Contribution of social isolation, restraint, and hindlimb unloading to changes in hemodynamic parameters and motion activity in rats

The most accepted animal model for simulation of the physiological and morphological consequences of microgravity on the cardiovascular system is one of head-down hindlimb unloading. Experimental conditions surrounding this model include not only head-down tilting of rats, but also social and restraint stresses that have their own influences on cardiovascular system function. Here, we studied levels of spontaneous locomotor activity, blood pressure, and heart rate during 14 days under the following experimental conditions: cage control, social isolation in standard rat housing, social isolation in special cages for hindlimb unloading, horizontal attachment (restraint), and head-down hindlimb unloading. General activity and hemodynamic parameters were continuously monitored in conscious rats by telemetry. Heart rate and blood pressure were both evaluated during treadmill running to reveal cardiovascular deconditioning development as a result of unloading. The main findings of our work are that: social isolation and restraint induced persistent physical inactivity, while unloading in rats resulted in initial inactivity followed by normalization and increased locomotion after one week. Moreover, 14 days of hindlimb unloading showed significant elevation of blood pressure and slight elevation of heart rate. Hemodynamic changes in isolated and restrained rats largely reproduced the trends observed during unloading. Finally, we detected no augmentation of tachycardia during moderate exercise in rats after 14 days of unloading. Thus, we concluded that both social isolation and restraint, as an integral part of the model conditions, contribute essentially to cardiovascular reactions during head-down hindlimb unloading, compared to the little changes in the hydrostatic gradient.     

Regulation of plasma vasopressin and renin activity in conscious hindlimb-unloaded rats

Cardiovascular deconditioning occurs in astronauts after spaceflight or in individuals subjected to bed rest. It is characterized by an increased incidence of orthostatic intolerance. The mechanisms responsible for orthostatic intolerance are likely multifactorial and may include hypovolemia, autonomic dysfunction, and vascular and cardiac alterations. The arterial baroreflex is an important compensatory mechanism in the response to an orthostatic stress. In a previous study, we demonstrated that arterial baroreflex mediated sympathoexcitation was blunted in hindlimb-unloaded (HU) rats, a model of cardiovascular deconditioning. The arterial baroreflex also contributes to the regulation of vasoactive hormones including vasopressin and angiotensin II. In the present study, we tested the hypothesis that the neurohumoral response to hypotension is also attenuated in rats after 14 days of hindlimb unloading. To test this hypothesis, the vasodilator diazoxide (15 or 25 mg/kg) or saline (0.9%) was administered to produce hypotension or control conditions, respectively, in conscious HU and control rats. Plasma samples were collected and assayed for vasopressin and plasma renin activity (PRA). Diazoxide (25 mg/kg) produced significant increases in vasopressin and PRA compared with saline controls. HU rats exhibited significantly higher levels of vasopressin at rest and the increase in vasopressin levels during hypotension was enhanced by hindlimb unloading. Neither resting nor hypotension-induced PRA was altered by hindlimb unloading. These data suggest that although baroreflex-mediated sympathoexcitation is blunted by hindlimb unloading, hypotension-induced vasopressin release is enhanced and hypotension-induced PRA is unaffected. Increased circulating vasopressin may serve to compensate for blunted baroreflex regulation of sympathetic nervous activity produced by hindlimb unloading or may actually contribute to it.     

The current state of bone loss research: Data from spaceflight and microgravity simulators

Bone loss is a well documented phenomenon occurring in humans both in short‐ and in long‐term spaceflights. This phenomenon can be also reproduced on the ground in human and animals and also modeled in cell‐based analogs. Since space flights are infrequent and expensive to study the biomedical effects of microgravity on the human body, much of the known pathology of bone loss comes from experimental studies. The most commonly used in vitro simulators of microgravity are clinostats while in vivo simulators include the bed rest studies in humans and hindlimb unloading experiments in animals. Despite the numerous reports that have documented bone loss in wide ranges in multiple crew members, the pathology remains a key concern and development of effective countermeasures is still a major task. Thus far, the offered modalities have not shown much success in preventing or alleviating bone loss in astronauts and cosmonauts. The objective of this review is to capture the most recent research on bone loss from spaceflights, bed rest and hindlimb unloading, and in vitro studies utilizing cellular models in clinostats. Additionally, this review offers projections on where the research has to focus to ensure the most rapid development of effective countermeasures. J. Cell. Biochem. 114: 1001–1008, 2013. © 2012 Wiley Periodicals, Inc.     

A brief review of bone adaptation to unloading

Weight-bearing bone is constantly adapting its structure and function to mechanical environments. Loading through routine exercises stimulates bone formation and prevents bone loss, but unloading through bed rest and cast immobilization as well as exposure to weightlessness during spaceflight reduces its mass and strength. In order to elucidate the mechanism underlying unloading-driven bone adaptation, ground-based in vitro and in vivo analyses have been conducted using rotating cell culturing and hindlimb suspension. Focusing on gene expression studies in osteoblasts and hindlimb suspension studies, this minireview introduces our recent understanding on bone homeostasis under weightlessness in space. Most of the existing data indicate that unloading has the opposite effects to loading through common signaling pathways. However, a question remains as to whether any pathway unique to unloading （and not to loading） may exist.     

Early Deсline in Rat Soleus Passive Tension with Hindlimb Unloading: Inactivation of Cross-bridges or Activation of Calpains?

The study was aimed at testing the hypotheses about the role of cross-bridges and calpains in reduction of rat soleus passive tension under conditions of hindlimb unloading. For this purpose, we used an inhibitor of μ-calpain PD 150606 as well as a blocker of actomyosin interaction (blebbistatin). It was found for the first time that a decrease in passive tension of rat soleus after 3-day hindlimb unloading is associated with the activity of μ-calpain and does not depend on the processes of cross-bridges formation.     

Atrophy of the soleus muscle by hindlimb unweighting

The unweighting model is a unique whole animal model that will permit the future delineation of the mechanism(s) by which gravity maintains contractile mass in postural (slow-twitch) skeletal muscle. Since the origination of the model of rodent hindlimb unweighting almost one decade ago, about half of the 59 refereed articles in which this model was utilized have been published in the Journal of Applied Physiology. Thus the purpose of this review is to provide, for those researchers with an interest in the hindlimb unweighting model, a summation of the data derived from this model to data and hopefully to stimulate research interest in aspects of the model for which data are lacking. The stress response of the animal to hindlimb unweighting is transient, minimal in magnitude, and somewhat variable. After 1 wk of unweighting, the animal exhibits no chronic signs of stress. The atrophy of the soleus muscle, a predominantly slow-twitch muscle, is emphasized because unweighting preferentially affects it compared with other calf muscles, which are mainly fast-twitch muscles. The review considers the following information about the unweighted soleus muscle: electromyogram activity, amount and type of protein lost, capillarization, oxidative capacity, glycolytic enzyme activities, fiber cross section, contractile properties, glucose uptake, sensitivity to insulin, protein synthesis and degradation rates, glucocorticoid receptor numbers, responses of specific mRNAs, and changes in metabolite concentrations.     

Hindlimb unloading increases oxidative stress and disrupts antioxidant capacity in skeletal muscle

Skeletal muscle disuse with space-flight and ground-based models (e.g., hindlimb unloading) results in dramatic skeletal muscle atrophy and weakness. Pathological conditions that cause muscle wasting (i.e., heart failure, muscular dystrophy, sepsis, COPD, cancer) are characterized by elevated "oxidative stress," where antioxidant defenses are overwhelmed by oxidant production. However, the existence, cellular mechanisms, and ramifications of oxidative stress in skeletal muscle subjected to hindlimb unloading are poorly understood. Thus we examined the effects of hindlimb unloading on hindlimb muscle antioxidant enzymes (e.g., superoxide dismutase, catalase, glutathione peroxidase), nonenzymatic antioxidant scavenging capacity (ASC), total hydroperoxides, and dichlorohydrofluorescein diacetate (DCFH-DA) oxidation, a direct indicator of oxidative stress. Twelve 6 month old Sprague Dawley rats were divided into two groups: 28 d of hindlimb unloading (n = 6) and controls (n = 6). Hindlimb unloading resulted in a small decrease in Mn-superoxide dismutase activity (10.1%) in the soleus muscle, while Cu,Zn-superoxide dismutase increased 71.2%. In contrast, catalase and glutathione peroxidase, antioxidant enzymes that remove hydroperoxides, were significantly reduced in the soleus with hindlimb unloading by 54.5 and 16.1%, respectively. Hindlimb unloading also significantly reduced ASC. Hindlimb unloading increased soleus lipid hydroperoxide levels by 21.6% and hindlimb muscle DCFH-DA oxidation by 162.1%. These results indicate that hindlimb unloading results in a disruption of antioxidant status, elevation of hydroperoxides, and an increase in oxidative stress.     

Inhibition of Xanthine Oxidase by Allopurinol Prevents Skeletal Muscle Atrophy: Role of p38 MAPKinase and E3 Ubiquitin Ligases

Alterations in muscle play an important role in common diseases and conditions. Reactive oxygen species (ROS) are generated during hindlimb unloading due, at least in part, to the activation of xanthine oxidase (XO). The major aim of this study was to determine the mechanism by which XO activation causes unloading-induced muscle atrophy in rats, and its possible prevention by allopurinol, a well-known inhibitor of this enzyme. For this purpose we studied one of the main redox sensitive signalling cascades involved in skeletal muscle atrophy i.e. p38 MAPKinase, and the expression of two well known muscle specific E3 ubiquitin ligases involved in proteolysis, the Muscle atrophy F-Box (MAFbx; also known as atrogin-1) and Muscle RING (Really Interesting New Gene) Finger-1 (MuRF-1). We found that hindlimb unloading induced a significant increase in XO activity and in the protein expression of the antioxidant enzymes CuZnSOD and Catalase in skeletal muscle. The most relevant new fact reported in this paper is that inhibition of XO with allopurinol, a drug widely used in clinical practice, prevents soleus muscle atrophy by ∼20% after hindlimb unloading. This was associated with the inhibition of the p38 MAPK-MAFbx pathway. Our data suggest that XO was involved in the loss of muscle mass via the activation of the p38MAPK-MAFbx pathway in unloaded muscle atrophy. Thus, allopurinol may have clinical benefits to combat skeletal muscle atrophy in bedridden, astronauts, sarcopenic, and cachexic patients.     

ERK is involved in the reorganization of somatosensory cortical maps in adult rats submitted to hindlimb unloading

Sensorimotor restriction by a 14-day period of hindlimb unloading (HU) in the adult rat induces a reorganization of topographic maps and receptive fields. However, the underlying mechanisms are still unclear. Interest was turned towards a possible implication of intracellular MAPK signaling pathway since Extracellular-signal-Regulated Kinase 1/2 (ERK1/2) is known to play a significant role in the control of synaptic plasticity. In order to better understand the mechanisms underlying cortical plasticity in adult rats submitted to a sensorimotor restriction, we analyzed the time-course of ERK1/2 activation by immunoblot and of cortical reorganization by electrophysiological recordings, on rats submitted to hindlimb unloading over four weeks. Immunohistochemistry analysis provided evidence that ERK1/2 phosphorylation was increased in layer III neurons of the somatosensory cortex. This increase was transient, and parallel to the changes in hindpaw cortical map area (layer IV). By contrast, receptive fields were progressively enlarged from 7 to 28 days of hindlimb unloading. To determine whether ERK1/2 was involved in cortical remapping, we administered a specific ERK1/2 inhibitor (PD-98059) through osmotic mini-pump in rats hindlimb unloaded for 14 days. Results demonstrate that focal inhibition of ERK1/2 pathway prevents cortical reorganization, but had no effect on receptive fields. These results suggest that ERK1/2 plays a role in the induction of cortical plasticity during hindlimb unloading.     

Alternative Splicing of Titin mRNA in Rat Soleus after Seven-Day Gravitational Unloading

Human Physiology - Changes in titin alternative splicing in the rat soleus after seven-day gravitational unloading (the hindlimb unloading model) were studied by long-fragment PCR and nanopore...     

O-GlcNAc level variations are associated with the development of skeletal muscle atrophy.

O-linked N-acetylglucosaminylation (O-GlcNAc) is a regulatory posttranslational modification of nucleocytoplasmic proteins, which consists of the attachment of N-acetylglucosamine to serine or threonine residues of a protein. This glycosylation is a ubiquitous posttranslational modification, which probably plays important roles in many aspects of protein function. Our laboratory has previously reported that, in skeletal muscle, proteins of the glycolytic pathway and energetic metabolism and contractile proteins were O-GlcNAc modified (Cieniewski-Bernard C, Bastide B, Lefebvre T, Lemoine J, Mounier Y, and Michalski JC. Mol Cell Proteomics 3: 577-585, 2004). O-GlcNAc has been recently demonstrated to play a role in modulating cellular function in response to nutrition and also in stress conditions. Therefore, we have investigated here the implication of the glycosylation/deglycosylation process in the development of atrophy in rat skeletal muscle after hindlimb unloading. The high O-GlcNAc level found in control soleus [compared with control extensor digitorum longus (EDL)] becomes lower in atrophied soleus. On the opposite side, the low rate of O-GlcNAc in control EDL reaches higher levels in EDL, not atrophied after hindlimb unloading. These variations in O-GlcNAc level are correlated with a variation of the O-GlcNAc process enzyme activities and could be associated with a differential expression of heat shock proteins. Our results suggest that O-GlcNAc variations could control the muscle protein homeostasis and be implicated in the regulation of muscular atrophy.     

Hindlimb unloading alters nitric oxide and autonomic control of resting arterial pressure in conscious rats

After periods of microgravity or bed rest, individuals often exhibit reduced Vo(2 max), hypovolemia, cardiac and vascular effects, and autonomic dysfunction. Recently, alterations in expression of vascular and central nervous system NO synthase (NOS) have been observed in hindlimb-unloaded (HU) rats, a model used to simulate physiological effects of microgravity or bed rest. We examined the effects of 14 days of hindlimb unloading on hemodynamic responses to systemic NOS inhibition in conscious control and HU rats. Because differences in NO and autonomic regulation might occur after hindlimb unloading, we also evaluated potential differences in resting autonomic tone and effects of NOS inhibition after autonomic blockade. Administration of nitro-L-arginine methyl ester (L-NAME; 20 mg/kg iv) increased mean arterial pressure (MAP) to similar levels in control and HU rats. However, the change in MAP in response to L-NAME was less in HU rats, that had an elevated baseline MAP. In separate experiments, atropine (1 mg/kg iv) increased heart rate (HR) in control but not HU rats. Subsequent administration of the ganglionic blocker hexamethonium (30 mg/kg iv) decreased MAP and HR to a greater extent in HU rats. Administration of L-NAME after autonomic blockade increased MAP in both groups to a greater extent compared with intact conditions. However, the pressor response to L-NAME was still reduced in HU rats. These data suggest that hindlimb unloading in rats reduces peripheral NO as well as cardiac parasympathetic tone. Along with elevations in sympathetic tone, these effects likely contribute to alterations in vascular control and changes in autonomic reflex function following spaceflight or bed rest.     

Muscle-specific up-regulation of rat UCP3 mRNA expression by long-term hindlimb unloading

The effect of long-term hindlimb unloading (2 or 5 week) on the expression of uncoupling protein-3 (UCP3) gene was investigated in rat skeletal muscles. The interaction of hindlimb unloading and thyroid status was also investigated at 2 weeks. Whatever the duration, mechanical unloading induced a similar increase in UCP3 mRNA relative abundance in the slow-twitch soleus (SOL) muscle (+80%, P < 0.05), whereas no effect was observed in the fast-twitch extensor digitorum longus (EDL) muscle. Hypothyroidism down-regulated while hyperthyroidism up-regulated UCP3 mRNA relative abundance in both SOL and EDL muscles, but thyroid status did not prevent the up-regulation of UCP3 induced by 2 weeks of suspension. These data therefore indicate for the first time that long-term hindlimb unloading up-regulates muscle UCP3 gene expression in a muscle-specific manner which is independent of thyroid status.     

改良的大鼠模拟失重模型制备方法*

目的: 介绍一种改良的尾部悬吊使后肢去负荷的制备大鼠模拟失重模型的方法。方法: 90只成年雄性SD大鼠随机分为对照组,经典尾吊组和改良尾吊组(每组30只)。经典尾吊组利用医用胶带和纱布制作大鼠尾套后悬吊大鼠尾部。改良尾吊组在上述操作基础之上,在尾套内增加聚乙烯发泡棉隔层,以缓冲纱布对尾部的挤压,保证远端血液循环。对照组尾部不做特殊处理。尾吊4周后观察尾部损伤和尾套脱落等并发症,测量大鼠体质量及右侧比目鱼肌湿重。结果: 与对照组相比,经典尾吊组的比目鱼肌湿重/体质量比值显著减少,但体质量无显著差异,大鼠尾部远端出现缺血坏死损伤的发生率为40.0%,尾套脱落的发生率为26.7%,模型成功率为33.3%;与经典尾吊组相比,改良尾吊组的尾部损伤程度明显降低,远端缺血坏死率为13.3%, 尾套脱落率为3.3%,模型成功率为83.3% (P均＜0.05)。结论: 采用改良尾吊法建立大鼠模拟失重模型能够显著减少鼠尾坏死和尾套脱落发生率,简单易行,提高了模型制备的成功率。     

Mechanical unloading impairs keratinocyte migration and angiogenesis during cutaneous wound healing.

Although initially thought to improve an individual's ability to heal, mechanical unloading promoted by extended periods of bed rest has emerged as a contributing factor to delayed or aberrant tissue repair. Using a rat hindlimb unloading (HLU) model of hypogravity, we mimicked some aspects of physical inactivity by removing weight-bearing loads from the hindlimbs and producing a systemic cephalic fluid shift. This model simulates bed rest in that the animal undergoes physiological adaptations, resulting in a reduction in exercise capability, increased frequency of orthostatic intolerance, and a reduction in plasma volume. To investigate whether changes associated with prior prolonged bed rest correlate with impaired cutaneous wound healing, we examined wound closure, angiogenesis, and collagen content in day 2 to day 21 wounds from rats exposed to HLU 2 wk before excisional wounding. Wound closure was delayed in day 2 wounds from HLU rats compared with ambulatory controls. Although the levels of proangiogenic growth factors, fibroblast growth factor-2 (FGF-2), and vascular endothelial growth factor (VEGF) were similar between the two groups, wound vascularity was significantly reduced in day 7 wounds from HLU animals. To further examine this disparity, total collagen content was assessed but found to be similar between the two groups. Taken together, these results suggest that keratinocyte and endothelial cell function may be impaired during the wound healing process under periods of prolonged inactivity or bed rest.     

Effects of disrupted beta1-integrin function on the skeletal response to short-term hindlimb unloading in mice.

The study was designed to determine whether beta1-integrin plays a role in mediating the acute skeletal response to mechanical unloading. Transgenic (TG) mice were generated to express a dominant negative form of beta1-integrin under the control of the osteocalcin promoter, which targets expression of the transgene to mature osteoblasts. At 63 days of age, wild-type (WT) and TG mice were subjected to hindlimb unloading by tail suspension for 1 wk. Pair-fed, normally loaded WT and TG mice served as age-matched controls. Bone samples from each mouse were processed for quantitative bone histomorphometry and biomechanical testing. The skeletal phenotype of TG mice was characterized by lower cancellous bone mass in the distal femoral metaphysis (-52%) and lumbar vertebral body (-20%), reduced curvature of the proximal tibia (-20%), and decreased bone strength (-20%) and stiffness (-23%) of the femoral diaphysis with relatively normal indexes of cancellous bone turnover. Hindlimb unloading for only 1 wk induced a 10% decline in tibial curvature and a 30% loss of cancellous bone in the distal femur due to a combination of increased bone resorption and decreased bone formation in both WT and TG mice. However, the strength and stiffness of the femoral diaphysis were unaffected by short-term hindlimb unloading in both genotypes. The observed increase in osteoclast surface was greater in unloaded TG mice (92%) than in unloaded WT mice (52%). Cancellous bone formation rate was decreased in unloaded WT (-29%) and TG (-15%) mice, but, in contrast to osteoclast surface, the genotype by loading interaction was not statistically significant. The results indicate that altered integrin function in mature osteoblasts may enhance the osteoclastic response to mechanical unloading but that it does not have a major effect on the development of cancellous osteopenia in mice during the early stages of hindlimb unloading.     

Effects of 5 weeks of lower limb suspension on muscle size and strength

Lack of weight-bearing, as occurs in space, appears to be associated with reductions in strength and mass of skeletal muscle. Very limited data, however, is at hand describing changes in skeletal muscle size and function following manned space missions. Our current knowledge therefore is mainly based on studies of space flown rats. It is obvious though that this information, only in part can be extrapolated to humans. A few bed rest studies have demonstrated that decreases in strength and muscle size are substantial. At this time, however, the magnitude or time course of such changes either in response to space flight or simulations of microgravity have not been defined. In the last few years we have employed a human model to simulate unloading of lower limb skeletal muscles that occurs in microgravity. This model was essentially adopted from the rat hindlimb suspension technique. The purpose of this study was to assess the magnitude of decreases in muscle strength and size as a result of five weeks of unilateral lower limb suspension.     

Age effects on rat hindlimb muscle atrophy during suspension unloading.

Disuse can induce numerous adaptive alterations in skeletal muscle. In the present study the effects of hindlimb unloading on muscle mass and biochemical responses were examined and compared in adult (450 g) and juvenile (200 g) rats after 1, 7, or 14 days of whole body suspension. Quantitatively and qualitatively the soleus (S), gastrocnemius (G), plantaris (P), and extensor digitorum longus (EDL) muscles of the hindlimb exhibited a differential sensitivity to suspension and weightlessness unloading in both adults and juveniles. The red slow-twitch soleus exhibited the most pronounced atrophy under both conditions, with juvenile responses being greater than adult. In contrast, the fast-twitch EDL hypertrophied during suspension and atrophied during weightlessness, with no significant difference between adults and juveniles. Determination of biochemical parameters (total protein, RNA, and DNA) indicated a less rapid rate of response in adult muscles. This was corroborated by assessment of muscle alpha-actin mRNA levels, which indicated a rapid (within 1 day) and significant (P less than 0.05) effect in juveniles but not in adults. The results of this investigation indicate 1) a qualitatively similar differential effect of unloading on muscles of adults and juveniles, 2) a quantitatively reduced and less rapid effect of suspension on adult muscles, and 3) a close similarity of adult and juvenile muscle responses during suspension and spaceflight, suggesting that this ground-based model simulates many of the unloading effects of weightlessness.     

Role of L-type Ca channels in Ca2+ accumulation and changes in distribution of myosin heavy chain and SERCA isoforms in rat M. soleus under gravitational unloading

It is known that hindlimb unloading brings about the intracellular Ca2+ accumulation and MyHC slow-to-fast shift in m.soleus. SERCA (sarcoendoplasmatic reticulum Ca ATPase) function as a Ca pump to uptake to sarcoendoplasmatic reticulum after skeletal muscle contraction, and can modulate intracellular resting Ca level. The study was aimed at investigation of the role of intracellular Ca2+ level for MyHC and SERCA isoforms transformation in m.soleus under hindlimb unloading. To determine role of intracellular Ca we administrated nifedipin--specific blocker of L-type calcium channel in myofibers. We hypothesized that decrease of intracellular calcium level prevented-NFATc1 nuclear translocation and MyHC slow-to-fast transformation. 42 male Wistar rats (180-200 g) were divided in 3 groups: cage control (C, n = 14), 14 days HU (HU, n = 14), 14 days HU with 7 mg/kg/day of nifedipin administration with water (HUN, n = 14). The study has shown that increase of intracellular Ca2+ level under HU leads to MHC slow-to-fast shift via activation of calcineurin-NFATc1 signaling pathway. Percentage of muscle fibers with SERCA I increased under hindlimb unloading, being dependent of intracellular calcium level, percentage of muscle fibers with SERCA II decreased under hindlimb unloading but did not depend on calcium. We suppose that nifedipin administration decreases intracellular Ca level, prevents MHC slow-to-fast shift via prevention of NFATcl accumulation in nuclear extract of m.soleus, and prevent increase of SERCAI expression. The work was supported by grants RFBR N05-04-49255a, 04-04-49044, 05-04-08200-ofi-a, contract with Federal Agency for Science and Iinnovation N02.467.11.3005, and Presidium of RAS program "Basic sciences for medicine".     

Preventive effect of nucleoprotein on hindlimb unloading-induced capillary regression in rat soleus muscle

We investigated the preventive effects of nucleoprotein on capillary regression and mitochondrial dysfunction induced by unloading in the soleus muscle of rats. Nucleoprotein is a supplement made from soft roe of salmon, and its major components are nucleotides and protamine. Adult male Sprague-Dawley rats were divided randomly into control, hindlimb unloading (HU), and hindlimb unloading plus nucleoprotein administration (HU+ NP) groups. Hindlimb unloading was carried out for 2 weeks in the rats belonging to the HU and the HU+ NP groups. The rats of the HU+ NP group were administered nucleoprotein (500 mg/kg) using a feeding needle twice a day for 2 weeks. Hindlimb unloading resulted in capillary regression, decreased succinate dehydrogenase activity of the muscle fiber, and decreased PGC-1α expression in the soleus muscle. These effects were prevented by administration of nucleoprotein. Nucleoprotein appears to prevent capillary regression and mitochondrial dysfunction caused by unloading of the skeletal muscle. Therefore, nucleoprotein supplementation may be an effective therapy for maintaining capillary network and mitochondrial metabolism of the muscle fiber during an unloading period.