Cells respond to space microgravity through cytoskeleton reorganization.

Decades of spaceflight studies have provided abundant evidence that individual cells in vitro are capable of sensing space microgravity and responding with cellular changes both structurally and functionally. However, how microgravity is perceived, transmitted, and converted to biochemical signals by single cells remains unrevealed. Here in this review, over 40 cellular biology studies of real space fights were summarized. Studies on cells of the musculoskeletal system, cardiovascular system, and immune system were covered. Among all the reported cellular changes in response to space microgravity, cytoskeleton (CSK) reorganization emerges as a key indicator. Based on the evidence of CSK reorganization from space flight research, a possible mechanism from the standpoint of "cellular mechanical equilibrium" is proposed for the explanation of cellular response to space microgravity. Cytoskeletal equilibrium is broken by the gravitational change from ground to space and is followed by cellular morphological changes, cell mechanical properties changes, extracellular matrix reorganization, as well as signaling pathway activation/inactivation, all of which ultimately lead to the cell functional changes in space microgravity.

Longitudinal metabolomic profiles reveal sex-specific adjustments to long-duration spaceflight and return to Earth.

Spaceflight entails a variety of environmental and psychological stressors that may have long-term physiological and genomic consequences. Metabolomics, an approach that investigates the terminal metabolic outputs of complex physiological alterations, considers the dynamic state of the human body and allows the identification and quantification of down-stream metabolites linked to up-stream physiological and genomic regulation by stress. Employing a metabolomics-based approach, this study investigated longitudinal metabolic perturbations of male (n = 40) and female (n = 11) astronauts on 4-6-month missions to the International Space Station (ISS). Proton nuclear magnetic resonance (1H-NMR) spectroscopy followed by univariate, multivariate and machine learning analyses were used on blood serum to examine sex-specific metabolic changes at various time points throughout the astronauts' missions, and the metabolic effects of long-duration space travel. Space travel resulted in sex-specific changes in energy metabolism, bone mineral and muscle regulation, immunity, as well as macromolecule maintenance and synthesis. Additionally, metabolic signatures suggest differential metabolic responses-especially during the recovery period-with females requiring more time to adjust to return to Earth. These findings provide insight into the perturbations in glucose and amino acid metabolism and macromolecule biosynthesis that result from the stressors of long-duration spaceflight. Metabolomic biomarkers may provide a viable approach to predicting and diagnosing health risks associated with prolonged space travel and other physiological challenges on Earth.

Effects of short-term -30° HDT on contrast sensitivity.

Potential cognitive and physiological alterations due to space environments have been investigated in long-term space flight and various microgravity-like conditions, for example, head-down tilt (HDT), confinement, isolation, and immobilization. However, little is known about the influence of simulated microgravity environments on visual function. Contrast sensitivity (CS), which indicates how much contrast a person requires to see a target, is a fundamental feature of human vision. Here, we investigated how the CS changed by 1-h -30° HDT and determined the corresponding mechanisms with a perceptual template model. A quick contrast sensitivity function procedure was used to assess the CS at ten spatial frequencies and three external noise levels. We found that (1) relative to the + 30° head-up tilt (HUT) position, 1-h -30° HDT significantly deteriorated the CS at intermediate frequencies when external noise was present; (2) CS loss was not detected in zero- or high-noise conditions; (3) HDT-induced CS loss was characterized by impaired perceptual template; and (4) self-reported questionnaires indicated that subjects felt less pleasure and more excitement, less comfort and more fatigued by screen light, less comfort in the area around the eye, and serious symptoms such as piercing pain, blur acid, strain, eye burning, and dizziness after HDT. These findings improve our understanding of the negative effects of simulated microgravity on visual function and elucidate the potential risks of astronauts during space flight.

Wearable Sensing System for NonInvasive Monitoring of Intracranial BioFluid Shifts in Aerospace Applications.

The alteration of the hydrostatic pressure gradient in the human body has been associated with changes in human physiology, including abnormal blood flow, syncope, and visual impairment. The focus of this study was to evaluate changes in the resonant frequency of a wearable electromagnetic resonant skin patch sensor during simulated physiological changes observed in aerospace applications. Simulated microgravity was induced in eight healthy human participants (n = 8), and the implementation of lower body negative pressure (LBNP) countermeasures was induced in four healthy human participants (n = 4). The average shift in resonant frequency was -13.76 ± 6.49 MHz for simulated microgravity with a shift in intracranial pressure (ICP) of 9.53 ± 1.32 mmHg, and a shift of 8.80 ± 5.2097 MHz for LBNP with a shift in ICP of approximately -5.83 ± 2.76 mmHg. The constructed regression model to explain the variance in shifts in ICP using the shifts in resonant frequency (R2 = 0.97) resulted in a root mean square error of 1.24. This work demonstrates a strong correlation between sensor signal response and shifts in ICP. Furthermore, this study establishes a foundation for future work integrating wearable sensors with alert systems and countermeasure recommendations for pilots and astronauts.

Cerebrovascular Effects of Lower Body Negative Pressure at 3T MRI: Implications for Long-Duration Space Travel.

BACKGROUND: Optic disc edema develops in most astronauts during long-duration spaceflight. It is hypothesized to result from weightlessness-induced venous congestion of the head and neck and is an unresolved health risk of space travel. PURPOSE: Determine if short-term application of lower body negative pressure (LBNP) could reduce internal jugular vein (IJV) expansion associated with the supine posture without negatively impacting cerebral perfusion or causing IJV flow stasis. STUDY TYPE: Prospective. SUBJECTS: Nine healthy volunteers (six women). FIELD STRENGTH/SEQUENCE: 3T/cine two-dimensional phase-contrast gradient echo; pseudo-continuous arterial spin labeling single-shot gradient echo echo-planar. ASSESSMENT: The study was performed with two sequential conditions in randomized order: supine posture and supine posture with 25 mmHg LBNP (LBNP25 ). LBNP was achieved by enclosing the lower extremities in a semi-airtight acrylic chamber connected to a vacuum. Heart rate, bulk cerebrovasculature flow, IJV cross-sectional area, fractional IJV outflow relative to arterial inflow, and cerebral perfusion were assessed in each condition. STATISTICAL TESTS: Paired t-tests were used to compare measurement means across conditions. Significance was defined as P < 0.05. RESULTS: LBNP25 significantly increased heart rate from 64 ± 9 to 71 ± 8 beats per minute and significantly decreased IJV cross-sectional area, IJV outflow fraction, cerebral arterial flow rate, and cerebral arterial stroke volume from 1.28 ± 0.64 to 0.56 ± 0.31 cm2 , 0.75 ± 0.20 to 0.66 ± 0.28, 780 ± 154 to 708 ± 137 mL/min and 12.2 ± 2.8 to 9.7 ± 1.7 mL/cycle, respectively. During LBNP25 , there was no significant change in gray or white matter cerebral perfusion (P = 0.26 and P = 0.24 respectively) and IJV absolute mean peak flow velocity remained ≥4 cm/sec in all subjects. DATA CONCLUSION: Short-term application of LBNP25 reduced IJV expansion without decreasing cerebral perfusion or inducing IJV flow stasis. LEVEL OF EVIDENCE: 1 TECHNICAL EFFICACY STAGE: 1.

Global transcriptomic analysis of a murine osteocytic cell line subjected to spaceflight.

Bone loss is a major health concern for astronauts during long-term spaceflight and for patients during prolonged bed rest or paralysis. Growing evidence suggests that osteocytes, the most abundant cells in the mineralized bone matrix, play a key role in sensing mechanical forces applied to the skeleton and integrating the orchestrated response into subcellular biochemical signals to modulate bone homeostasis. However, the precise molecular mechanisms underlying both mechanosensation and mechanotransduction in late-osteoblast-to-osteocyte cells under microgravity (µG) have yet to be elucidated. To unravel the mechanisms by which late osteoblasts and osteocytes sense and respond to mechanical unloading, we exposed the osteocytic cell line, Ocy454, to 2, 4, or 6 days of µG on the SpaceX Dragon-6 resupply mission to the International Space Station. Our results showed that µG impairs the differentiation of osteocytes, consistent with prior osteoblast spaceflight experiments, which resulted in the downregulation of key osteocytic genes. Importantly, we demonstrate the modulation of critical glycolysis pathways in osteocytes subjected to microgravity and discovered a set of mechanical sensitive genes that are consistently regulated in multiple cell types exposed to microgravity suggesting a common, yet to be fully elucidated, genome-wide response to microgravity. Ground-based simulated microgravity experiments utilizing the NASA rotating-wall-vessel were unable to adequately replicate the changes in microgravity exposure highlighting the importance of spaceflight missions to understand the unique environmental stress that microgravity presents to diverse cell types. In summary, our findings demonstrate that osteocytes respond to µG with an increase in glucose metabolism and oxygen consumption.

Hepatology in space: Effects of spaceflight and simulated microgravity on the liver.

Microgravity as experienced during spaceflight affects a number of physiological processes in various organs. However, effects on the liver have yet been poorly documented. Nevertheless, the liver is a metabolically highly active organ involved in carbohydrate metabolism, lipid metabolism and xenobiotic biotransformation. The present paper provides an overview of the effects of microgravity on the liver observed in experimental animals during actual spaceflight and upon simulation of microgravity on Earth. These include (i) induction of liver injury and inflammation associated with apoptosis and oxidative stress, (ii) changes in liver carbohydrate metabolism resulting in the onset of a diabetogenic phenotype, (iii) modifications in hepatic lipid metabolism leading to early non-alcoholic fatty liver disease and (iv) alterations of the hepatic xenobiotic biotransformation machinery. Although most of these observations remain to be fully validated in humans, appropriate measures to counteract liver pathogenesis should be considered, especially in view of long-term space missions.

Effect of space flight on the behavior of human retinal pigment epithelial ARPE-19 cells and evaluation of coenzyme Q10 treatment.

Astronauts on board the International Space Station (ISS) are exposed to the damaging effects of microgravity and cosmic radiation. One of the most critical and sensitive districts of an organism is the eye, particularly the retina, and > 50% of astronauts develop a complex of alterations designated as spaceflight-associated neuro-ocular syndrome. However, the pathogenesis of this condition is not clearly understood. In the current study, we aimed to explore the cellular and molecular effects induced in the human retinal pigment ARPE-19 cell line by their transfer to and 3-day stay on board the ISS in the context of an experiment funded by the Agenzia Spaziale Italiana. Treatment of cells on board the ISS with the well-known bioenergetic, antioxidant, and antiapoptotic coenzyme Q10 was also evaluated. In the ground control experiment, the cells were exposed to the same conditions as on the ISS, with the exception of microgravity and radiation. The transfer of ARPE-19 retinal cells to the ISS and their living on board for 3 days did not affect cell viability or apoptosis but induced cytoskeleton remodeling consisting of vimentin redistribution from the cellular boundaries to the perinuclear area, underlining the collapse of the network of intermediate vimentin filaments under unloading conditions. The morphological changes endured by ARPE-19 cells grown on board the ISS were associated with changes in the transcriptomic profile related to the cellular response to the space environment and were consistent with cell dysfunction adaptations. In addition, the results obtained from ARPE-19 cells treated with coenzyme Q10 indicated its potential to increase cell resistance to damage.

Microbial biospherics: The experimental study of ecosystem function and evolution.

Awareness that our planet is a self-supporting biosphere with sunlight as its major source of energy for life has resulted in a long-term historical fascination with the workings of self-supporting ecological systems. However, the studies of such systems have never entered the canon of ecological or evolutionary tools and instead, have led a fringe existence connected to life support system engineering and space travel. We here introduce a framework for a renaissance in biospherics based on the study of matter-closed, energy-open ecosystems at a microbial level (microbial biospherics). Recent progress in genomics, robotics, and sensor technology makes the study of closed systems now much more tractable than in the past, and we argue that the time has come to emancipate the study of closed systems from this fringe context and bring them into a mainstream approach for studying ecosystem processes. By permitting highly replicated long-term studies, especially on predetermined and simplified systems, microbial biospheres offer the opportunity to test and develop strong hypotheses about ecosystem function and the ecological and evolutionary determinants of long-term system failure or persistence. Unlike many sciences, ecosystem ecology has never fully embraced a reductionist approach and has remained focused on the natural world in all its complexity. We argue that a reductionist approach to ecosystem ecology, using microbial biospheres, based on a combination of theory and the replicated study of much simpler self-enclosed microsystems could pay huge dividends.

How plants grow under gravity conditions besides 1 g: perspectives from hypergravity and space experiments that employ bryophytes as a model organism.

Plants have evolved and grown under the selection pressure of gravitational force at 1 g on Earth. In response to this selection pressure, plants have acquired gravitropism to sense gravity and change their growth direction. In addition, plants also adjust their morphogenesis in response to different gravitational forces in a phenomenon known as gravity resistance. However, the gravity resistance phenomenon in plants is poorly understood due to the prevalence of 1 g gravitational force on Earth: not only it is difficult to culture plants at gravity > 1 g(hypergravity) for a long period of time but it is also impossible to create a < 1 genvironment (µg, micro g) on Earth without specialized facilities. Despite these technical challenges, it is important to understand how plants grow in different gravity conditions in order to understand land plant adaptation to the 1 g environment or for outer space exploration. To address this, we have developed a centrifugal device for a prolonged duration of plant culture in hypergravity conditions, and a project to grow plants under the µg environment in the International Space Station is also underway. Our plant material of choice is Physcomitrium (Physcomitrella) patens, one of the pioneer plants on land and a model bryophyte often used in plant biology. In this review, we summarize our latest findings regarding P. patens growth response to hypergravity, with reference to our on-going "Space moss" project. In our ground-based hypergravity experiments, we analyzed the morphological and physiological changes and found unexpected increments of chloroplast size and photosynthesis rate, which might underlie the enhancement of growth and increase in the number of gametophores and rhizoids. We further discussed our approaches at the cellular level and compare the gravity resistance in mosses and that in angiosperms. Finally, we highlight the advantages and perspectives from the space experiments and conclude that research with bryophytes is beneficial to comprehensively and precisely understand gravitational responses in plants.

Plasticity of the human IgM repertoire in response to long-term spaceflight.

Immune dysregulation is among the main adverse outcomes of spaceflight. Despite the crucial role of the antibody repertoire in host protection, the effects of spaceflight on the human antibody repertoire are unknown. Consequently, using high-throughput sequencing, we examined the IgM repertoire of five cosmonauts 25 days before launch, after 64 ± 11 and 129 ± 20 days spent on the International Space Station (ISS), and at 1, 7, and 30 days after landing. This is the first study of this kind in humans. Our data revealed that the IgM repertoire of the cosmonauts was different from that of control subjects (n = 4) prior to launch and that two out the five analyzed cosmonauts presented significant changes in their IgM repertoire during the mission. These modifications persisted up to 30 days after landing, likely affected the specificities of IgM binding sites, correlated with changes in the V(D)J recombination process responsible for creating antibody genes, and coincided with a higher stress response. These data confirm that the immune system of approximately half of the astronauts who spent 6 months on the ISS is sensitive to spaceflight conditions, and reveal individual responses indicating that personalized approaches should be implemented during future deep-space exploration missions that will be of unprecedented durations.

Spaceflight induces oxidative damage to blood-brain barrier integrity in a mouse model.

Many factors contribute to the health risks encountered by astronauts on missions outside Earth's atmosphere. Spaceflight-induced potential adverse neurovascular damage and late neurodegeneration are a chief concern. The goal of the present study was to characterize the effects of spaceflight on oxidative damage in the mouse brain and its impact on blood-brain barrier (BBB) integrity. Ten-week-old male C57BL/6 mice were launched to the International Space Station (ISS) for 35 days as part of Space-X 12 mission. Ground control (GC) mice were maintained on Earth in flight hardware cages. Within 38 ± 4 hours after returning from the ISS, mice were euthanized and brain tissues were collected for analysis. Quantitative assessment of brain tissue demonstrated that spaceflight caused an up to 2.2-fold increase in apoptosis in the hippocampus compared to the control group. Immunohistochemical analysis of the mouse brain revealed an increased expression of aquaporin4 (AQP4) in the flight hippocampus compared to the controls. There was also a significant increase in the expression of platelet endothelial cell adhesion molecule-1 (PECAM-1) and a decrease in the expression of the BBB-related tight junction protein, Zonula occludens-1 (ZO-1). These results indicate a disturbance of BBB integrity. Quantitative proteomic analysis showed significant alterations in pathways responsible for neurovascular integrity, mitochondrial function, neuronal structure, protein/organelle transport, and metabolism in the brain after spaceflight. Changes in pathways associated with adhesion and molecular remodeling were also documented. These data indicate that long-term spaceflight may have pathological and functional consequences associated with neurovascular damage and late neurodegeneration.

Irisin prevents microgravity-induced impairment of osteoblast differentiation in vitro during the space flight CRS-14 mission.

Understanding molecular mechanisms responsible for bone cells unbalance in microgravity would allow the development of better countermeasures for astronauts, and eventually advancing terrestrial osteoporosis treatments. We conduct a unique investigation by using a controlled 3D in vitro cell model to mimic the bone microenvironment in microgravity aboard the SpaceX Dragon cargo ferry to the ISS. Osteoblasts (OBs), osteoclasts (OCs), and endothelial cells (ECs), seeded on Skelite discs, were cultured w/ or w/o rec-Irisin and exposed to 14 days of microgravity in the eOSTEO hardware. Gene expression analysis was assessed, and results were compared to ground controls treated within identical payloads. Our results show that the microgravity-induced downregulation of mRNA levels of genes encoding for OB key transcription factors (Atf4 -75%, P < .01; RunX2 -87%, P < .001, Osterix -95%, P < .05 vs ground) and proteins (Collagen I -84%, P < .05; Osteoprotegerin -94%, P < .05) were prevented by irisin. Despite it was not effective in preventing Trap and Cathepsin K mRNA increase, irisin induced a 2.8-fold increase of Osteoprotegerin (P < .05) that might act for reducing osteoclastogenesis in microgravity. Our results provide evidence that irisin supports OB differentiation and activity in microgravity and it might represent a countermeasure to prevent bone loss in astronauts.

Genomic Changes Driven by Radiation-Induced DNA Damage and Microgravity in Human Cells.

The space environment consists of a complex mixture of different types of ionizing radiation and altered gravity that represents a threat to humans during space missions. In particular, individual radiation sensitivity is strictly related to the risk of space radiation carcinogenesis. Therefore, in view of future missions to the Moon and Mars, there is an urgent need to estimate as accurately as possible the individual risk from space exposure to improve the safety of space exploration. In this review, we survey the combined effects from the two main physical components of the space environment, ionizing radiation and microgravity, to alter the genetics and epigenetics of human cells, considering both real and simulated space conditions. Data collected from studies on human cells are discussed for their potential use to estimate individual radiation carcinogenesis risk from space exposure.

Spaceflight Modulates the Expression of Key Oxidative Stress and Cell Cycle Related Genes in Heart.

Spaceflight causes cardiovascular changes due to microgravity-induced redistribution of body fluids and musculoskeletal unloading. Cardiac deconditioning and atrophy on Earth are associated with altered Trp53 and oxidative stress-related pathways, but the effects of spaceflight on cardiac changes at the molecular level are less understood. We tested the hypothesis that spaceflight alters the expression of key genes related to stress response pathways, which may contribute to cardiovascular deconditioning during extended spaceflight. Mice were exposed to spaceflight for 15 days or maintained on Earth (ground control). Ventricle tissue was harvested starting ~3 h post-landing. We measured expression of select genes implicated in oxidative stress pathways and Trp53 signaling by quantitative PCR. Cardiac expression levels of 37 of 168 genes tested were altered after spaceflight. Spaceflight downregulated transcription factor, Nfe2l2 (Nrf2), upregulated Nox1 and downregulated Ptgs2, suggesting a persistent increase in oxidative stress-related target genes. Spaceflight also substantially upregulated Cdkn1a (p21) and cell cycle/apoptosis-related gene Myc, and downregulated the inflammatory response gene Tnf. There were no changes in apoptosis-related genes such as Trp53. Spaceflight altered the expression of genes regulating redox balance, cell cycle and senescence in cardiac tissue of mice. Thus, spaceflight may contribute to cardiac dysfunction due to oxidative stress.

Early Deconditioning of Human Skeletal Muscles and the Effects of a Thigh Cuff Countermeasure.

Muscle deconditioning is a major consequence of a wide range of conditions from spaceflight to a sedentary lifestyle, and occurs as a result of muscle inactivity, leading to a rapid decrease in muscle strength, mass, and oxidative capacity. The early changes that appear in the first days of inactivity must be studied to determine effective methods for the prevention of muscle deconditioning. To evaluate the mechanisms of muscle early changes and the vascular effect of a thigh cuff, a five-day dry immersion (DI) experiment was conducted by the French Space Agency at the MEDES Space Clinic (Rangueil, Toulouse). Eighteen healthy males were recruited and divided into a control group and a thigh cuff group, who wore a thigh cuff at 30 mmHg. All participants underwent five days of DI. Prior to and at the end of the DI, the lower limb maximal strength was measured and muscle biopsies were collected from the vastus lateralis muscle. Five days of DI resulted in muscle deconditioning in both groups. The maximal voluntary isometric contraction of knee extension decreased significantly. The muscle fiber cross-sectional area decreased significantly by 21.8%, and the protein balance seems to be impaired, as shown by the reduced activation of the mTOR pathway. Measurements of skinned muscle fibers supported these results and potential changes in oxidative capacity were highlighted by a decrease in PGC1-α levels. The use of the thigh cuff did not prevent muscle deconditioning or impact muscle function. These results suggest that the major effects of muscle deconditioning occur during the first few days of inactivity, and countermeasures against muscle deconditioning should target this time period. These results are also relevant for the understanding of muscle weakness induced by muscle diseases, aging, and patients in intensive care.

Molecular Basis to Integrate Microgravity Signals into the Photoperiodic Flowering Pathway in Arabidopsis thaliana under Spaceflight Condition.

Understanding the effects of spaceflight on plant flowering regulation is important to setup a life support system for long-term human space exploration. However, the way in which plant flowering is affected by spaceflight remains unclear. Here, we present results from our latest space experiments on the Chinese spacelab Tiangong-2, in which Arabidopsis wild-type and transgenic plants pFT::GFP germinated and grew as normally as their controls on the ground, but the floral initiation under the long-day condition in space was about 20 days later than their controls on the ground. Time-course series of digital images of pFT::GFP plants showed that the expression rhythm of FT in space did not change, but the peak appeared later in comparison with those of their controls on the ground. Whole-genome microarray analysis revealed that approximately 16% of Arabidopsis genes at the flowering stage changed their transcript levels under spaceflight conditions in comparison with their controls on the ground. The GO terms were enriched in DEGs with up-regulation of the response to temperature, wounding, and protein stabilization and down-regulation of the function in circadian rhythm, gibberellins, and mRNA processes. FT and SOC1 could act as hubs to integrate spaceflight stress signals into the photoperiodic flowering pathway in Arabidopsis in space.

Reciprocal Homer1a and Homer2 Isoform Expression Is a Key Mechanism for Muscle Soleus Atrophy in Spaceflown Mice.

The molecular mechanisms of skeletal muscle atrophy under extended periods of either disuse or microgravity are not yet fully understood. The transition of Homer isoforms may play a key role during neuromuscular junction (NMJ) imbalance/plasticity in space. Here, we investigated the expression pattern of Homer short and long isoforms by gene array, qPCR, biochemistry, and laser confocal microscopy in skeletal muscles from male C57Bl/N6 mice (n = 5) housed for 30 days in space (Bion-flight = BF) compared to muscles from Bion biosatellite on the ground-housed animals (Bion ground = BG) and from standard cage housed animals (Flight control = FC). A comparison study was carried out with muscles of rats subjected to hindlimb unloading (HU). Gene array and qPCR results showed an increase in Homer1a transcripts, the short dominant negative isoform, in soleus (SOL) muscle after 30 days in microgravity, whereas it was only transiently increased after four days of HU. Conversely, Homer2 long-form was downregulated in SOL muscle in both models. Homer immunofluorescence intensity analysis at the NMJ of BF and HU animals showed comparable outcomes in SOL but not in the extensor digitorum longus (EDL) muscle. Reduced Homer crosslinking at the NMJ consequent to increased Homer1a and/or reduced Homer2 may contribute to muscle-type specific atrophy resulting from microgravity and HU disuse suggesting mutual mechanisms.

Hypercapnia augments resistive exercise-induced elevations in intraocular pressure in older individuals.

NEW FINDINGS: What is the central question of this study? Astronauts on-board the International Space Station (ISS) perform daily exercises designed to prevent muscle atrophy and bone demineralization: what is the effect of resistive exercise performed by subjects while exposed to the same level of hypercapnia as on the ISS on intraocular pressure (IOP)? What is the main finding and its importance? The static exercise-induced elevation in IOP during 6° prone head-down tilt (simulating the headward shift of body fluids in microgravity) is augmented by hypercapnia and exceeds the ocular hypertension threshold. ABSTRACT: The present study assessed the effect of 6° head-down (establishing the cephalad fluid displacement noted in astronauts in microgravity) prone (simulating the effect on the eye) tilt during rest and exercise (simulating exercise performed by astronauts to mitigate the sarcopenia induced by unloading of weight-bearing limbs), in normocapnic and hypercapnic conditions (the latter simulating conditions on the International Space Station) on intraocular pressure (IOP). Volunteers (mean age = 57.8 ± 6 years, n = 10) participated in two experimental sessions, each comprising: (i) 10 min rest, (ii) 3 min static handgrip exercise (30% max), and (iii) 2 min recovery, inspiring either room air (NCAP) or a hypercapnic mixture (1% CO2 , HCAP). We measured IOP in the right eye, cardiac output (CO), stroke volume (SV), heart rate (HR) and mean arterial pressure (MAP) at regular intervals. Baseline IOP in the upright seated position while breathing room air was 14.1 ± 2.9 mmHg. Prone 6° head-down tilt significantly (P < 0.01) elevated IOP in all three phases of the NCAP (rest: 27.0 ± 3.7 mmHg; exercise: 32.2 ± 4.8 mmHg; recovery: 27.4 ± 4.0 mmHg) and HCAP (rest: 27.3 ± 4.3 mmHg; exercise: 34.2 ± 6.0 mmHg; recovery: 29.1 ± 5.8 mmHg) trials, with hypercapnia augmenting the exercise-induced elevation in IOP (P < 0.01). CO, SV, HR and MAP were significantly increased during handgrip dynamometry, but there was no effect of hypercapnia. The observed IOP measured during prone 6° HDT in all phases of the NCAP and HCAP trials exceeded the threshold pressure defining ocular hypertension. The exercise-induced increase in IOP is exacerbated by hypercapnia.