inhibition of bone formation and trabecular osteopenia induced by skeletal unloading in rats are associated with reduced proliferation of osteoblastic cells lining the bone surface. In this study, we examined the effects of insulin-like growth factor-I (IGF-I) on trabecular bone formation, bone mineral density, and proliferation of marrow-derived osteoblastic cells in unloaded rats. Skeletal unloading of hind limbs was induced by tail suspension, and recombinant human IGF-I was administered at two different doses (1.3 or 2.0 mg/kg.day) in control and unloaded rats by continuous infusion for 14 days. Treatment with IGF-I had no effect on plasma glucose levels, body weight, or longitudinal bone growth. The double calcein-labeled surface, bone formation rate, and trabecular number measured at the tibial metaphysis were lower in unloaded rats compared to controls and were increased after IGF-I treatment. The increased number of bone-forming sites induced by IGF-I was associated with partial prevention of trabecular bone loss in unloaded rats. In contrast to the beneficial effects of IGF-I on bone formation and bone mineral content in unloaded rats, IGF-I had no effect in control rats. To evaluate the cellular mechanisms of action of IGF-I, marrow stromal cells were derived from the tibia of unloaded and control rats and studied in vitro. Unloading was associated with a decreased proliferation of alkaline phosphatase-positive (ALP+) marrow stromal cells. Treatment with IGF-I increased the number of ALP+ cells in unloaded rats, but not in control rats. IGF-I treatment increased ALP activity and osteocalcin production by marrow-derived cells in suspended and control rats, suggesting that IGF-I stimulated the proliferation and differentiation of osteoblast precursor cells. These results indicate that IGF-I infusion enhanced the recruitment of osteoblastic cells, increased trabecular bone formation, and partially prevented trabecular bone loss in unloaded rats, which supports the hypothesis that IGF-I may mediate in part the effects of loading on bone formation.

Effects of exogenous growth hormone on bone mineralization and remodeling and on plasma calcitriol in intact pigs.

Growing pigs were given subcutaneous (SC) injections of 40 micrograms porcine GH/kg body weight (BW) or its vehicle twice daily for 2 months. Animals were pair-fed with a diet containing 1.1% Ca, 0.6% P, and 1000 IU vitamin D3/kg. At slaughter, several bone characteristics including histomorphometric data (using double tetracycline labeling) were measured on tibia and metacarpals. GH accelerated growth, with greater (p < 0.01) tibial and metacarpal weights, greater tibial length (p < 0.01) and diameters (outside and inside, p < 0.01), and greater tibial ash weight (p < 0.02) in GH-treated pigs than in controls. The similar values of apparent bone density (weight/volume) and ash/bone volume or ash/dry matter in the two groups suggest adequate coupling between bone growth and mineralization in GH-treated pigs. Histomorphometric data for the distal metacarpal metaphysis indicated greater trabecular bone volume (p < 0.01), osteoblastic surface (p < 0.01), and mineral apposition rate (p < 0.05) in GH-treated pigs. The osteoclast surface, lacuna depth, and osteoid-related parameters in GH-treated and control pigs were similar. The plasma PTH of the two groups of pigs were similar throughout the experiment. These data and the elevated plasma alkaline phosphatase activity (p < 0.05) in GH-treated pigs suggest that GH specifically affects bone formation. GH had no effect on the plasma 25-OH vitamin D3 but 1,25(OH)2 vitamin D3 (calcitriol) was higher (p < 0.01) in treated pigs throughout the experiment. This suggests that calcitriol may help adapt bone mineralization to accelerated bone formation during growth hormone treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo bone metabolism and ex vivo bone marrow osteoprogenitors in vitamin D-deprived pigs.

Vitamin D insufficiency is still a concern in countries where there is no routine food supplementation, such as France. A low vitamin D status is clearly associated with an increased risk of fracture in the elderly, but the long-term consequences of latent vitamin D insufficiency in young people and adults are not known. We fed 26 growing pigs a high calcium diet (1.1%) with a 1000 IU cholecalciferol/kg diet (controls), or without vitamin D (0D) for 4 months. We then analyzed the overall impact of low vitamin D status on osteotropic hormones (calcitriol and immunoreactive parathyroid hormone), plasma markers of bone remodeling (alkaline phosphatase [ALP] activity, carboxyterminal propeptide of type I procollagen [PICP], osteocalcin, hydroxyproline), whole bone parameters (ash content, bending moment), histomorphometry, and the populations of marrow osteoblastic and osteoclastic precursors by ex vivo cultures. The fall in plasma 25-dihydroxyvitamin [25(OH)D] in the 0D pigs indicated severe depletion of their vitamin D stores. However, they remained normocalcemic, were mildly hyperparathyroid after 2 months of vitamin D deprivation, and showed only a slight decrease in plasma calcitriol. The bone mineral content and bending moment of metatarsals decreased and they had increased osteoblastic (+59%, p < 0.05 0D vs. controls) and osteoclastic (+31%, p < 0.1 0D vs. controls) surfaces. This was not paralleled by increased bone turnover, because plasma hydroxyproline and ALP were unchanged and PICP and osteocalcin were decreased. The adherent fraction of bone marrow cells showed a great increase in the number of total stromal colony-forming units (CFU-F; +93%, p < 0.05 0D vs. controls) and in the percent of ALP(+) CFU-F (+58%, p < 0.01 0D vs. controls) in cultures from 0D pigs. More tartrate-resistant acid phosphatase-positive (TRAP(+)) multinucleated cells were generated in cultures of nonadherent marrow cells from 0D pigs, and the area of resorption was 345% greater than in controls. Thus, vitamin D deprivation caused only moderate hormonal changes in growing pigs fed a high-calcium diet, but affected their bone characteristics and greatly enhanced the pool of osteoblasts and osteoclasts by stimulating the commitment of their precursors in bone marrow.

Electrical stimulation of leg muscles increases tibial trabecular bone formation in unloaded rats.

Rat head-down hindlimb suspension (HS) has been shown to induce hindlimb cancellous bone loss. As HS is known to associate unloading with progressive disappearance of hindlimb muscle contractions, we investigated whether persisting muscle motion could modify suspension-induced bone disorders or even prevent them. Chronic electrical stimulation (ES) was applied to leg muscles of rats during 3-wk hindlimb suspension, the lack of support for hindlimbs maintaining a hypodynamic situation. The histomorphometric characteristics of the proximal tibial metaphysis were analyzed. At the end of this protocol of combined suspension and stimulation, trabecular bone loss remained similar to that of nonstimulated HS animals. However, trabecular bone cell activity parameters showed greater bone formation after muscle stimulation in unloaded animals, with significantly increased osteoblastic, osteoid, and mineralizing surfaces. In addition, periosteal mineral apposition rate and cancellous bone formation rate, markedly decreased by suspension, were not significantly different in suspended stimulated compared with normal loaded animals. This enhanced formation activity could be related to persistence of muscle activity, as shown by partial preservation of muscle mass. However, direct electrical effects on bone cannot be excluded. Thus, despite muscle stimulation, with enhanced bone formation, isolated suppression of hypokinesia has not been able to counteract bone effects of unloading. This finding supports the hypothesis of the importance of mechanical loading to maintain bone architecture.

Effects of spaceflight and recovery on rat humeri and vertebrae: histological and cell culture studies.

Skeletal changes associated with spaceflight in the rat have been well documented, but few data are available on bone tissue and bone cell metabolism after subsequent on-Earth recovery. We therefore investigated the effects of microgravity and subsequent recovery on trabecular bone morphology and cellular activities in rat humeri and thoracic vertebrae and compared histomorphometric parameters in caudal vertebrae with the behavior of vertebral osteoblastic cells in culture. We report here that humeral weight showed normal growth during the experiment but was unaffected by spaceflight or recovery from spaceflight. However, the 14-day spaceflight resulted in inhibition of static indexes of bone formation in humeral proximal metaphyses and thoracic vertebral bodies. This was associated with a decrease in bone volume in humeral metaphyses. After 14 days of on-Earth recovery, osteoblastic and osteoid surfaces returned toward normal and bone volume was normalized in humeri, whereas the static bone formation parameters were not restored in thoracic vertebrae. In addition, histological indexes of bone formation and osteoblastic cell growth in vitro were not affected by spaceflight in caudal vertebrae. This study shows that rat humeri and thoracic and caudal vertebrae exhibit different patterns of response to spaceflight and subsequent on-Earth recovery, which could be due, at least in part, to the different loading pattern of these bones, and also to differences in bone turnover rate.

Effects of spaceflight on bone mineralization in the rhesus monkey.

We combined dual-photon absorptiometry, iliac crest histomorphometry, and backscattered electrons analysis to characterize bone mineralization effects of a spaceflight on young monkeys. Two 4- to 5-kg male rhesus monkeys (Macaca mulatta) were flown during a 11.5-day spaceflight that took place onboard Cosmos 2229 biosatellite (Bion 10). Vivarium (n = 4) and Earth-based chair (n = 4) control situations were studied for comparison. Flight monkeys exhibited lower values of iliac cancellous bone volume, associated with nonsignificantly thinner trabeculae. Bone mineralization rate and the proportion of trabecular bone surface involved in mineralization processes were found markedly reduced after spaceflight. Analysis of embedded sections by backscattered electrons imaging showed a nonsignificant shift to lower mineralization in the flight biopsies vs. postflight mock-up biopsies. These results were in accordance with dual-photon absorptiometry evaluations showing a tendency for decreased bone mineral content during flight and recovery thereafter. The ground simulation experiment performed on the same monkeys more than 1 mo after landing suggests that the observed effects were specifically related to spaceflight and that the animals had only partially recovered. Additional animals on future flights will be required to confirm these findings.

Changes in rat atrial ANF granules induced by hindlimb suspension.

It is well known that the heart releases a factor called ANF (atrial natriuretic factor) or ANP (atrial natriuretic peptide) capable of inducing rapid diuretic and natriuretic actions. This factor is stored in secretory granules mainly located in myocytes in both atria. The main secretory stimulus is the distention of the atrial cavity resulting, for example, from enhanced venous return. However, the cellular events which occur after the stimulation remain to be clarified. The aim of this investigation was to study the intra-cellular events preceding the ANF release, using the rat hindlimb suspension as model of stimulation. In this model, Wistar rats were placed in a 30 degrees anti-orthostatic position and a blood shift towards the heart was obtained. Different durations (1/4 h, 1/2 h, 3/4 h, 1 h, 2 h and 6 h) were studied. The ANF plasma level was investigated by Radio Immuno Assay and granule immunoreactivity was measured by counting gold particles on micrographs. The ANF plasma level was significantly increased (+60%) after 1 h of suspension. The response was transient and then decreased to basal values. Morphological criteria established at the beginning of this study, and measured throughout the experiment, were found transiently modified after suspension. The surface of the perinuclear area was transitory enlarged by 36% 30 min after suspension. Moreover, in the same time immunoreactivity of the secretory granules was enhanced without changes in granule size. These results suggest an increase in the ANF synthesis and storage in the granules during the stimulation. However, the cellular regulatory mechanism of the ANF synthesis which could explain the transitory aspect of these events, requires further investigation.

Effects of BMP-2 on osteoblastic cells and on skeletal growth and bone formation in unloaded rats.

A previous study showed that skeletal unloading induced by hindlimb suspension for 14 days in rats reduces osteoblastic cell proliferation, inhibits skeletal growth and bone formation and induces metaphyseal bone loss. This study investigated the effect of recombinant human bone morphogenetic protein-2 (rhBMP-2) in this model. In vitro analysis showed that rhBMP-2 (25-100 ng/ml, 48-96 h) increased alkaline phosphatase activity, an early marker of osteoblast differentiation, in rat neonatal calvaria cells and adult marrow stromal cells, showing that rhBMP-2 induced the differentiation of osteoblast precursor cells in vitro. In contrast, rhBMP-2 did not increase rat calvaria or marrow stromal cell proliferation. Biochemical and histomorphometric analysis showed that systemic infusion with rhBMP-2 (2 microg/kg/day) in unloaded rats had no significant effect on serum osteocalcin levels and on histomorphometric indices of bone formation. Accordingly, rhBMP-2 infusion did not prevent the decreased skeletal growth, trabecular bone bone volume and bone mineral content induced by unloading. The present data indicate that, although rhBMP-2 stimulates osteoblastic cell differentiation, rhBMP-2 infusion is not effective in increasing bone formation and in preventing trabecular bone loss induced by unloading in rats.

TGF-beta2 prevents the impaired chondrocyte proliferation induced by unloading in growth plates of young rats.

Growth plate width and cartilage organization are altered during skeletal unloading in growing rats. Immunohistochemical studies have identified TGF-beta in calcified cartilage, and TGF-beta is known to induce mitogenic effects on chondrocytes in vitro. On the other hand, IGF-1 was shown to be expressed in the proximal tibial growth plate and to mediate GH-induced longitudinal bone growth in rats. We therefore investigated the effect of recombinant human (rh) IGF-1 and rhTGF-beta2 infusion on the changes induced by unloading in the cellular organization of the growth plate in growing rats. Hindlimb unloading for 14 days induced a 13% reduction in growth cartilage height in the proximal tibia. This effect was mostly related to a 17% and 14% decrease in the proliferative zone height and chondrocyte number, respectively. In unloaded rats treated with a systemic infusion of rhTGF-beta2 (2microg/kg/day) the number of chondrocytes in the proliferative zone was not different from those of normal loaded animals. In contrast, rhIGF-1 treatment at a 2mg/kg/day dose was not effective in counteracting the effects of unloading on growth plate height and chondrocyte number. These results show that systemic administration of rhTGF-beta2 prevents in large part the reduced growth of chondrocytes in the proliferative zone and the reduced epiphyseal growth plate growth induced by unloading in rats.

Bone mass increases in less than 4 wk of voluntary exercising in growing rats.

INTRODUCTION: Physical exercise is known to augment bone mass, but the specific mechanisms by which physical activity influences skeletal metabolism is still not thoroughly understood. So far, time related events of bone tissue adaptation to physical training have not been investigated. We, therefore, studied the time-course effects of daily spontaneous exercise training on bone metabolism in the rat by biochemical and tissue analyses. METHODS: Forty-five 4-wk-old female Dark Agouti rats were used, randomly assigned to voluntary exercising groups of five animals for 1, 2, 3, and 4 wk, or to age-matched sedentary controls. At sacrifice, blood was sampled for determination of parameters of bone and calcium metabolism (osteocalcin, alkaline phosphatases, total and ionized calcium, phosphorus, parathyroid hormone). Right and left tibiae were removed for bone mass (dry and ash mass, mineral content) and histomorphometric analyses. RESULTS: We found that the animals performed large amounts of exercise (reaching 50 +/- 8 km x wk(-1)), and that this voluntary physical training induced significant changes in bone metabolism. An increase (approximately 32%) in serum alkaline phosphatase activity was already measurable at the end of the first week of exercising and persisted throughout the experiment, accompanied by a concomitant increase in osteoblastic bone formation (approximately 27%), as evaluated in bone tissue. These changes were associated with consecutive increases in tibial bone mass (approximately 7.6%), mineral content (approximately 7.4%) and metaphyseal bone volume (approximately 17%) measurable 4 wk after the onset of exercise, with interesting positive correlations between tibial bone mass parameters and soleus muscle mass. Conversely, no change was evidenced in biochemical parameters of calcium metabolism, except for an increase in phosphorus serum levels in trained animals. CONCLUSION: Daily spontaneous exercise training induced an increase in bone mass and bone volume in less than 4 wk. Further studies are needed to fully investigate these changes during the very first days of exercising.

Is there a relationship between physical activity and dietary calcium intake? A survey in 10,373 young French subjects.

PURPOSE: The purpose of this study was to discuss, in the light of the results of a survey, the calcium ration of a sample of French youth and to determine whether various sports activities can be related to dietary calcium intake. METHODS: Physical activity was evaluated using Baecke's questionnaire. Calcium intake was evaluated using a food frequency oriented questionnaire. The survey was performed on a population of 10,373 subjects (6,966 males and 3,407 females) including three different groups of subjects: school children and college students, military personnel, and athletes registered in sports federations. The mean age of this population was 19 +/- 9 yr, ages ranging between 7 and 50 yr. RESULTS: The mean amount of declared calcium intake (DCI) for the total population was 1242 +/- 843 mg per 24 hr (mg x d(-1)). Fifty percent of this population consumed less than 1000 mg x d(-1) and 13% less than 500 mg x d(-1). There was no significant relationship between the index of activity and declared calcium intake. Calcium intake decreased with age and was lower in females compared to males. CONCLUSIONS: The subjects trained in individual endurance sports such as triathlon, biking, and road running have a lower DCI than subjects trained in team sports such as volley ball, handball, or basketball. This survey, performed on a large population, does indicate that for half of them daily calcium intake is below the threshold of 1,000 mg x d(-1) considered the daily requirement covering the needs of a population without age or gender distinction and that calcium intake is not related to the level of physical activity.

Effect of endurance training on postexercise parathyroid hormone levels in elderly men.

The present study was designed to evaluate the effects of 6-wk endurance training on serum parathyroid hormone (PTH) levels and on other parameters at rest and after a maximal exercise test (MET) in 24 55- to 73-yr-old men. Before training, MET was found to induce a significant increase in PTH levels as compared with resting values. This MET-induced rise in PTH was accompanied by enhanced total calcium, phosphate, alkaline phosphatase (ALP), osteocalcin, and albumin levels. After the training period (75-80% maximal heart rate, 1 h.d-1, 4 d.wk-1), the changes induced by MET in calcium, phosphate, ALP, and albumin levels followed the same pattern as before training. Conversely, the MET-induced increase in PTH levels was found markedly more pronounced after training than in untrained conditions (+21.9% vs +11.1%, respectively, P < 0.05). Furthermore, lower values of osteocalcin were found after training as compared with pretraining values, both at rest and after maximal exercise. These findings indicate that 6 wk of endurance training enhanced exercise-related release of PTH and reduced osteocalcin levels in elderly men. This might be of importance regarding bone status in the elderly, as exercise is proposed as a preventive measure against osteopenia.

Effects of microgravity on bone and calcium homeostasis.

Mechanical function is known to be of crucial importance for the maintenance of bone tissue. Gravity on one hand and muscular effort on the other hand are required for normal skeletal structure. It has been shown by numerous experimental studies that loss of total-body calcium, and marked skeletal changes occur in people who have flown in space. However, most of the pertinent investigations have been conducted on animal models, including rats and non-human primates, and a reasonably clear picture of bone response to spaceflight has emerged during the past few years. Osteopenia induced by microgravity was found to be associated with reduction in both cortical and trabecular bone formation, alteration in mineralization patterns and disorganization of collagen, and non-collagenous protein metabolism. Recently, cell-culture techniques have offered a direct approach of altered gravity effects at the osteoblastic-cell level. But the fundamental mechanisms by which bone and calcium are lost during spaceflight are not yet fully known. Infrequency and high financial cost of flights have created the necessity to develop on-Earth models designed to mimic weightlessness effects. Antiorthostatic suspension devices are now commonly used to obtain hindlimb unloading in rats, with skeletal effects similar to those observed after spaceflight. Therefore, actual and "simulated" spaceflights, with investigations conducted at whole body and cellular levels, are needed to elucidate pathogeny of bone loss in space, to develop effective countermeasures, and to study recovery processes of bone changes after return to Earth.

Enhancement of insulin receptor mediated endocytosis in cultured osteoblastic cells under hypergravity.

NASA: Many cellular ligands are internalized by the cells after binding to a specific membrane receptor. This process, called Receptor-Mediated Endocytosis (RME) is an ubiquitous mechanism. It has been demonstrated on osteoblastic cells for many ligands, like insulin. The different steps of RME can be divided in 3 main phases. First takes place the binding phase corresponding to the interaction between the ligand and its specific membrane receptor. The second phase corresponds to the clathrin-mediated internalization of the ligand-receptor complex, by the mean of coated pits and coated vesicles. In the third phase are found the intracytoplasmic events, recycling or degradation, through endosomes and lysosomes. Insulin has been shown to bind to osteoblastic cells on specific receptors and to stimulate bone formation. The receptor-ligand complex is then internalized by RME. Visualization of this complex in transmission electron microscopy can be made effective by coupling the ligand to colloidal gold particles. In previous studies, we demonstrated that microgravity had no apparent effect on insulin binding to its specific receptor, but that RME process was accelerated by altered gravitational fields induced by parabolic flight. The aim of the present work was to determine, by the mean of a quantitative analysis of the three phases previously described, whether RME is sensitive to a continuous 2G exposition.^ieng

[The suspension device for hindlimb unloading in the rat--results of a video monitoring study]. / Le dispositif de suspension et de privation d'appui chez le rat--resultat d'une surveillance video.

Several physiological modifications reported in rats after spaceflight have been shown to be mimicked by an experimental procedure which permits skeletal unloading of the animal. Therefore, the model of hindlimb suspension is used by many authors interested in spaceflight and in hypokinesia studies. The aim of our work was to describe this model and propose a new device for headdown suspension and hindlimb unloading purposes. Using a video monitoring system, we observed that animals adapt to their new situation. Indeed, we have verified that the apparatus allows most of animals' functions. The growth curve which was not significantly modified, confirmed the present observations showing that rat feeding activity seems to not be altered. The rat is also able to move in the cage, to rest and sleep, to groom. Moreover, most of problems encountered during previous experiments have been understood and a new device with the same principles, but protected against disfunction, is proposed.