Characteristics of physiological responses to +Gx loads in non-professional crew members of short-term missions to the ISS

Analysis of physiological responses to +Gx loads during space flights (SFs) 8–12 days in duration was carried out in ten non-professional crew members of the International Space Station (ISS), many of whom were accepted to participate in SFs while having partial health deficiency. The results were compared to the data on ten apparently healthy crew members of the Mir Orbital Station (OS) who participated in SFs of a similar duration. The age (29–60 years) of the subjects from the ISS crew was older than the age (31–49 years) of the subjects from the Mir OS crew. Objective data showed that subjects from the ISS crew had significantly higher levels of sinus tachycardia and tachypnea during settling into orbit and re-entry than subjects from the Mir OS crew. In some cases, extrasystolic arrhythmias were recorded in subjects from the ISS crew during the pre-launch period. The arrhythmias persisted during the ascent to orbit, and usually became more considerable during the descent to Earth. During the descent to Earth, the subjects from the ISS crew demonstrated, in general, a higher rate of heart rhythm disorders, which were characterized by more pronounced polymorphism and a greater severity. Particularly significant changes in electrocardiograms (ECGs) were observed in two subjects from the ISS crew aged 60 years. These disorders were sporadic in all the cosmonauts. The results of the study show that individual characteristics of cardiac rhythm regulation observed before the start may lead to marked changes in ECG during +Gx loads during the descent from the orbit to Earth. This should be taken into account in preparing individual medical programs for non-professional members of ISS crews in order to minimize health risks. The study has also demonstrated the necessity for stricter criteria of selection for ISS missions for older candidates with partial health deficiency.     

Vestibular Function after Repeated Space Flights

This paper presents the results from testing the vestibular function on return from repeated space flights (SF) in 32 cosmonauts of the International Space Station that were in long SFs of 125–215 days. The cosmonauts were tested twice before the flight (baseline data collection) and on days 1–2, 4–5, and 8–9 after landing. The testing was made using two methods for recording eye movements (with simultaneous recording of head movements): electro-oculography and video-oculography. It is shown that the repeated stay in the long SF leads to a considerable statistically significant reduction in the de-adaptation period. Atypical vestibular disorders and changed patterns of the otolith-semicircular canal interaction are observed mostly in the cosmonauts who have made their maiden flights to microgravity.     

Metabolic features of cosmonauts after ballistic descent from the Earth orbit

The features of metabolic reactions in five cosmonauts after long-term flights on the International Space Station (ISS) and landing along a ballistic trajectory and in the cosmonauts returning to Earth in the mode of automatic controlled descent were studied. Venous blood samples were collected, and 50 biochemical parameter values that reflect the functional state of organs and tissues and characterize the main metabolic pathways were determined. On the first day of the recovery period after ballistic descent, the activity of the myocardial, liver, and gastrointestinal enzymes in the blood serum of cosmonauts was increased 1.3- to 2.1-fold; a number of the parameter values exceeded the upper normal limit. The level of C-reactive protein increased fivefold as compared with the preflight values. Marked signs of glycolysis, glycogenolysis and lipolysis activation as well as disorders of acid–base balance were observed. Changes in the biochemical parameter values in cosmonauts after landing along a ballistic trajectory differed significantly from those revealed in the same cosmonauts after long-term missions followed by automatic controlled descent to Earth. Negative metabolic changes tendency after landing along a ballistic trajectory remained for at least 14 days of the recovery period. It was concluded that changes in the metabolic reactions of cosmonauts after long-term missions to the ISS depend on the flights final stage conditions. After landing on Soyuz spaceships in the ballistic descent mode, the cosmonauts had adverse prognosis changes in the biochemical values characterizing the state of the cardiovascular system and marked shifts in the activity of the liver and gastrointestinal constellation enzymes. The dynamics of carbohydrate, lipid, and protein metabolism as well as acid–base balance indicates a significant tension of all body systems and exhaustion of its functional reserves.     

Vestibular function and space motion sickness

The vestibular system plays an important role in intersensory interactions and gravitation is a natural stimulus for its receptors. Weightlessness alters the input signals of the otoliths and their effect on the pattern and dynamics of changes in the vestibular function (VF), which is accompanied by development of space adaptation syndrome (SAS) and space motion sickness (SMS). These changes occur both during the spaceflight (SF) and after returning to Earth, but the mechanisms of their development are still poorly understood and require special studies. In total, 47 Russian cosmonauts (crewmembers of long-term International Space Station (ISS) missions) have participated in the studies into VF before and after SF and nine of them, in onboard studies during SF (129–215 days) as a part of the Virtual space experiment (stage 1). Electro- and video-oculography are used to record spontaneous eye movements (SpEM), static vestibular–ocular responses during head tilts to the right or left shoulder (static otolith–cervical–ocular reflex, OCOR), and dynamic vestibular-ocular response during the head rotation around the longitudinal axis of the body. The examination is accompanied by personal and questionnaire survey on subjective responses and complaints of cosmonauts about SAS and SMS. Significant changes in SpEM (drifts of eyes, spontaneous and gaze-evoked nystagmus, and arbitrary saccades) and a decrease in OCOR (statistically significant decrease in the amplitude of ocular counter-rolling in response to head tilts up to its absence or inversion, an atypical OCOR) are observed during SF. An atypical OCOR is observed at the beginning of adaptation to weightlessness in seven of the nine cosmonauts (the first one to two weeks of SF) and repeatedly throughout the flight in all cosmonauts regardless of whether it is their first flight or not. Atypical vestibular responses after SF, similar to the responses during SF, are observed in several cosmonauts by day 9 after flight. It has been shown that atypical OCOR variants are more frequently observed in the subjects lacking any previous space experience, as well as a more pronounced decrease in this response with a concurrent increase in the response of the semicircular canals. It is also demonstrated that repeated SFs lead to a considerable shortening in the after-flight readaptation to terrestrial conditions and a considerable decrease in the degree of vestibular disorders. In the initial period of SF, the changes in VF are correlated with the complaints and manifestations of SAS and SMS; however, the complaints and the corresponding symptoms are unobservable during the further flight despite significant changes in the VF state. The patterns of the VF disorders associated with the impact of weightlessness and observed during and after SF are very similar, allowing these disorders to be regarded as SAS and SMS of different severities (intensities).     

Calcium metabolism characteristics in microgravity]

The results of research of calcium exchange parameters at cosmonauts taken part in long space flights (SF) onboard of orbital stations "SALUT" and "MIR" within 1978-1998 were generalized. The analysis of data received during observation of 44 cosmonauts (18 of them have taken part in long SF twice) was done. The observation was carried out before and after SF by duration 30-438 days. The content of a total calcium in blood serum was increased basically by the increase of its ionized fraction after flights of moderate (3-6 months) and large duration (6-14 months) along with the significant increase of PTH and decrease of calcitonin levels. The content of osteocalcin after SF was increased. Three cosmonauts participated in research of calcium kinetics using stable isotopes before, in time and after a 115-day SF. Reduction of intestinal absorption, excretion through a gastrointestinal tract, and increase of calcium excretion with urine were marked in time of SF. In early postflight period a level of intestinal absorption, on the average, was much lower than in SF, and the calcium removal through intestine was increased. Both renal and intestinal excretion of calcium were not normalized in 3.5-4.5 months after end of SF. Increase of resorbtive processes in bone tissues which induced negative bone balance during flight was observed in all test subjects, proceeding from estimations of speed of the basic calcium flows made on the basis of mathematical modeling. The conclusion about decrease in speed of bone tissue remodeling and strengthening of its resorption proves to be true by data of research of biochemical and endocrine markers.     

Autonomic cardiovascular and respiratory control during prolonged spaceflights aboard the International Space Station.

Impaired autonomic control represents a cardiovascular risk factor during long-term spaceflight. Little has been reported on blood pressure (BP), heart rate (HR), and heart rate variability (HRV) during and after prolonged spaceflight. We tested the hypothesis that cardiovascular control remains stable during prolonged spaceflight. Electrocardiography, photoplethysmography, and respiratory frequency (RF) were assessed in eight male cosmonauts (age 41-50 yr, body-mass index of 22-28 kg/m2) during long-term missions (flight lengths of 162-196 days). Recordings were made 60 and 30 days before the flight, every 4 wk during flight, and on days 3 and 6 postflight during spontaneous and controlled respiration. Orthostatic testing was performed pre- and postflight. RF and BP decreased during spaceflight (P < 0.05). Mean HR and HRV in the low- and high-frequency bands did not change during spaceflight. However, the individual responses were different and correlated with preflight values. Pulse-wave transit time decreased during spaceflight (P < 0.05). HRV reached during controlled respiration (6 breaths/min) decreased in six and increased in one cosmonaut during flight. The most pronounced changes in HR, BP, and HRV occurred after landing. The decreases in BP and RF combined with stable HR and HRV during flight suggest functional adaptation rather than pathological changes. Pulse-wave transit time shortening in our study is surprising and may reflect cardiac output redistribution in space. The decrease in HRV during controlled respiration (6 breaths/min) indicates reduced parasympathetic reserve, which may contribute to postflight disturbances.     

Mechanisms of changes in human hemodynamics under the conditions of microgravity and prognosis of postflight orthostatic stability

The mechanisms of hemodynamic responses to orthostatic stresses and orthostatic stability (OS) of cosmonauts were studied before and after short-and long-term spaceflights (SFs) using orthostatic tests, as well as before, during, and after SFs using ultrasonic methods in tests with exposure to lower body negative pressure (LBNP). The capacitance and distensibility of the veins of the lower extremities were studied using occlusive air plethysmography before, during, and after SFs of different durations. A stay in microgravity has been proved to result in detraining of, mainly, the vascular mechanisms of compensating orthostatic perturbations. It has been established that the decrease in OS under the influence of microgravity is determined by a reduction of the vasoconstrictive ability of large blood vessels of the lower extremities; an increase in venous distensibility and capacitance of the legs; and an impairment of blood flow regulation, which leads to a cerebral blood flow deficit in orthostatic stresses, and of the initial individual OS before the flight. The results of preflight studies of hemodynamics by ultrasonic methods at LBNP and the data of orthostatic tests before SFs make it possible to predict the degree of decrease of OS after an SF proceeding in the normal mode. At the same time, the data of ultrasonic blood flow examination provide more a accurate estimation of OS and make it possible to assess the physiological reserves of hemodynamic regulation and to reveal the loss of regulation capacity even in cases where integrated indices (heart rate and blood pressure) are within the normal ranges.     

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.     

Nine months in space: effects on human autonomic cardiovascular regulation.

We studied three Russian cosmonauts to better understand how long-term exposure to microgravity affects autonomic cardiovascular control. We recorded the electrocardiogram, finger photoplethysmographic pressure, and respiratory flow before, during, and after two 9-mo missions to the Russian space station Mir. Measurements were made during four modes of breathing: 1) uncontrolled spontaneous breathing; 2) stepwise breathing at six different frequencies; 3) fixed-frequency breathing; and 4) random-frequency breathing. R wave-to-R wave (R-R) interval standard deviations decreased in all and respiratory frequency R-R interval spectral power decreased in two cosmonauts in space. Two weeks after the cosmonauts returned to Earth, R-R interval spectral power was decreased, and systolic pressure spectral power was increased in all. The transfer function between systolic pressures and R-R intervals was reduced in-flight, was reduced further the day after landing, and had not returned to preflight levels by 14 days after landing. Our results suggest that long-duration spaceflight reduces vagal-cardiac nerve traffic and decreases vagal baroreflex gain and that these changes may persist as long as 2 wk after return to Earth.     

Dependence of the circulation system functioning on cosmonaut age according to the results of physical loading tests on a veloergometer

Age-related hemodynamic reactions to the standard incremental physical loading tests on a cycle ergometer were assessed in cosmonauts before and during extended space missions. Analysis of the data from 353 tests performed with 63 cosmonauts differentiated into three age groups (30–39, 40–49, and 50–55 years old) showed changes in adaptive-compensatory hemodynamic responses to microgravity and physical loading depending on age. The consistent gradual degradation of the heart chronotropic function with age can be interpreted as a symptom of declining cardiovascular reactivity. In orbit, the cardiac output volume depended mainly on heart rate and blood pressure (i.e., vascular tone).     

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.     

Impaired cerebrovascular autoregulation and reduced CO₂ reactivity after long duration spaceflight

Long duration habitation on the International Space Station (ISS) is associated with chronic elevations in arterial blood pressure in the brain compared with normal upright posture on Earth and elevated inspired CO(2). Although results from short-duration spaceflights suggested possibly improved cerebrovascular autoregulation, animal models provided evidence of structural and functional changes in cerebral vessels that might negatively impact autoregulation with longer periods in microgravity. Seven astronauts (1 woman) spent 147 ± 49 days on ISS. Preflight testing (30-60 days before launch) was compared with postflight testing on landing day (n = 4) or the morning 1 (n = 2) or 2 days (n = 1) after return to Earth. Arterial blood pressure at the level of the middle cerebral artery (BP(MCA)) and expired CO(2) were monitored along with transcranial Doppler ultrasound assessment of middle cerebral artery (MCA) blood flow velocity (CBFV). Cerebrovascular resistance index was calculated as (CVRi = BP(MCA)/CBFV). Cerebrovascular autoregulation and CO(2) reactivity were assessed in a supine position from an autoregressive moving average (ARMA) model of data obtained during a test where two breaths of 10% CO(2) were given four times during a 5-min period. CBFV and Doppler pulsatility index were reduced during -20 mmHg lower body negative pressure, with no differences pre- to postflight. The postflight indicator of dynamic autoregulation from the ARMA model revealed reduced gain for the CVRi response to BP(MCA) (P = 0.017). The postflight responses to CO(2) were reduced for CBFV (P = 0.056) and CVRi (P = 0.047). These results indicate that long duration missions on the ISS impaired dynamic cerebrovascular autoregulation and reduced cerebrovascular CO(2) reactivity.     

Orthostatic blood pressure control before and after spaceflight, determined by time-domain baroreflex method.

Reduction in plasma volume is a major contributor to orthostatic tachycardia and hypotension after spaceflight. We set out to determine time- and frequency-domain baroreflex (BRS) function during preflight baseline and venous occlusion and postflight orthostatic stress, testing the hypothesis that a reduction in central blood volume could mimic the postflight orthostatic response. In five cosmonauts, we measured finger arterial pressure noninvasively in supine and upright positions. Preflight measurements were repeated using venous occlusion thigh cuffs to impede venous return and "trap" an increased blood volume in the lower extremities; postflight sessions were between 1 and 3 days after return from 10- to 11-day spaceflight. BRS was determined by spectral analysis and by PRVXBRS, a time-domain BRS computation method. Although all completed the stand tests, two of five cosmonauts had drastically reduced pulse pressures and an increase in heart rate of approximately 30 beats/min or more during standing after spaceflight. Averaged for all five subjects in standing position, high-frequency interbeat interval spectral power or transfer gain did not decrease postflight. Low-frequency gain decreased from 8.1 (SD 4.0) preflight baseline to 6.8 (SD 3.4) postflight (P = 0.033); preflight with thigh cuffs inflated, low-frequency gain was 9.4 (SD 4.3) ms/mmHg. There was a shift in time-domain-determined pulse interval-to-pressure lag, Tau, toward higher values (P < 0.001). None of the postflight results were mimicked during preflight venous occlusion. In conclusion, two of five cosmonauts showed abnormal orthostatic response 1 and 2 days after spaceflight. Overall, there were indications of increased sympathetic response to standing, even though we can expect (partial) restoration of plasma volume to have taken place. Preflight venous occlusion did not mimic the postflight orthostatic response.     

Effect of directional switching frequency on toluene degradation in a vapor-phase bioreactor

A potential method to improve biomass distribution and the stability of vapor-phase bioreactors is to operate them in a directionally switching mode such that the contaminant air stream direction is periodically reversed through the reactor. In this study, the effect of switching frequency (SF) on bioreactor performance and biodegradation activity was investigated at 1-, 3- and 7-day SFs using toluene as a model compound. Rapid losses of biodegradation capacity and serious bioreactor instability were observed in the bioreactor operated at a 1-day SF. It is hypothesized that the frequent dynamic loading conditions at the 1-day SF hindered biofilm development and ultimately bioreactor stability. In contrast, bioreactors operated at the 3- and 7-day SFs achieved overall removal efficiencies of greater than 99% for 72 and 59 days of operation, respectively. Following each air-stream reversal, the bioreactor operated at the 7-day SF required 48 h to fully restore biodegradation capacity in the inlet bioreactor section. The 1-day SF bioreactor required no such reacclimation period. The toluene-degrading activity in the inlet section of the 7-day SF bioreactor dropped by 71% during the 7-day cycle, whereas it decreased by only 11% in the inlet of the 3-day SF bioreactor. These declines suggest that continuous or near-continuous exposure to toluene can inhibit microbial activity. Of the three SFs examined, the 3-day SF yielded the most efficient bioreactor performance by balancing reacclimation requirements with biodegradation activity losses.     

Visual–manual tracking after long spaceflights

This study presents the results of the pre- and postflight clinical and physiological examination (CPE) and scientific experiment “Sensory Adaptation-2” at the Gagarin Research and Test Cosmonaut Training Center, which involved 14 Russian cosmonauts, crewmembers of long-term international spaceflights ISS-28/29 to ISS 36/37, who were in microgravity from 159 to 195 days. The cosmonauts were aged 35–50 years. The studies were conducted twice before the spaceflight (the background), as well as on days R+1(2), R+4(5), and R+8(9) after landing. In the study of visual–manual tracking (VMT), eye movements were recorded by the electrooculography method (EOG), and hand movements were recorded by a joystick (the screen represented the current tilt angle of a joystick handle). The examinations were conducted using stimulation computer programs, were presented to an examined subject on the screen of the Sensomotor hardware–software complex. The examinations took place in the dialog mode and included the EOG calibration; VMT within ±10° on the screen with blank background (the smooth linear and sinusoidal movement of a point target with a frequency of 0.16 Hz in the vertical and horizontal directions). The study estimated the time, amplitude, and velocity characteristics of visual and manual tracking (VT and MT), including the effectiveness (ec) and gain (gc) coefficients as the ratios of the amplitude and velocity of eye/hand movements to the amplitude and velocity of the visual stimulus. The study of the vestibular function (VF) was performed before and after the spaceflight using videooculography. The static torsion otolith–cervical–ocular reflex (OCOR), dynamic vestibular–cervical–ocular reactions (VCOR), vestibular reactivity, and spontaneous eye movements were assessed. The study of VF in the first postflight days has shown a sharp decrease (up to its complete absence) of static vestibular excitability accompanied by the increased dynamic reactivity of the vestibular system. The study of VTM in the first postflight days has shown a significant decrease in the ec and gc of VT as well as correlations between the parameters of VT and MT and between the parameters of VF and VT and has not found a correlation between the parameters of VF and MT. The conditions of the spaceflight have been revealed to affect the accuracy of VT more strongly than the accuracy of MT. A complete return of the characteristics of VMT and VF to the baseline was observed on R+8(9) days after the spaceflight.     

Movement of stress fibers away from focal adhesions identifies focal adhesions as sites of stress fiber assembly in stationary cells

Force generated in contractile actin filament bundles (stress fibers-SFs) is transmitted to the extracellular matrix (ECM) via linker proteins and transmembrane integrins at focal adhesions (FAs). Though it has long been known that actin is rapidly exchanged in FAs, the connection between SFs and FAs has not been studied in detail. We introduced fiduciary marks on SFs by expressing GFP-palladin or GFP-alpha-actinin-1, which are both FA and dense body proteins, and by pattern bleaching of GFP-actin. Following fiduciary marks on SFs over time by time-lapse fluorescence microscopy, we detected assembly of SFs at FAs in stationary cells resulting in movement of SFs away from FAs with a velocity of 0.2-0.4 microm/min. Visualization of FAs in GFP-palladin/DsRed-paxillin double transfected cells showed that SF elongation was not accompanied by a change in FA length. SF elongation at FAs depended on actin polymerization and force as demonstrated by inhibitors of actin polymerization (cytochalasin D, jasplakinolide) and inhibitors of myosin-dependent contraction (blebbistatin, Y-27632), respectively. Our finding of SF assembly at FAs has important implications for SF formation, force transmission, and tension distribution within the actin cytoskeletal network of stationary cells.     

Calcium metabolism and its regulation in man during adaptation to microgravitation

This work generalizes the results of studies of calcium metabolism in the participants of long-term space flights of 30 to 438 days on the Salyut and Mir orbital stations during 1978–1998. The results of pre- and postflight examination of 44 cosmonauts (18 subjects participated twice in long-term space flights) were analyzed. After space flights of medium (of 3 to 6 months) and long (of 6 to 14 months) duration, the total blood calcium content was increased, mainly due to its ionized fraction; the blood level of parathyroid hormone was significantly increased and the level of calcitonin was decreased. The content of osteocalcin was increased after space flights. Calcium kinetics was studied using stable isotopes in three cosmonauts before, during, and after the 115-day flight. During the flight, intestinal absorption of calcium and its gastrointestinal excretion were decreased, whereas its renal excretion was increased. Early postflight intestinal absorption was, on average, lower than during the flight, whereas intestinal excretion increased. Both renal and intestinal excretion of calcium were not normalized 3.5 to 4.5 months after the glight. The mathematical models used for evaluating the rates of main calcium flows revealed increased bone tissue resorption that resulted in the negative bone balance during the flight. The conclusion about the decreased rate of bone tissue remodeling and its increased resorption was confirmed by biochemical data, including endocrine markers.     

Strain waveform dependence of stress fiber reorientation in cyclically stretched osteoblastic cells: effects of viscoelastic compression of stress fibers

Actin stress fibers (SFs) of cells cultured on cyclically stretched substrate tend to reorient in the direction in which a normal strain of substrate becomes zero. However, little is known about the mechanism of this reorientation. Here we investigated the effects of cyclic stretch waveform on SF reorientation in osteoblastic cells. Cells adhering to silicone membranes were subjected to cyclic uniaxial stretch, having one of the following waveforms with an amplitude of 8% for 24 h: triangular, trapezoid, bottom hold, or peak hold. SF reorientation of these cells was then analyzed. No preferential orientation was observed for the triangular and the peak-hold waveforms, whereas SFs aligned mostly in the direction with zero normal strain (~55°) with other waveforms, especially the trapezoid waveform, which had a hold time both at loaded and unloaded states. Viscoelastic properties of SFs were estimated in a quasi-in situ stress relaxation test using intact and SF-disrupted cells that maintained their shape on the substrate. The dynamics of tension F(SFs) acting on SFs during cyclic stretching were simulated using these properties. The simulation demonstrated that F(SFs) decreased gradually during cyclic stretching and exhibited a compressive value (F(SFs) < 0). The magnitude and duration time of the compressive forces were relatively larger in the group with a trapezoid waveform. The frequency of SF orientation had a significant negative correlation with the applied compressive forces integrated with time in a strain cycle, and the integrated value was largest with the trapezoid waveform. These results may indicate that the applied compressive forces on SFs have a significant effect on the stretch-induced reorientation of SFs, and that SFs realigned to avoid their compression. Stress relaxation of SFs might be facilitated during the holding period in the trapezoid waveform, and depolymerization and reorientation of SFs were significantly accelerated by their viscoelastic compression.     

Adaptation of the water-electrolyte metabolism to space flight and at its imitation

A fifty-year study of water-electrolyte metabolism, the condition of the water medium of the body, and hormonal regulation during space flights and the postflight period or their on-ground modeling (hypokinesia, bed rest, immersion, etc.) has shown the important role of water-salt homeostasis in adaptation of the human and animal body to weightlessness. It has been revealed that, in weightlessness, the conditions for the development of a negative balance of a liquid (hydrohydration) and basic electrolytes are created. After termination of long space flights, attributes of the development of adaptive reactions, compensating for the loss of extracellular liquid volume come to light. In order to assess the state of the kidneys and water-electrolyte metabolism in cosmonauts and investigators, functional load tests, and special methods of diagnostics were developed. This is the basis for the research aimed at improving the scheme of correction of the water balance of the body of cosmonauts at different stages of a flight.     

Sympathetic nervous activity decreases during head-down bed rest but not during microgravity.

We tested the hypothesis that sympathoadrenal activity in humans is low during spaceflight and that this effect can be simulated by head-down bed rest (HDBR). Platelet norepinephrine and epinephrine were measured as indexes of long-term changes in sympathoadrenal activity. Ten normal healthy subjects were studied before and during HDBR of 2-wk duration, as well as during an ambulatory study period of a similar length. Platelet norepinephrine concentrations (half-life = 2 days) were studied in five cosmonauts, 2 wk before launch, within 12 h after landing after 11-12 days of flight, and at least 2 wk after return to Earth. Because of the long half-life of platelet norepinephrine, data obtained early after landing would still reflect the microgravity state. Platelet norepinephrine decreased markedly during HDBR (P < 0.001), whereas there were no significant changes when subjects were ambulatory. Platelet epinephrine did not change during HDBR. During microgravity, platelet norepinephrine and epinephrine increased in four of the five cosmonauts. Platelet norepinephrine concentrations expressed in percentage of preflight and pre-HDBR values, respectively, were significantly different during microgravity compared with HDBR [153 +/- 28% (mean +/- SE) vs. 60 +/- 6%, P < 0.004]. Corresponding values for platelet epinephrine were also significant (293 +/- 85 vs. 90 +/- 12%, P < 0.01). The mechanism of the platelet norepinephrine and epinephrine response during spaceflight flight is most likely related to the concomitant decrease in plasma volume. HDBR cannot be applied to simulate changes in sympathoadrenal activity during microgravity.