Effect of real and simulated weightlessness on the characteristics of the static otolith reflex

To determine the role of the support-proprioceptive factor in functioning of the vestibular system, in particular, the role of static torsional otolith-cervical-ocular reflex (OCOR), the latter was studied in 16 subjects after a seven-day “dry” horizontal immersion and in 14 cosmonauts after a prolonged exposure to weightlessness (for 126–195 days). OCOR was studied by the videooculography method during alternately tilting the head towards the right or left shoulder by an angle of 30° in the frontal plane before the flight and before immersion, as well as on days 1, 3, and 7 after the completion of the immersion experiment and on days 1 (2), 4 (5), and 8 (9) after the spaceflight. For the first time it was demonstrated that elimination of the support and minimizing the proprioceptive afferentation may lead to the absence or inversion of the static torsional OCOR, as well as to a positional nystagmus against the background of the inverted reflex. Comparison of OCOR in cosmonauts after prolonged exposure to weightlessness and in the subjects examined after immersion revealed similarity in this reaction. However, changes in OCOR after immersion were encountered only in 60% of the subjects, whereas after the spaceflight, in 90% of the cosmonauts examined. The post-flight changes in OCOR were more pronounced and long-lasting.     

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).     

Velocity of head movements and sensory-motor adaptation during and after short spaceflight

To investigate to time course of sensory-motor adaptation to microgravity, we tested spatially-directed voluntary head movements before, during and after short spaceflight. We also tested the re-adaptation of postural responses to sensory stimulation after space flight. The cosmonaut performed in microgravity six cycles of voluntary head rotation in pitch, roll and yaw directions. During the first days of weightlessness the angular velocity of head movements increased. Over the next days of microgravity the velocity of head movements gradually decreased. On landing day a significant decrease of head rotation velocity was observed compared to the head movement velocity before spaceflight. Re-adaptation to Earth condition measured by body sway on soft support showed similar time course, but re-adaptation measured by postural responses to vestibular galvanic stimulation was prolonged. These results showed that the angular velocity of aimed head movements of cosmonauts is a good indicator of sensory-motor adaptation in altered gravity conditions.     

Effects of long-term space flights on organization of horizontal gaze fixation reaction

Results of Russian-Austrian space experiment "Monimir" which was a part of international space program "Austromir" are presented in this paper. Characteristics of horizontal gaze fixation reaction (hGFR) to visual targets were analyzed. Seven crewmembers of "Mir" space station expeditions took part in the experiment. Experiments were carried out 4 times before space flight, 5 times in flight and 3-4 times after landing. There were revealed significant alterations in characteristics of gaze fixation reaction during flight and after its accomplishing, namely: an increase of the time of gaze fixation to the target, changes of eye and head movements' velocity and increase of the gain of vestibular-ocular reflex, that pointed out to the disturbances of the control mechanisms of vestibular-ocular reflex in weightlessness caused by changes of vestibular input's activity. There was discovered also the difference in the strategies of adaptation to microgravity conditions among the cosmonauts of flight and non-flight occupation: in the first group exposure to weightlessness was accompanied by gaze hypermetry and inhibition of head movements; in the second one--on the contrary--by increase of head movement velocity and decrease of saccades' velocity.     

Effect of a real and simulated weightlessness on characteristics of the static torsional otolith-cervical-ocular reflex

To determine the role of the support-proprioceptive factor in the functioning of the vestibular system, in particular the static torsional otolith-cervical-ocular reflex (OCOR), comparative OCOR studies with videooculography recording were performed after a 7-day "dry" horizontal immersion (16 immersion subjects) and after a prolonged (126 to 195 days) exposure to weightlessness (14 ISS cosmonauts). For the first time it was demonstrated that minimization of the support and propripceptive afferentation may results in an inversion or absence of the static torsional OCOR and the development of a positional nystagmus with an inverted reflex. A comparative OCOR data analysis of cosmonauts and immersion subjects has revealed similarity of responses. However, changes in OCOR after immersion were noted in only 60% of subjects, while after space fight, 90% of cosmonauts showed them. Post-flight changes were more frequent, marked and long-lasting.     

Characteristics of postural corrective responses before and after long-term spaceflight

Balance function is dramatically deteriorated after exposure to microgravity. The purpose of the present study was to investigate the role and the contribution of different gravity sensory systems to the development of balance impairment after long-term spaceflights. Postural perturbations (pushes to the chest) of the threshold, medium, and sub-maximal intensities were produced in eight cosmonauts before, and on the day 3, 7, and 11 following spaceflight. Postural corrective responses were analyzed by anterior-posterior body sway fluctuation and electromyographic activity of leg muscles. The characteristics of the postural corrective responses changed significantly on the day 3 following spaceflight: the amplitude of posterior sway caused by perturbation of threshold intensity was increased reaching 135% ofpreflight value; the corrective responses lasted more than 6 s in 50% of all trials, while it did not last more than 4 s in 96% before spaceflight. The EMG responses were characterized by increased contribution of medium- and long-latency reactions. On the day 11 following spaceflight, most of the characteristics of postural corrective responses were close to preflight values. We assumed that the balance alterations after spaceflight are caused by changes in weightlessness of functions of two main gravity sensory systems, namely, weight-bearing and vestibular one. The deficit of weight-bearing afferentation triggers a decline of the extensors' muscle tone, while changes of vestibular function cause a decline of accuracy of postural corrections.     

Characteristics of postural corrective responses before and after long-term spaceflights

Balance function is dramatically deteriorated after exposure to microgravity. The purpose of the present study was to investigate the role and the contribution of different gravity sensory systems to the development of balance impairment after long-term spaceflights. Postural perturbations (pushes to the chest) of the threshold, medium, and sub-maximal intensities were produced in eight cosmonauts before, and on the day 3, 7, and 11 following spaceflight. Postural corrective responses were analyzed by anterior-posterior body sway fluctuation and electromyographic activity of leg muscles. The characteristics of the postural corrective responses changed significantly on the day 3 following spaceflight: the amplitude of posterior sway caused by perturbation of threshold intensity was increased reaching 135% of preflight value; the corrective responses lasted more than 6 s in 50% of all trials, while it did not last more than 4 s in 96% before spaceflight. The EMG responses were characterized by increased contribution of medium- and long-latency reactions. On the day 11 following spaceflight, most of the characteristics of postural corrective responses were close to preflight values. We assumed that the balance alterations after spaceflight are caused by changes in weightlessness of functions of two main gravity sensory systems, namely, weight-bearing and vestibular one. The deficit of weight-bearing afferentation triggers a decline of the extensors’ muscle tone, while changes of vestibular function cause a decline of accuracy of postural corrections.     

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.     

Behavioral and functional vestibular disorders after space flight: I. Mammals

The review presents data on functional disorders in mammals caused by changes in the vestibular system after space flight. These data show that the mammalian vestibular system responds to weightlessness dissimilarly at different ontogenetic stages. During the embryonic period, orbital space flight conditions have a little effect on the developing vestibular system and even promote normal fetal development. During the early postnatal period, when optimal sensorymotor tactics arise, long-term exposure to space flight conditions leads to the development of novel, “extraterrestrial”, sensory-motor programs that may fixate in CNS for life. In adult individuals, substantial vestibular changes and disorders may occur immediately after landing depending on the weightlessness duration. An adult organism has to solve two concurrent and mutually conflicting problems: to adapt to weightlessness and not to adapt to it in order to facilitate readaptation after return. Thus, individuals have to counteract weightlessness to retain a maximum of their pre-flight health status. The means of such a counteraction have to be adjusted according to the weightlessness duration. It is noteworthy, however, that not all functional changes occurring in adult individuals under weightlessness can be adequately accounted for. Some of them can assume a chronic or even pathological character. The review raises for the first time the question of necessity to include into the scope of studies the effect of weightlessness on a senile (senescent) organism and its vestibular system. We believe that development of space gerontology as a special branch of space biology and medicine is undoubtedly of interest and may become practically important in the future in view of the ever-growing age of space explorers.     

Structural and functional organization of the vestibular apparatus in rats maintained under weightless conditions for 19.5 days aboard the satellite "Cosmos-782"]

Vestibular apparatus was investigated in rats subjected to weightlessness for 19.5 days in the satelite "Cosmos-782" and experienced acceleration on launching and landing. Some structural and functional changes were noted. They were seen in otolith clinging to the utricular receptor surface and in the peripheral arrangement of the nucleolus in the nuclei of the receptor cells. It is also possible that increased edema of the vestibular tissue resulted in destruction of some receptor cells, and within the otolith--changes in the form and structure of otoconia. In the horizontal crista the cupula was separated.     

Behavioral lateralization and otolith asymmetry

Lateralized behavior is widespread among vertebrate animals and is determined primarily by structural-functional brain asymmetry as well as by the presence of somatic and visceral asymmetry. Some kinds of asymmetric reactions are suggested to be due to the presence of asymmetry at the level of sense organs, in particular, of otolith organs. This review presents data on values and characters of otolith asymmetry (OA) in animals of various species and classes and on the effect of weightlessness and hypergravity on OA; the issue of the effect of OA on vestibular and auditory functions also is considered. In symmetric vertebrates, OA was shown to be fluctuating, and its coefficient χ ranges from ?0.2 to +0.2; in the overwhelming majority of individuals, |χ| < 0.06. The low OA level enables the paired otolith organs to work in coordination; this is why the OA level is equally low regardless of the individual taxonomic and ecological position, size, age, and otolith growth rate. Individuals with the abnormally high OA level can experience difficulties in analyzing auditory and vestibular stimuli; therefore, most of such individuals are eliminated by natural selection. Unlike symmetric vertebrates, labyrinths of many Pleuronectiformes have pronounced OA-otoliths in the lower labyrinth, on average, are significantly heavier than those in the upper labyrinth. The organs of flatfish represent the only example when OA, being directional, seem to play an essential role in lateralized behavior and are suggested to be used in the spatial localization of the source of sound. The short-term weightlessness and relatively weak hypergravity (≤ 2g) do not affect OA. However, it cannot be ruled out that the long-term weightlessness and hypergravity ≥ 3g as well as some diseases and age-related changes can enhance OA and cause some functional disturbances.     

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.     

Rocking or Rolling – Perception of Ambiguous Motion after Returning from Space

The central nervous system must resolve the ambiguity of inertial motion sensory cues in order to derive an accurate representation of spatial orientation. Adaptive changes during spaceflight in how the brain integrates vestibular cues with other sensory information can lead to impaired movement coordination, vertigo, spatial disorientation, and perceptual illusions after return to Earth. The purpose of this study was to compare tilt and translation motion perception in astronauts before and after returning from spaceflight. We hypothesized that these stimuli would be the most ambiguous in the low-frequency range (i.e., at about 0.3 Hz) where the linear acceleration can be interpreted either as a translation or as a tilt relative to gravity. Verbal reports were obtained in eleven astronauts tested using a motion-based tilt-translation device and a variable radius centrifuge before and after flying for two weeks on board the Space Shuttle. Consistent with previous studies, roll tilt perception was overestimated shortly after spaceflight and then recovered with 1–2 days. During dynamic linear acceleration (0.15–0.6 Hz, ±1.7 m/s²) perception of translation was also overestimated immediately after flight. Recovery to baseline was observed after 2 days for lateral translation and 8 days for fore–aft translation. These results suggest that there was a shift in the frequency dynamic of tilt-translation motion perception after adaptation to weightlessness. These results have implications for manual control during landing of a space vehicle after exposure to microgravity, as it will be the case for human asteroid and Mars missions.     

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.     

Effect of spaceflight on the subcutaneous venoarteriolar reflex in the human lower leg.

Whenever the legs are lowered in humans, a venoarteriolar reflex is activated by the hydrostatic distension of the venules. Through local axon reflexes, the adjacent arterioles are contracted to decrease blood flow and prevent formation of edema. Because the venoarteriolar reflex is activated by gravity, we tested the hypothesis that long-term weightlessness would attenuate it. The reduction in subcutaneous blood flow was measured by the (133)Xe washout technique just proximal to the ankle joint in dependent lower legs of eight supine astronauts, where the knee joint was passively bent by 90 degrees . The measurements were conducted before spaceflight and 3-6 h on landing following 4-6.5 mo in space. Activation of the venoarteriolar reflex reduced subcutaneous blood flow by 37 +/- 9% (P = 0.016) before flight and by 64 +/- 8% (P < 0.001) following landing with no statistical significant difference between the two reductions (P = 0.062). Therefore, our results show that the venoarteriolar reflex is not attenuated by weightlessness and therefore does not need the everyday stimulus of gravity to maintain efficiency.     

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.     

Altered gravity affects ventral root activity during fictive swimming and the static vestibuloocular reflex in young tadpoles (Xenopus laevis)

During early periods of life, modifications of the gravitational environment affect the development of sensory, neuronal and motor systems. The vestibular system exerts significant effects on motor networks that control eye and body posture as well as swimming. The objective of the present study was to study whether altered gravity (AG) affects vestibuloocular and spinal motor systems in a correlated manner. During the French Soyuz taxi flight Andromède to the International Space Station ISS (launch: October 21, 2001; landing: October 31, 2001) Xenopus laevis embryos were exposed for 10 days to microgravity (microg). In addition, a similar experiment with 3g-hypergravity (3g) was performed in the laboratory. At onset of AG, embryos had reached developmental stages 24 to 27. After exposure to AG, each tadpole was tested for its roll-induced vestibuloocular reflex (rVOR) and 3 hours later it was tested for the neuronal activity recorded from the ventral roots (VR) during fictive swimming. During the post-AG recording periods tadpoles had reached developmental stages 45 to 47. It was observed that microgravity affected VR activity during fictive swimming and rVOR. In particular, VR activity changes included a significant decrease of the rostrocaudal delay and a significant increase of episode duration. The rVOR-amplitude was transiently depressed. Hypergravity was less effective on the locomotor pattern; occurring effects on fictive swimming were the opposite of microg effects. As after microgravity, the rVOR was depressed after 3g-exposure. All modifications of the rVOR and VR-activity recovered to normal levels within 4 to 7 days after termination of AG. Significant correlations between the rVOR amplitude and VR activity of respective tadpoles during the recording period have been observed in both tadpoles with or without AG experience. The data are consistent with the assumptions that during this period of life which is characterized by a progressive development of vestibuloocular and vestibulospinal projections (i) microgravity retards the development of VR activity while hypergravity weakly accelerates it; (ii) that microgravity retards the rVOR development while hypergravity caused a sensitization, and that (iii) AG-induced changes of VR activity during fictive swimming have a vestibular origin.     

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.     

Decreased non-MHC-restricted (CD56+) killer cell cytotoxicity after spaceflight.

Cytotoxic activity of non-major histocompatibility complex-restricted (CD56+) (NMHC) killer cells and cell surface marker expression of peripheral blood mononuclear cells were determined before and after spaceflight. Ten astronauts (9 men, 1 woman) from two space shuttle missions (9- and 10-day duration) participated in the study. Blood samples were collected 10 days before launch, within 3 h after landing, and 3 days after landing. All peripheral blood mononuclear cell preparations were cryopreserved and analyzed simultaneously in a 4-h cytotoxicity (51)Cr release assay using K562 target cells. NMHC killer cell lytic activity was normalized per 1,000 CD56+ cells. When all 10 subjects were considered as one study group, NMHC killer cell numbers did not change significantly during the three sampling periods, but at landing lytic activity had decreased by approximately 40% (P < 0.05) from preflight values. Nine of ten astronauts had decreased lytic activity immediately after flight. NMHC killer cell cytotoxicity of only three astronauts returned toward preflight values by 3 days after landing. Consistent with decreased NMHC killer cell cytotoxicity, urinary cortisol significantly increased after landing compared with preflight levels. Plasma cortisol and ACTH levels at landing were not significantly different from preflight values. No correlation of changes in NMHC killer cell function or hormone levels with factors such as age, gender, mission, or spaceflight experience was found. After landing, expression of the major lymphocyte surface markers (CD3, CD4, CD8, CD14, CD16, CD56), as determined by flow cytometric analysis, did not show any consistent changes from measurements made before flight.