Pre-flight exercise and bone metabolism predict unloading-induced bone loss due to spaceflight.

OBJECTIVES: Bone loss remains a primary health concern for astronauts, despite in-flight exercise. We examined changes in bone microarchitecture, density and strength before and after long-duration spaceflight in relation to biochemical markers of bone turnover and exercise. METHODS: Seventeen astronauts had their distal tibiae and radii imaged before and after space missions to the International Space Station using high-resolution peripheral quantitative CT. We estimated bone strength using finite element analysis and acquired blood and urine biochemical markers of bone turnover before, during and after spaceflight. Pre-flight exercise history and in-flight exercise logs were obtained. Mixed effects models examined changes in bone and biochemical variables and their relationship with mission duration and exercise. RESULTS: At the distal tibia, median cumulative losses after spaceflight were -2.9% to -4.3% for bone strength and total volumetric bone mineral density (vBMD) and -0.8% to -2.6% for trabecular vBMD, bone volume fraction, thickness and cortical vBMD. Mission duration (range 3.5-7 months) significantly predicted bone loss and crewmembers with higher concentrations of biomarkers of bone turnover before spaceflight experienced greater losses in tibia bone strength and density. Lower body resistance training volume (repetitions per week) increased 3-6 times in-flight compared with pre-spaceflight. Increases in training volume predicted preservation of tibia bone strength and trabecular vBMD and thickness. CONCLUSIONS: Findings highlight the fundamental relationship between mission duration and bone loss. Pre-flight markers of bone turnover and exercise history may identify crewmembers at greatest risk of bone loss due to unloading and may focus preventative measures.

Use of Quantitative Computed Tomography to Assess for Clinically-relevant Skeletal Effects of Prolonged Spaceflight on Astronaut Hips.

INTRODUCTION: In 2010, experts in osteoporosis and bone densitometry were convened by the Space Life Sciences Directorate at NASA Johnson Space Center to identify a skeletal outcome in astronauts after spaceflight that would require a clinical response to address fracture risk. After reviewing astronaut data, experts expressed concern over discordant patterns in loss and recovery of bone mineral density (BMD) after spaceflight as monitored by dual-energy X-ray absorptiometry (DXA) and quantitative computed tomography (QCT). The pilot study described herein demonstrates the use of QCT to evaluate absence of recovery in hip trabecular BMD by QCT as an indicator of a clinically actionable response. METHODOLOGY: QCT and DXA scans of both hips were performed on 10 astronauts: once preflight and twice postflight about 1 wk and 1 yr after return. If trabecular BMD had not returned to baseline (i.e., within QCT measurement error) in 1 or both hips 1 yr after flight, then another QCT hip scan was obtained at 2 yr after flight. RESULTS: Areal BMD by DXA recovered in 9 of 10 astronauts at 1 yr postflight while incomplete recovery of trabecular BMD by QCT was evident in 5 of 10 astronauts and persisted in 4 of the 5 astronauts 2 yr postflight. CONCLUSION: As an adjunct to DXA, QCT is needed to detect changes to hip trabecular BMD after spaceflight and to confirm complete recovery. Incomplete recovery at 2 yr should trigger the need for further evaluation and possible intervention to mitigate premature fragility and fractures in astronauts following long-duration spaceflight.

Combined Effects of Simulated Microgravity and Radiation Exposure on Osteoclast Cell Fusion.

The loss of bone mass and alteration in bone physiology during space flight are one of the major health risks for astronauts. Although the lack of weight bearing in microgravity is considered a risk factor for bone loss and possible osteoporosis, organisms living in space are also exposed to cosmic radiation and other environmental stress factors. As such, it is still unclear as to whether and by how much radiation exposure contributes to bone loss during space travel, and whether the effects of microgravity and radiation exposure are additive or synergistic. Bone is continuously renewed through the resorption of old bone by osteoclast cells and the formation of new bone by osteoblast cells. In this study, we investigated the combined effects of microgravity and radiation by evaluating the maturation of a hematopoietic cell line to mature osteoclasts. RAW 264.7 monocyte/macrophage cells were cultured in rotating wall vessels that simulate microgravity on the ground. Cells under static 1g or simulated microgravity were exposed to γ rays of varying doses, and then cultured in receptor activator of nuclear factor-κB ligand (RANKL) for the formation of osteoclast giant multinucleated cells (GMCs) and for gene expression analysis. Results of the study showed that radiation alone at doses as low as 0.1 Gy may stimulate osteoclast cell fusion as assessed by GMCs and the expression of signature genes such as tartrate resistant acid phosphatase (Trap) and dendritic cell-specific transmembrane protein (Dcstamp). However, osteoclast cell fusion decreased for doses greater than 0.5 Gy. In comparison to radiation exposure, simulated microgravity induced higher levels of cell fusion, and the effects of these two environmental factors appeared additive. Interestingly, the microgravity effect on osteoclast stimulatory transmembrane protein (Ocstamp) and Dcstamp expressions was significantly higher than the radiation effect, suggesting that radiation may not increase the synthesis of adhesion molecules as much as microgravity.

Evaluation of body shape as a human body composition assessment in isolated conditions and remote environments.

Individuals in isolated and extreme environments can experience debilitating side-effects including significant decreases in fat-free mass (FFM) from disuse and inadequate nutrition. The objective of this study was to determine the strengths and weaknesses of three-dimensional optical (3DO) imaging for monitoring body composition in either simulated or actual remote environments. Thirty healthy adults (ASTRO, male = 15) and twenty-two Antarctic Expeditioners (ABCS, male = 18) were assessed for body composition. ASTRO participants completed duplicate 3DO scans while standing and inverted by gravity boots plus a single dual-energy X-ray absorptiometry (DXA) scan. The inverted scans were an analog for fluid redistribution from gravity changes. An existing body composition model was used to estimate fat mass (FM) and FFM from 3DO meshes. 3DO body composition estimates were compared to DXA with linear regression and reported with the coefficient of determination (R2) and root mean square error (RMSE). ABCS participants received only duplicate 3DO scans on a monthly basis. Standing ASTRO meshes achieved an R2 of 0.76 and 0.97 with an RMSE of 2.62 and 2.04 kg for FM and FFM, while inverted meshes achieved an R2 of 0.52 and 0.93 with an RMSE of 2.84 and 3.23 kg for FM and FFM, respectively, compared to DXA. For the ABCS arm, mean weight, FM, and FFM changes were -0.47, 0.06, and -0.54 kg, respectively. Simulated fluid redistribution decreased the accuracy of estimated body composition values from 3DO scans. However, FFM stayed robust. 3DO imaging showed good absolute accuracy for body composition assessment in isolated and remote environments.

Spaceflight-induced bone loss: is there an osteoporosis risk?

Currently, the measurement of areal bone mineral density (aBMD) is used at NASA to evaluate the effects of spaceflight on the skeletal health of astronauts. Notably, there are precipitous declines in aBMD with losses >10 % detected in the hip and spine in some astronauts following a typical 6-month mission in space. How those percentage changes in aBMD relate to fracture risk in the younger-aged astronaut is unknown. Given the unique set of risk factors that could be contributing to this bone loss (eg, adaptation to weightlessness, suboptimal diet, reduced physical activity, perturbed mineral metabolism), one might not expect skeletal changes due to spaceflight to be similar to skeletal changes due to aging. Consequently, dual-energy X-ray absorptiometry (DXA) measurement of aBMD may be too limiting to understand fracture probability in the astronaut during a long-duration mission and the risk for premature osteoporosis after return to Earth. Following a brief review of the current knowledge-base, this paper will discuss some innovative research projects being pursued at NASA to help understand skeletal health in astronauts.

Effects of exercise countermeasures on multisystem function in long duration spaceflight astronauts.

Exercise training is a key countermeasure used to offset spaceflight-induced multisystem deconditioning. Here, we evaluated the effects of exercise countermeasures on multisystem function in a large cohort (N = 46) of astronauts on long-duration spaceflight missions. We found that during 178 ± 48 d of spaceflight, ~600 min/wk of aerobic and resistance exercise did not fully protect against multisystem deconditioning. However, substantial inter-individual heterogeneity in multisystem response was apparent with changes from pre to postflight ranging from -30% to +5%. We estimated that up to 17% of astronauts would experience performance-limiting deconditioning if current exercise countermeasures were used on future spaceflight missions. These findings support the need for refinement of current countermeasures, adjunct interventions, or enhanced requirements for preflight physiologic and functional capacity for the protection of astronaut health and performance during exploration missions to the moon and beyond.

Incomplete recovery of bone strength and trabecular microarchitecture at the distal tibia 1 year after return from long duration spaceflight.

Determining the extent of bone recovery after prolonged spaceflight is important for understanding risks to astronaut long-term skeletal health. We examined bone strength, density, and microarchitecture in seventeen astronauts (14 males; mean 47 years) using high-resolution peripheral quantitative computed tomography (HR-pQCT; 61 µm). We imaged the tibia and radius before spaceflight, at return to Earth, and after 6- and 12-months recovery and assessed biomarkers of bone turnover and exercise. Twelve months after flight, group median tibia bone strength (F.Load), total, cortical, and trabecular bone mineral density (BMD), trabecular bone volume fraction and thickness remained - 0.9% to - 2.1% reduced compared with pre-flight (p ≤ 0.001). Astronauts on longer missions (> 6-months) had poorer bone recovery. For example, F.Load recovered by 12-months post-flight in astronauts on shorter (< 6-months; - 0.4% median deficit) but not longer (- 3.9%) missions. Similar disparities were noted for total, trabecular, and cortical BMD. Altogether, nine of 17 astronauts did not fully recover tibia total BMD after 12-months. Astronauts with incomplete recovery had higher biomarkers of bone turnover compared with astronauts whose bone recovered. Study findings suggest incomplete recovery of bone strength, density, and trabecular microarchitecture at the weight-bearing tibia, commensurate with a decade or more of terrestrial age-related bone loss.

Bone metabolism during strict head-down tilt bed rest and exposure to elevated levels of ambient CO2.

Astronauts on the International Space Station are exposed to levels of atmospheric carbon dioxide (CO2) above typical terrestrial levels. We explored the possibility that increased levels of ambient CO2 further stimulate bone resorption during bed rest. We report here data from 2 ground-based spaceflight analog studies in which 12 male and 7 female subjects were placed in a strict 6° head-down tilt (HDT) position for either 30 days at 0.5% ambient CO2 or 60 days with nominal environmental exposure to CO2. Bone mineral density (BMD) and bone mineral content (BMC) were determined using dual-energy X-ray absorptiometry (DXA). Blood and urine were collected before and after HDT for biochemical analysis. No change was detected in either BMD or BMC, as expected given the study duration. Bone resorption markers increased after bed rest as expected; however, elevated CO2 had no additive effect. Elevated CO2 did not affect concentrations of minerals in serum and urine. Serum parathyroid hormone and 1,25-dihydroxyvitamin D were both reduced after bed rest, likely secondary to calcium efflux from bone. In summary, exposure to 0.5% CO2 for 30 days did not exacerbate the typical bone resorption response observed after HDT bed rest. Furthermore, results from these strict HDT studies were similar to data from previous bed rest studies, confirming that strict 30-60 days of HDT can be used to evaluate changes in bone metabolism. This is valuable in the continuing effort to develop and refine efficacious countermeasure protocols to mitigate bone loss during spaceflight in low-Earth orbit and beyond.

Bisphosphonate Use May Reduce the Risk of Urolithiasis in Astronauts on Long-Term Spaceflights.

Long-duration spaceflight is associated with an increased risk of urolithiasis, and the pain caused by urinary calculi could result in loss of human performance and mission objectives. The present study investigated the risk of urolithiasis in astronauts during 6 months on the International Space Station, and evaluated whether the suppression of bone resorption by the bisphosphonate, alendronate (ALN), can reduce the risk. A total of 17 astronauts were included into the analysis: exercise using the advanced resistive exercise device (ARED) plus weekly oral 70 mg alendronate (ARED+ALN group, n = 7) was compared to resistive exercise alone (ARED group, n = 10). Urine volume decreased in both groups during spaceflight but recovered after return. The ARED group showed increased urinary calcium excretion from the 15th to 30th day of spaceflight, whereas urinary calcium was slightly decreased in the ARED+ALN group. Urinary N-terminal telopeptide (NTX) and helical peptide (HP) of type I collagen, as bone resorption markers, were elevated in the ARED group during and until 0 days after spaceflight, while there was no elevation in these parameters in the ARED+ALN group. Urinary oxalate and uric acid excretion tended to be higher in the ARED group than in the ARED+ALN group during spaceflight. These results demonstrate that astronauts on long-duration spaceflights may be at high risk for the formation of urinary calcium oxalate and calcium phosphate stones through increased urinary excretion of oxalate and uric acid, from degraded type I collagen, as well as of calcium from enhanced bone resorption. Our findings suggest that increased bone resorption during spaceflight, as a risk factor for urinary calculus formation, could be effectively prevented by an inhibitor of bone resorption. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Human Health during Space Travel: State-of-the-Art Review.

The field of human space travel is in the midst of a dramatic revolution. Upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX, Virgin Galactic) have already started the process of preparing for long-distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s. With the emergence of space tourism, space travel has materialized as a potential new, exciting frontier of business, hospitality, medicine, and technology in the coming years. However, current evidence regarding human health in space is very limited, particularly pertaining to short-term and long-term space travel. This review synthesizes developments across the continuum of space health including prior studies and unpublished data from NASA related to each individual organ system, and medical screening prior to space travel. We categorized the extraterrestrial environment into exogenous (e.g., space radiation and microgravity) and endogenous processes (e.g., alteration of humans' natural circadian rhythm and mental health due to confinement, isolation, immobilization, and lack of social interaction) and their various effects on human health. The aim of this review is to explore the potential health challenges associated with space travel and how they may be overcome in order to enable new paradigms for space health, as well as the use of emerging Artificial Intelligence based (AI) technology to propel future space health research.

Weekly Bone Loading Exercise Effects on a Healthy Subjects Strength, Bone Density, and Bone Biomarkers.

BACKGROUND: Bone density loss affects astronauts in long-duration spaceflight. The OsteoStrong Company has shown increased hip (14.95%) and lumbar (16.6%) area bone mineral density (aBMD) after 6 mo of exercises with their loading devices. The devices were tested on one subject as a pilot study.CASE REPORT: The subject performed 15 min of osteogenic exercises weekly for 24 wk. Total and regional aBMD, BAP (bone formation biomarker), NTX (bone resorption biomarker), forces exerted on devices, and weekly maximum weights lifted were collected. The control data was the subjects own lifting records 1.5 yr prestudy. The subject increased forces exerted on the devices in the upper extremity (97%, 197 to 390 kg; 435 to 859 lb), lower extremity (43%, 767 to 1097 kg; 1690 to 2418 lb), and spinal compression (22%, 275 to 336 kg; 607 to 740 lb). The monthly strength gain rate increased for snatch (2.3 vs. 0.71 kg; 5 vs. 1.56 lb), clean and jerk (2.5 vs. 0.4 kg; 5.5 vs. 0.88 lb), back squat (3.74 vs. 0 kg; 8.25 vs. 0 lb), front squat (2.15 vs. 0.2 kg; 4.75 vs. 0.47 lb), and deadlift (3.97 vs. 1.09 kg; 8.75 vs. 2.4 lb). The BAP increased by 39% (10.4 to 14.5 4 ug L1) and NTX decreased by 41% (13.4 to 7 nmol L1 BME). aBMD increased in the head (6%), arms (4.3%), trunk (6.3%), ribs (3.8%), and pelvis (11%). There were no differences in body weight, legs, spine, and whole-body aBMD on the full-body dual-energy X-ray absorptiometry (DXA). There were no differences in lumbar, hip, and femoral neck aBMD on the regional DXA.DISCUSSION: The osteogenic loading apparatus used for 15 min weekly increased strength for the one individual in this preliminary study. Future studies on astronauts and other healthy populations are necessary.Tsung A, Jupiter D, Jaquish J, Sibonga J. Weekly bone loading exercise effects on a healthy subjects strength, bone density, and bone biomarkers. Aerosp Med Hum Perform. 2021;92(3):201206.

Relations Between Bone Quantity, Microarchitecture, and Collagen Cross-links on Mechanics Following In Vivo Irradiation in Mice.

Humans are exposed to ionizing radiation via spaceflight or cancer radiotherapy, and exposure from radiotherapy is known to increase risk of skeletal fractures. Although irradiation can reduce trabecular bone mass, alter trabecular microarchitecture, and increase collagen cross-linking, the relative contributions of these effects to any loss of mechanical integrity remain unclear. To provide insight, while addressing both the monotonic strength and cyclic-loading fatigue life, we conducted total-body, acute, gamma-irradiation experiments on skeletally mature (17-week-old) C57BL/6J male mice (n = 84). Mice were administered doses of either 0 Gy (sham), 1 Gy (motivated by cumulative exposures from a Mars mission), or 5 Gy (motivated by clinical therapy regimens) with retrieval of the lumbar vertebrae at either a short-term (11-day) or long-term (12-week) time point after exposure. Micro-computed tomography was used to assess trabecular and cortical quantity and architecture, biochemical composition assays were used to assess collagen quality, and mechanical testing was performed to evaluate vertebral compressive strength and fatigue life. At 11 days post-exposure, 5 Gy irradiation significantly reduced trabecular mass (p < 0.001), altered microarchitecture (eg, connectivity density p < 0.001), and increased collagen cross-links (p < 0.001). Despite these changes, vertebral strength (p = 0.745) and fatigue life (p = 0.332) remained unaltered. At 12 weeks after 5 Gy exposure, the trends in trabecular bone persisted; in addition, regardless of irradiation, cortical thickness (p < 0.01) and fatigue life (p < 0.01) decreased. These results demonstrate that the highly significant effects of 5 Gy total-body irradiation on the trabecular bone morphology and collagen cross-links did not translate into detectable effects on vertebral mechanics. The only mechanical deficits observed were associated with aging. Together, these vertebral results suggest that for spaceflight, irradiation alone will likely not alter failure properties, and for radiotherapy, more investigations that include post-exposure time as a positive control and testing of both failure modalities are needed to determine the cause of increased fracture risk. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research. This article has been contributed to by US Government employees and their work is in the public domain in the USA.

Selected discoveries from human research in space that are relevant to human health on Earth.

A substantial amount of life-sciences research has been performed in space since the beginning of human spaceflight. Investigations into bone loss, for example, are well known; other areas, such as neurovestibular function, were expected to be problematic even before humans ventured into space. Much of this research has been applied research, with a primary goal of maintaining the health and performance of astronauts in space, as opposed to research to obtain fundamental understanding or to translate to medical care on Earth. Some people-scientists and concerned citizens-have questioned the broader scientific value of this research, with the claim that the only reason to perform human research in space is to keep humans healthy in space. Here, we present examples that demonstrate that, although this research was focused on applied goals for spaceflight participants, the results of these studies are of fundamental scientific and biomedical importance. We will focus on results from bone physiology, cardiovascular and pulmonary systems, and neurovestibular studies. In these cases, findings from spaceflight research have provided a foundation for enhancing healthcare terrestrially and have increased our knowledge of basic physiological processes.

Quantitative computed tomography reveals the effects of race and sex on bone size and trabecular and cortical bone density.

To examine the effects of race and sex on bone density and geometry at specific sites within the proximal femur and lumbar spine, we used quantitative computed tomography to image 30 Caucasian American (CA) men, 25 African American (AA) men, 30 CA women, and 17 AA women aged 35-45 yr. Volumetric integral bone mineral density (BMD), trabecular BMD (tBMD), and cross sectional area were measured in the femoral neck, trochanter, total femur, and L1/L2 vertebrae. Volumetric cortical BMD (cBMD) was also measured in the femur regions of interest. Differences were ascertained using a multivariate regression model. Overall, AA subjects had denser bones than CA subjects, but there were no racial differences in bone size. Men had larger femoral necks but not larger vertebrae than women. The AA men had higher tBMD and cBMD in the femur than CA men, whereas AA women had higher femoral tBMD but not higher femoral cBMD than CA women. These data support the idea that higher hip fracture rates in women compared with men are associated with smaller bone size. Lower fracture rates in AA elderly compared with CA elderly are consistent with higher peak bone density, particularly in the trabecular compartment, and potentially lower rates of age-related bone loss rather than larger bone size.

Skeletal effects of long-duration head-down bed rest.

INTRODUCTION: Skeletal unloading during spaceflight causes regional loss of bone mineral density (BMD), primarily in the spine and lower body regions. This loss of skeletal mass could adversely affect crew health during and after spaceflight and jeopardize mission success. Bed rest has long been used as a spaceflight analog to study the effects of disuse on many body systems, including the skeleton. This study was undertaken by the NASA Flight Analogs Project (FAP) to collect control data for upcoming countermeasure studies. METHODS: There were 13 subjects who participated in 42, 44, 49, 52, 60, or 90 d of continuous, head-down bed rest. DXA scans (dual-energy X-ray absorptiometry) were obtained before and after bed rest to measure changes in BMD of the whole body, lumbar spine, hip, heel, and wrist; the 90-d subjects were also scanned at the 60-d time point. Follow-up DXA scans were performed after 6 mo and 12 mo of reambulation to assess BMD recovery. RESULTS: BMD changes were consistent with earlier bed rest and spaceflight studies, with statistically significant losses averaging 1% per month in the hip, pelvis, and heel. Recovery data were also consistent with data obtained after spaceflight. Bone biomarker data are described, and support the findings of previous studies. Specifically, the process of normal bone remodeling is uncoupled: increased bone resorption with no concomitant change in bone formation. CONCLUSION: The FAP appears to be a valid test bed for skeletal disuse studies, and should provide a useful research platform for evaluating countermeasures to spaceflight-induced bone loss.

Hip load capacity cut-points for Astronaut Skeletal Health NASA Finite Element Strength Task Group Recommendations.

Concerns raised at a 2010 Bone Summit held for National Aeronautics and Space Administration Johnson Space Center led experts in finite element (FE) modeling for hip fracture prediction to propose including hip load capacity in the standards for astronaut skeletal health. The current standards for bone are based upon areal bone mineral density (aBMD) measurements by dual X-ray absorptiometry (DXA) and an adaptation of aBMD cut-points for fragility fractures. Task Group members recommended (i) a minimum permissible outcome limit (POL) for post-mission hip bone load capacity, (ii) use of FE hip load capacity to further screen applicants to astronaut corps, (iii) a minimum pre-flight standard for a second long-duration mission, and (iv) a method for assessing which post-mission physical activities might increase an astronaut's risk for fracture after return. QCT-FE models of eight astronaut were analyzed using nonlinear single-limb stance (NLS) and posterolateral fall (NLF) loading configurations. QCT data from the Age Gene/Environment Susceptibility (AGES) Reykjavik cohort and the Rochester Epidemiology Project were analyzed using identical modeling procedures. The 75th percentile of NLS hip load capacity for fractured elderly males of the AGES cohort (9537N) was selected as a post-mission POL. The NLF model, in combination with a Probabilistic Risk Assessment tool, was used to assess the likelihood of exceeding the hip load capacity during post-flight activities. There was no recommendation to replace the current DXA-based standards. However, FE estimation of hip load capacity appeared more meaningful for younger, physically active astronauts and was recommended to supplement aBMD cut-points.

Microporous polysaccharide hemospheres do not inhibit bone healing compared to bone wax or microfibrillar collagen.

Topical hemostats may reduce bone bleeding but the presence of residual matter may inhibit bone growth. We compared a recently approved hemostat, microporous polysaccharide hemospheres (Arista AH, Medafor Inc, Minneapolis, Minnesota), with conventional hemostatic agents in a rabbit calvarial model. Standard defects were created and microfibrillar collagen (Avitene; C.R. Bard Inc, Murray Hill, New Jersey), bonewax, or microporous polysaccharide hemospheres was applied. Bone growth was evaluated after fluorescent labeling the mineralization front. At 7 weeks bonewax and microfibrillar collagen had reduced bone growth compared to control or microporous polysaccharide hemospheres. Microporous polysaccharide hemospheres and control animals also had reduced rabbit calvarial defects compared to bonewax or microfibrillar collagen.

Effects of parathyroid hormone (1-34) on tibia in an adult rat model for chronic alcohol abuse.

Chronic alcohol abuse is a risk factor for osteoporosis in men. Human recombinant parathyroid hormone (1-34) (PTH) therapy increases bone mass in patients with osteoporosis. The purpose of the present study was to determine whether PTH is effective in increasing bone formation and bone mass in a rat model for established osteopenia caused by chronic alcohol abuse. Eight-month-old male Sprague Dawley rats were fed the Lieber-DeCarli liquid diet in which 35% of the calories were derived from either maltose-dextran or ethanol. Measurements were performed 16 weeks later to establish the magnitude of bone changes in the rats fed alcohol. High dose PTH (80 microg/kg/day) was administered 5 days/week for 6 weeks to establish the differential efficacy of hormone therapy on bone formation in alcohol consuming and alcohol withdrawn rats. The effects of alcohol and PTH on cancellous and cortical bone mass, architecture and turnover were determined by densitometry and histomorphometry. Rats fed alcohol had reduced bone mineral contents and densities, cancellous and cortical bone areas and cancellous bone formation rates compared to pair-fed controls. Following the withdrawal of alcohol, indices of bone formation increased compared to baseline values. PTH treatment increased bone mineral content and density, bone formation rates, cortical bone area, cancellous bone area and trabecular number and thickness, but several indices of bone formation were reduced in the presence of continued alcohol consumption. These results suggest that alcohol consumption, in addition to inducing bone loss, may reduce the efficacy of PTH therapy to reverse osteoporosis.

Periosteal remodeling at the femoral neck in nonhuman primates.

UNLABELLED: Periosteal bone turnover is poorly understood. We documented intramembranous periosteal bone turnover in the femoral neck in intact nonhuman primates and an increase in osteoclast numbers at the periosteal surface in sex steroid-deficient animals. Our studies are the first to systematically document periosteal turnover at the femoral neck. INTRODUCTION: Bone size is an important determinant of bone strength, and cellular events at the periosteal surface could alter bone dimensions. We characterized periosteal cellular activity with dynamic histomorphometric studies of nonhuman primate femoral neck and shaft. MATERIALS AND METHODS: Femur specimens from 16 intact adult male and female nonhuman primates (Rhesus [Macaca mulatta, n = 9] and Japanese Macaque [Macaca fuscata, n = 7]) were analyzed. Animals were double-labeled with tetracycline, and necropsy was performed 2-7 days after the last dose. We characterized periosteal resorptive activity in an additional group of five intact and four castrate female animals. Multiple group comparisons in intact animals were performed by one-way ANOVA followed by a Fisher PLSD posthoc test. In gonadectomized animals, Fisher's exact test was used for dichotomous and Mann-Whitney U-test for continuous variables. RESULTS: Bone turnover in the periosteum of the femoral neck in intact animals was more rapid than at the femoral shaft but slower than in femoral neck cancellous bone. Similarly, in these intact animals, the eroded surface of cortical bone at the femoral neck periosteal surface was significantly greater than in the cancellous bone compartment (p < 0.0001) or on the femoral shaft (p < 0.0001). Gonadectomized female animals showed an increase in osteoclast number on the periosteal surface compared with intact controls (p < 0.01). CONCLUSIONS: We documented intramembranous periosteal bone turnover in the femoral neck by histomorphometric analyses. The tissue level bone formation rate was sufficient to add substantively to femoral neck size over time. Periosteal osteoclastic activity was not the result of the emergence of intracortical tunneling at the bone surface. Sex steroid deficiency produced an increase in osteoclast numbers at the periosteal surface. This is the first systematic documentation of periosteal turnover at the femoral neck.

Evidence that the cells responsible for marrow fibrosis in a rat model for hyperparathyroidism are preosteoblasts.

We examined proliferation of cells associated with PTH-induced peritrabecular bone marrow fibrosis in rats as well as the fate of those cells after withdrawal of PTH. Time-course studies established that severe fibrosis was present 7 d after initiation of a continuous sc PTH infusion (40 microg/kg.d). To ascertain cell proliferation, rats were coinfused for 1 wk with PTH (treated) or vehicle (control) and [3H]thymidine (1.5 mCi/rat). Groups of control and treated rats were killed immediately (d 0) and 1 wk (d 7) later. Few osteoblasts (Obs) and osteocytes in treated and control groups were radiolabeled on d 0. Peritrabecular cells expressing a fibroblastic (Fb) phenotype and surrounded by an extracellular matrix were not present in controls on either d 0 or d 7. Multiple cell layers of Fbs lined most (70%) of the bone surface on d 0 in treated rats and nearly all (85%) of the Fbs were radiolabeled. Fbs had entirely disappeared from bone surfaces on d 7. Eighty-five percent of the Obs on and 73% of the osteocytes within the active remodeling sites were radiolabeled. Immunohistochemistry revealed that Fbs induced by PTH treatment produced osteocalcin, osteonectin, and core binding factor-alpha1. These data provide compelling evidence that Fbs recruited to bone surfaces in response to a continuous PTH infusion undergo extensive proliferation, express osteoblast-specific proteins, and produce an extracellular matrix that is similar to osteoid. After restoration of normal PTH levels, Fbs differentiated to Obs, providing further evidence that Fbs are preosteoblasts.