Oxidative phosphorylation in rat skeletal muscles after space flight on board biosatellites

Polarographic analysis of biological oxidation in rat's skeletal muscles after the 18- and 22-day flights revealed changes specific for the flight animals: oxidative phosphorylation uncoupling, distinct inertness of energy accumulation after 10 hrs of landing. Tissue respiration's inhibition was observed in both flight and synchronous rats suggesting the effect of other than microgravity factors. Energy metabolism in muscles of flight animals returned to the pre-flight level later (29 d) compared to the synchronous rats (6 d). Muscles of different functions (predominance of fast or slow fibers) showed similar responses of energy metabolism to weightlessness, i.e. inhibition of the intensity and decline of the energy efficiency of oxidative processes. A decrease in dehydrogenase activity has been found in the first day of recovery. The effects may be caused by the inhibition of both aerobic and anaerobic metabolism after space flight.     

Variable lymphocyte responses in rats after space flight.

Most studies of human blood lymphocyte function following space flight have indicated that microgravity suppresses T cell proliferation. However, several other postflight experiments with animals have shown no decrease in proliferation of lymphocytes from peripheral lymphatic tissues, suggesting that different tissues may be variably affected by microgravity. Therefore, we examined the proliferation of lymphocytes from both spleen and lymph nodes of rats following a 4-day flight aboard the Space Shuttle. The experiments were designed to investigate tissue variability as well as potential mechanisms involved in suppressing proliferation. We found that proliferation of lymph node lymphocytes (LNL) from flight (FLT) animals stimulated with the antigen receptor-dependent T cell mitogen concanavalin A was depressed and could not be restored by supplementing cultures with interleukin 1 or interleukin 2 (IL-2). Response to another receptor-dependent mitogen, phytohemagglutinin, was not decreased. However, proliferation of FLT LNL following stimulation with the receptor-independent, mitogenic combination of phorbol ester and ionomycin was depressed. LNL IL-2 activity, cell surface marker expression, and B cell responses to mitogen were normal. Thus, deficits in antigen receptor/ligand interactions, cell surface marker expression, or IL-2 did not account for the suppressed lymphocyte proliferation observed postflight. In contrast to LNL, FLT splenocyte proliferation was not depressed. Assayable IL-2, IL-2 receptor expression, and cell surface marker expression likewise were unaffected by space flight. The differences between lymph node and splenic responses demonstrate the tissue-specific nature of microgravity effects on individual lymphatic tissues.     

Spaceflight affects bone formation in rhesus monkeys: a histological and cell culture study.

Using analyses of iliac crest cell and tissue, back-scattered electron imaging, and biochemical techniques, we characterized the effects of a 14-day spaceflight (Bion 11) on bone structure and bone formation in two 3- to 4-yr-old male rhesus monkeys compared with eight age-matched Earth-control monkeys. We found that postflight bone volume was 35% lower than preflight values in flight monkeys. This was associated with reduced osteoid (-40%) and mineralizing (-32%) surfaces and decreased bone formation rate (-53%). Moreover, flight monkeys exhibited trends to lower values of mineralization profile in iliac bone (back-scattered electron imaging) and to decreased osteocalcin serum levels (P = 0.08). The initial number of trabecular bone cells yielded in cultures did not differ in flight and control animals before or after the flight. However, osteoblastic cell proliferation was markedly lower in postflight vs. preflight at 9 and 14 days of culture in one flight monkey. This study suggests that a 14-day spaceflight reduces iliac bone formation, osteoblastic activity, and/or recruitment in young rhesus monkeys, resulting in decreased trabecular bone volume.     

Changes of insulin in plasma and receptor for insulin in various tissues after the exposure of rats to space flights and hypokinesia

The explanation of the mechanism of the response to gravity changes is of great importance for the determination of the capacity of human subjects to adapt to the load of gravitational stress. Therefore several studies were performed to investigate the activity of endocrine system, since the hormones are involved in the regulation of physiological functions and metabolic processes. However the studies of endocrine system activity during altered gravity conditions, especially during the weightlessness are influenced by the several interventions in biomedical observations due to operational program of astronauts, wide variability in individual response and tolerance, use of extensive countermeasures, differences in the type of space missions and in the studies after landing also a hypergravity effect at landing and variability in postflight readaptation process. The significant changes of plasma insulin and glucose levels were observed in astronauts during space flights and in the first days of recovery period. In the first inflight period plasma insulin levels were increased, unchanged or decreased however after 4-5 weeks of exposure to weightlessness a decrease of insulin plasma levels were noted. After space flights an increase of plasma insulin levels were demonstrated in experimental animals and in human subjects. Since plasma insulin level is considered as most important factor involved in the regulation for insulin receptors in target tissues, an investigation of insulin receptors in various tissues was performed in rats exposed to space flight or to hypokinesia (model used for simulation of some effects of microgravity).     

Effects of spaceflight on human calf hemodynamics.

Chronic microgravity may modify adaptations of the leg circulation to gravitational pressures. We measured resting calf compliance and blood flow with venous occlusion plethysmography, and arterial blood pressure with sphygmomanometry, in seven subjects before, during, and after spaceflight. Calf vascular resistance equaled mean arterial pressure divided by calf flow. Compliance equaled the slope of the calf volume change and venous occlusion pressure relationship for thigh cuff pressures of 20, 40, 60, and 80 mmHg held for 1, 2, 3, and 4 min, respectively, with 1-min breaks between occlusions. Calf blood flow decreased 41% in microgravity (to 1.15 +/- 0.16 ml x 100 ml(-1) x min(-1)) relative to 1-G supine conditions (1.94 +/- 0.19 ml x 100 ml(-1) x min(-1), P = 0.01), and arterial pressure tended to increase (P = 0.05), such that calf vascular resistance doubled in microgravity (preflight: 43 +/- 4 units; in-flight: 83 +/- 13 units; P < 0.001) yet returned to preflight levels after flight. Calf compliance remained unchanged in microgravity but tended to increase during the first week postflight (P > 0.2). Calf vasoconstriction in microgravity qualitatively agrees with the "upright set-point" hypothesis: the circulation seeks conditions approximating upright posture on Earth. No calf hemodynamic result exhibited obvious mechanistic implications for postflight orthostatic intolerance.     

Plasticity of arterial vasculature during simulated weightlessness and its possible role in the genesis of postflight orthostatic intolerance

Even after several decades of extensive research, the basic mechanism of postflight cardiovascular dysfunction has not yet been fully elucidated. It is now well recognized that multiple mechanisms might account for the frequent occurrence of significant postflight orthostatic intolerance. It has been found that all tissues adapt their design when exposed to sustained alteration in local activity and/or stress. The most obvious example is the musculo-skeletal system, structure and function of which might be severely affected during microgravity exposure. In an attempt to elucidate whether structure and function of cardiac and vascular smooth muscle might be affected by simulated by microgravity, a serial work was started several years ago. In this paper, we present our more recent findings on plasticity of arterial vasculature and its innervation state during and after simulated microgravity and its time course.     

Recruitment of the Rhesus soleus and medial gastrocnemius before, during and after spaceflight

Electromyograms were recorded from the soleus and medial gastrocnemius muscles and tendon force from the medial gastrocnemius muscle of 2 juvenile Rhesus monkeys before, during and after Cosmos flight 2229 and of ground control animals. Recording sessions were made while the Rhesus were performing a foot pedal motor task. Preflight testing indicated normal patterns of recruitment between the soleus and medial gastrocnemius, i.e. a higher level of recruitment of the soleus compared to the medial gastrocnemius during the task. Recording began two days into the spaceflight and showed that the media gastrocnemius was recruited preferentially over the soleus. This observation persisted throughout the flight and for the 2 week period of postflight testing. These data indicate a significant change in the relative recruitment of slow and fast extensor muscles under microgravity conditions. The appearance of clonic-like activity in one muscle of each Rhesus during flight further suggests a reorganization in the neuromotor system in a microgravity environment.     

Cell interactions in microgravity: cytotoxic effects of natural killer cells in vitro

To study cell-to-cell interactions in microgravity we examined the functional activity of natural killer cells on board of the ISS. NK cells are the effector cells with direct cytotoxic activity to oncogenic, virus-infected cells and cells with modified differentiation. Ground-based experiments have shown that the examination of target cell lysis after incubation with NK cells is a simple and informative model for studying the influence of microgravity. NK cytotoxicity was measured as the value of non-degradeted labeled myeloblasts (K-562) after 24 hrs exposure with human lymphocytes in suspension. A special device was developed for space flight experiments. Human cultured lymphocytes and labeled K-562 cells were loaded into special syringes and delivered to the Russian segment of the ISS. Cosmonauts prepared co-cultured suspensions during the first day of microgravity, exposed them at 37 degrees C for 24 hrs and then separated H3-labeled cells on special filters. The results of ISS-8 mission showed that human NK cells in vitro remain lysis activity toward target cells in microgravity. The basal level of NK cytotoxicity was low and we did not found significant differences between "control" and "flight" values. Interferon production during the interaction between immune and target cells (ratio 10:1) in microgravity did not differ compared with ground-based control experiments. Ground exposure of the same lymphocyte samples with K-562 cells to 24 hrs clinorotation also did not lead to significant differences. These experiments paved the way for understanding the cell interaction mechanisms in space flight and the obtained results suggest that microgravity does not disrupt the interaction of NK cells with tumor cells.     

Contractile characteristics of the triceps surae muscle in healthy males during 120-days head-down tilt (HDT) and countermeasures

To reveal mechanisms responsible for changes in muscle contractility during microgravity, it seems expedient to perform similar studies under microgravity or conditions simulating microgravity. Among standard methods for simulating microgravity, hypokinesia modelling support unloading (or rather its redistribution), and hypodynamia are employed. Absence of weight loading, decreased muscular effort characteristic of the Earth conditions due to counteracting gravity, results in a general muscle underloading and therefore in lowered activity of the proprioceptive input. This may be one of the reasons not only for a resetting of motor coordination and control, but also for a gradual development of a persistent change in the motor control system. The basis of countermeasures against negative consequences of microgravity (hypokinesia) is the correct choice of countermeasures. In this connection of specific interest is a study of the magnitude of change in skeletal muscle contractility in humans after a variety of countermeasures when functional activity is lowered by a long-term 120-days HDT which is an adequate simulation of physiological microgravity-induced effects.     

Circadian rhythms of salivary cortisol content during long-term space flight

Adaptation mechanisms of adrenal function related to secretion of cortisol were studied under conditions of microgravity. Parameters of diurnal rhythms of salivary cortisol were studied by Russian cosmonauts on board orbital station Mir during long-term space flights (SF). The preflight circadian rhythms of salivary cortisol in cosmonauts were characterized by the morning maximum occurring at 9∶43 a.m., the fluctuation amplitude 6.05 nmol/1, and the daily average concentration 8.79 nmol/l. The characteristics of cortisol diurnal rhythm changed under conditions of long-term space flight. On average, the rhythm measure and amplitude decreased after two months of flight. The postflight maximum concentration of free cortisol tended to occur later in the day. Evidently, the motor activity during SF, i.e., prophylactic exercises along with other factors, significantly influenced the parameters of cortisol circadian rhythm that was revealed by the individual variability of findings during the flight. After the long-term SF, individual ratios of salivary and plasma cortisol levels increased against the background of increased plasma content of the hormone, i.e., the fraction of free, physiologically active hormone in the total pool of circulating molecules decreased.     

Microgravity alters respiratory sinus arrhythmia and short-term heart rate variability in humans

We studied heart rate (HR), heart rate variability (HRV), and respiratory sinus arrhythmia (RSA) in four male subjects before, during, and after 16 days of spaceflight. The electrocardiogram and respiration were recorded during two periods of 4 min controlled breathing at 7.5 and 15 breaths/min in standing and supine postures on the ground and in microgravity. Low (LF)- and high (HF)-frequency components of the short-term HRV (< or =3 min) were computed through Fourier spectral analysis of the R-R intervals. Early in microgravity, HR was decreased compared with both standing and supine positions and had returned to the supine value by the end of the flight. In microgravity, overall variability, the LF-to-HF ratio, and RSA amplitude and phase were similar to preflight supine values. Immediately postflight, HR increased by approximately 15% and remained elevated 15 days after landing. LF/HF was increased, suggesting an increased sympathetic control of HR standing. The overall variability and RSA amplitude in supine decreased postflight, suggesting that vagal tone decreased, which coupled with the decrease in RSA phase shift suggests that this was the result of an adaptation of autonomic control of HR to microgravity. In addition, these alterations persisted for at least 15 days after return to normal gravity (1G).     

Permanent depression of plasma cGMP during long-term space flight

The purpose of this study was to investigate plasma concentrations of cyclic guanosine monophosphate (cGMP) and atrial natriuretic peptide (ANP) during and after real and simulated space flight. Venous blood was obtained 3 min after the beginning and 2 min after the lower body negative pressure maneuver in two cosmonauts preflight (supine), inflight, and postflight (supine) and in five other subjects before, at the end, and 4 days after a 5-day head-down tilt (-6 degrees) bed rest. In cosmonaut 1 (10 days in space), plasma cGMP fell from preflight 4.3 to 1.4 nM on flight day 6, and was 3.0 nM on the fourth day after landing. In cosmonaut 2 (438 days in space), it fell from preflight 4.9 to 0.5 nM on on flight day 3, and stayed <0.1 nM with 5, 9, and 14 months in space, as well as on the fourth day after landing. Three months after the flight his plasma cGMP was back to normal (6.3 nM). Cosmonaut 2 also displayed relatively low inflight ANP values but returned to preflight level immediately after landing. In a ground-based simulation on five other persons, supine plasma cGMP was reduced by an average of 30% within 5 days of 6 degrees head-down tilt bed rest. The data consistently demonstrate lowered plasma cGMP with real and simulated weightlessness, and a complete disappearance of cGMP from plasma during, and shortly after long-duration space flight.     

Venous distensibility and venous emptying in the legs during a 42-day -6 degrees head-down bedrest

Exposure to actual or simulated microgravity is known to result in changes in lower limb venous compliance or distensibility which may play a role in post-bedrest or postflight orthostatic intolerance. Venous deconditioning has only been described in terms of changes in vascular compliance or distensibility. But a complete understanding of changes in venous hemodynamics and cardiovascular regulation occurring under these conditions has to take into account changes in emptying capacities of the veins which influence venous return, cardiac filling, and cardiac output regulation. Moreover, few data are available about the course of changes in venous hemodynamics for periods of simulated microgravity longer than 4 weeks. The purpose of this investigation was to measure parameters of venous compliance and venous emptying before, during, and after a 42-day period of bedrest at -6 degrees head-down tilt for a better understanding of long term venous physiological adaptation to microgravity.     

Time course and reversibility of arterial vasoreactivity changes in simulated microgravity rats

Recent works have shown that postflight orthostatic intolerance involves multiple alterations in physiological function during actual or simulated microgravity. In our previous work, we demonstrated that 14-day tail-suspension resulted in an impaired ability of vascular smooth muscle to develop tension in arteries confined to the hindquarter, which have been suggested as an important factor accounting for the occurrence of orthostatic intolerance. To our knowledge, data on arterial vasoreactivity alterations induced by simulated microgravity longer than two weeks are not found. The aim of the present work was to characterize the time course of alterations in vasoconstrictor properties of hindquarter arteries during tail-suspension up to eight weeks, and to examine whether these alterations are reversible.     

The calcium endocrine system of adolescent rhesus monkeys and controls before and after spaceflight

The calcium endocrine system of nonhuman primates can be influenced by chairing for safety and the weightless environment of spaceflight. The serum of two rhesus monkeys flown on the Bion 11 mission was assayed pre- and postflight for vitamin D metabolites, parathyroid hormone, calcitonin, parameters of calcium homeostasis, cortisol, and indexes of renal function. Results were compared with the same measures from five monkeys before and after chairing for a flight simulation study. Concentrations of 1,25-dihydroxyvitamin D were 72% lower after the flight than before, and more than after chairing on the ground (57%, P < 0.05). Decreases in parathyroid hormone did not reach significance. Calcitonin showed modest decreases postflight (P < 0.02). Overall, effects of spaceflight on the calcium endocrine system were similar to the effects of chairing on the ground, but were more pronounced. Reduced intestinal calcium absorption, losses in body weight, increases in cortisol, and higher postflight blood urea nitrogen were the changes in flight monkeys that distinguished them from the flight simulation study animals.     

Effect of ecdysterone and juvenoid on the developmental involution of flight muscles in Acheta domestica

Morphogenesis and degeneration of the flight muscles in Acheta domestica was studied. The dorso-longitudinal flight muscles (DLMs) degenerate during the fourth day after adult ecdysis and the dorso-ventral flight muscles (DVMs) on the fifteenth day. In the presence of an intact innervation the degeneration of the DLMs can be retarded for 2 days by the injection of ecdysterone into very young adults. This retardation may also result in hypertrophy of the muscle fibres. The injection of ecdysterone, even in high doses, did not affect the flight muscle remnants. No notable changes have been found in the degeneration of DLMs by ovarectomy. Thus, the degeneration of flight muscles and the development of ovaries appear to be independent processes.The DLMs are homogeneous in fibre pattern in respect to succinic dehydrogenase, an important oxidative enzyme, and to ATPase activity, but the muscle fibres do not show any phosphorylase activity.     

Motor reactions and vestibular reflexes in cats and monkey in weightlessness

Close morpho-functional relationships of the cerebellum and vestibular system at all stages of phylogenesis of vertebrates suggest that cerebellum can be regarded as an important center of gravireceptive function. Direct examination of electrical activity of the labyrinth in cats during transient (1-2 sec) state of weightlessness produced by free fall has shown that there was an almost two fold increase in both the rate and amplitude of electrical activity in the vestibular ganglion. It is commonly accepted at present time that the conditions of orbital flight around Earth closely connect with weightlessness that usually manifests itself as undesirable factor of flight. It is known, that vestibular, proprioceptive, visual and other sensory modalities are converted on the cerebellum, which would indicate that this information is used for motor coordination and spatial orientation. Undoubtedly, origin of many vestibulo-motor disturbances during flight and in postflight period to a considerable degree depends on weightlessness. On the whole the visual illusions, motor discoordination, and space sickness, including vomiting are referred to the "space adaptation syndrome." But nature of these disturbances still is not well understood. This investigation was dedicated to study of vestibular and motor reactions of cats and monkey in short-term microgravity.     

Functional changes in the snail statocyst system elicited by microgravity

Background

The mollusk statocyst is a mechanosensing organ detecting the animal''s orientation with respect to gravity. This system has clear similarities to its vertebrate counterparts: a weight-lending mass, an epithelial layer containing small supporting cells and the large sensory hair cells, and an output eliciting compensatory body reflexes to perturbations.

Methodology/Principal Findings

In terrestrial gastropod snail we studied the impact of 16- (Foton M-2) and 12-day (Foton M-3) exposure to microgravity in unmanned orbital missions on: (i) the whole animal behavior (Helix lucorum L.), (ii) the statoreceptor responses to tilt in an isolated neural preparation (Helix lucorum L.), and (iii) the differential expression of the Helix pedal peptide (HPep) and the tetrapeptide FMRFamide genes in neural structures (Helix aspersa L.). Experiments were performed 13–42 hours after return to Earth. Latency of body re-orientation to sudden 90° head-down pitch was significantly reduced in postflight snails indicating an enhanced negative gravitaxis response. Statoreceptor responses to tilt in postflight snails were independent of motion direction, in contrast to a directional preference observed in control animals. Positive relation between tilt velocity and firing rate was observed in both control and postflight snails, but the response magnitude was significantly larger in postflight snails indicating an enhanced sensitivity to acceleration. A significant increase in mRNA expression of the gene encoding HPep, a peptide linked to ciliary beating, in statoreceptors was observed in postflight snails; no differential expression of the gene encoding FMRFamide, a possible neurotransmission modulator, was observed.

Conclusions/Significance

Upregulation of statocyst function in snails following microgravity exposure parallels that observed in vertebrates suggesting fundamental principles underlie gravi-sensing and the organism''s ability to adapt to gravity changes. This simple animal model offers the possibility to describe general subcellular mechanisms of nervous system''s response to conditions on Earth and in space.     

Specific Features of Metabolism in Humans Performing a Physical Exercise Test after 7-Day Dry Immersion

Healthy male volunteers (n = 5) aged 27–42 years participated in experimental studies (7-day immersion) designed to simulate the effects of microgravity. To study the metabolic changes caused by decreased weight bearing on the musculoskeletal system and a change in the position of the body relative to the gravity vector, a 15-min load test before and after immersion was used. A wide set of biochemical parameters characterizing the state of the energy metabolism, substrate levees, and enzyme activities, as well as the blood level of hormones, was measured in the blood plasma. Multifactor analysis was used in processing the experimental data. After immersion, a significant decrease in the blood plasma activity of isocitrate dehydrogenase, creatine phosphokinase, and lactate dehydrogenase was noted, whereas the growth hormone and insulin levels exceeded the baseline values. The physical exercise test increased the differences in the metabolic status before and after 7 days of immersion. The factor analysis allowed us to reveal the most significant biochemical variables for identifying a new metabolic state of the physiological systems after exposure to short-term simulated microgravity. Changes in the creatine phosphokinase activity and the human blood plasma levels of cortisol, triglycerides, insulin, and inorganic phosphate made the most significant contributions to these differences, and the direction of biochemical shifts in response to exercise was different before and after immersion. The results obtained are indicate that energy and substrate metabolism changes in response to a decrease in weight bearing and an altered body position relative to the gravity vector and that these changes are especially pronounced when an exercise test is used.     

Ontogenesis of mammals and gravity

The results of the experiments with Wistar rats in microgravity and 2G hypergravity are summarized. Their analysis allows to conclude that adaptive potentials of adult animals in space flights lasting up to 1/50 of their life span are enough for maintenance of adequate reactions to acute and chronic stressors in the postflight period, rapid elimination of space-induced metabolic and structural alterations on return to Earth, maintenance of normal reproductive function after space flight. In embryological experiments it was demonstrated that during space flight it is possible not only to maintain physiological functions of an adult organism, but to form functions of a developing fetus. The animals that spent the portion of their prenatal development in space flight were capable to go through the entire cycle of postnatal development, up to sexual maturity and reproduction. In ground based centrifuge experiments with 2G it was demonstrated the possibility of realizing, under hypergravity, of all the main stages of prenatal and early postnatal development of rats: fertilization, embryon implantation, fetal development, birth and lactation of progeny. Exposure of rats to microgravity did not reduce their life span post flight. Alterations in biological age of animals were small.