Seven-day dry immersion: relations between water-electrolyte and cardiovascular responses

The paper presents the results of the experiment with 7-day dry immersion. Eight healthy men were studied before, during and after exposure. It is shown that the primary response involves hemodynamic and water-electrolyte changes. The mechanisms of cardiovascular adaptation to immersion conditions are revealed. In particular, some electrophysiological shifts in propagation of myocardial excitation are found, leading to increase in "myocardium" index, estimated by analysis of natural small oscillations in the amplitude of the PQRST complex with dispersion ECG mapping. Revealed significant reduction of functional reserves of the cardiovascular regulatory mechanisms in the process of adaptation to dry immersion is of great practical importance. The results suggest that water-electrolyte shifts induced by dry immersion are the basis and the first phase of the further changes in autonomic regulation and myocardial functional state.     

Functional load tests in the assessment of the state of the kidneys and water-electrolyte metabolism

The use of functional load tests to assess the specific features of water-electrolyte metabolism under extreme conditions is considered, with special emphasis on their implications for space physiology and medicine. Water and mineral metabolism, the kidney function, and their hormonal regulation during simulation experiments, as well as in spaceflights and in the readaptation period, play an important role in human adaptation to new conditions of vital activity. In order to assess the state of the kidneys and water-electrolyte metabolism in cosmonauts and investigators, functional load tests were developed. They enabled us not only to gain insight into the mechanisms of osmotic and volumetric regulation but also to develop countermeasures to correct unfavorable shifts in water-salt homeostasis.     

The water-salt balance and renal function in space flights and in model experiments

Study of a condition of mineral and water-electrolyte metabolism, function of kidneys, and their hormonal regulation during model experiments (hypokinesia, bed rest, immersion etc.), and also in space flights and in readaptation period, has shown a major role of water-electrolyte homeostasis during general adaptation of humans and animals to new conditions of life and to conditions of weightlessness in particular. The change in regulation of volumes of fluid milieu in an initial period of weightlessness was shown to be the consequence of redistribution of blood and hemodynamics of the shifts resulting in change of production of volume-regulation hormones, formation of negative water balance, and redistribution of fluid in the organism among various fluid compartments. At later stages of flight or long-term hypokinesia, a change of water-electrolyte homeostasis occurs with a decrease in the kidneys excretion of sodium, and diuresis, but with an increased excretion of calcium and production of ADH and RAAS hormones. Following returning to earth gravitation, the majority of astronauts have adaptive reactions, compensating for the loss extracellular fluid and mineral substances and formation of "earth" water-electrolyte homeostasis. For estimation of water-electrolyte homeostasis and the functions of kidneys in astronauts, various functional loading tests have been developed. The developed system of preventive maintenance is successfully used for abolition of adverse changes at various stages of space flight and in readaptation period.     

Physiological [correction of Physilogical] cost of standard physical load after 7-day dry immersion

Working capacity and cardiovascular response to exercise were studied in men subjected to 7-days of dry immersion. Heart rate, ventilation, and oxygen uptake and consumption were measured before immersion and on the 4th and 7th days of immersion. No significant changes in heart rate were found, but some changes in ventilation and oxygen uptake dynamics indicate that gas exchange may be altered during immersion. Physiological adaptation and individual differences are discussed.     

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.     

Vertical and horizontal posture during suited immersion: similarity and differences according to some body functions

The role of axial body orientation was weighed during acute adaptation to suit immersion (SI) in 3 hr vertical (SIv, n=4) exposure series, and 3 d experiments in which six volunteers participated in both SIv and SIh. Various types of SI recruited different hormonal mechanisms to induce similar reactions of the water-electrolyte metabolism (WEM). Changes in the body position relative to the gravity vector modified equally strength and dynamics of the WEM regulation. The experimental data suggest that osmo- and volume-regulation reactions to SIv shown the response to the actual volumetric stimulus against the "distributed" weight-bearing reaction. They are less vigorous as compared to SIh and do not reach maximum within 3 hrs. Physiological reactions to SIh were marked by greater effects of ADH on the kidney when compared with dry immersion, and an additional stimulation of aldosterone secretion by the sympathetic nervous system and/or ACTH. SIh caused rapid and more heavy symptoms of motion sickness that correlated with body mass reduction, negative water balance, degradation of tolerance of classic physical tests executed after SI. Three-days SIv, as opposed to SIh, resulted in more profound hypervolemia in the upper body of the test-subjects.     

Adaptations of heart activity to physical exercise in female physical education teachers in the European North

Characteristics of heart activity were studied in female residents of the European North with ages between 25 and 59 years who were adapted to physical exercise. The results of studies of the biological age and the degree of training are presented. The mechanisms of adaptation and the specific features of the cardiovascular system (CVS) response to physical exercise depending on the duration of professional experience of subjects were determined. The greatest changes in the functional CVS characteristics were observed in the youngest (25–35 years) and oldest (46–59 years) subjects. This was shown to depend on the type of adaptation, age-specific characteristics, and moderately severe climatic conditions. Comparison of groups of subjects with different durations of professional experience showed that the speed of nervous processes decreased, the regulation of blood circulation became inert, and age-related shifts in some of the variables accelerated. These changes are considered to be the result of an increased expenditure of the physiological reserves of the body under different environmental conditions.     

Adaptation of the Human Body to Simulated Weightlessness: Clinical Studies

Problems of adaptation of functional systems of the human body to conditions of continuous weightlessness are considered (prolonged stay under conditions of antiorthostatic hypokinesia and in an immersion medium). It was revealed that, during adaptation to these conditions, polymorphic clinicofunctional disorders develop, transforming into clinicophysiological syndromes, the most frequently observed being autonomic vascular malfunction, asthenoneurotic syndrome, detraining of the blood circulatory system, trophic and neuromuscular disorders, statokinetic syndrome, pain syndrome, and metabolic and hormonal disorders. The severity of the specified disorders and the duration of the recovery period depend on the duration of hypokinesia and the intensity and regularity of application of preventive measures during hypokinesia. The most probable pathophysiological mechanisms of adverse effects of continuous hypokinesia (maladaptation) on functional systems of the human body are described.     

Biomechanics and regulation of external respiration under conditions of five-day dry immersion

The functional state of external respiration and the features of its regulation in healthy persons were studied under conditions of microgravity simulated using dry immersion. The lung volume, the ratio of thoracic and abdominal components during quiet breathing and performing various respiratory maneuvers, as well as the parameters that characterize the regulation of breathing (the duration of breath holding and the ability to voluntarily control respiratory movements), were recorded during the baseline period, on days 2 and 4 of dry immersion, and after the end of the dry immersion. It has been shown that the breathing pattern did not significantly change under conditions of dry immersion compared to the baseline period; however, the inspiratory reserve volume increased (p < 0.05), while the expiratory reserve volume decreased (p < 0.01). Dry immersion did not alter pulmonary ventilation, yet most of the subjects trended toward an increase in the contribution of the abdominal component of breathing movements during quiet breathing and demonstrated a statistically significant increase in this parameter during the lung vital capacity maneuver. The durations of the inspiratory and expiratory maximal breath holding under conditions of immersion did not differ from the background values. During the immersion, the accuracy of voluntary control of breathing increased. We believe that immersion, similar to microgravity, leads to changes in the reserve lung volume, which are partly because of changes in the body position; changes in relative contributions of the thoracic and abdominal components in the breathing movements; and changes in voluntary breath regulation.     

Dry immersion effects on the mechanisms of metaboreflex regulation of hemodynamics during exercise

The effects of four-day dry immersion on metaboreflex regulation of hemodynamics were evaluated during local static exercise (30% of the maximum voluntary contraction) of the calf plantar flexors. One group of immersed subjects received low-frequency electrostimulation of their leg muscles to decrease the immersion effect on the EMG of exercising muscles. Metaboreflex regulation was evaluated by comparison of cardiovascular responses to physical loads with and without post-exercise circulatory occlusion. Immersion slightly increased the heart rate (HR) and reduced the systolic blood pressure in resting subjects; however, it did not have a distinct effect on blood pressure (BP) and HR during exercise or metaboreflex potentiation of hemodynamic shifts.     

Fundamental and applied objectives of investigation in dry immersion

The results of extended comparative research in the effect of hypogravity on the motor system in space flights and ground-based experiments have shown that “dry” immersion (DI) is the most adequate model of microgravity—the time of development, and the volume and depth of structural and functional motor disorders in DI are very close to what is observed in real microgravity. The high intensity and speed of development of hypogravity effects during immersion hypokinesia in comparison with bed rest hypokinesia, differing from DI only by the level of support deafferentation, promoted an insight into the leading (triggering) role of support lessness in the genesis of microgravity-induced syndromes of muscular deconditioning and hypogravitational ataxia. The involvement and pathways of support afferentation within the muscular system were experimentally studied and verified. The mechanisms of the development of changes in the activity of the system mechanisms remain much less investigated. These issues, as well as some new approaches for the elimination of the negative effects of hypogravity, were the subjects of investigation in the program of a complex dry immersion experiment, the results of which are presented in this issue of the journal.     

Identification of proteins of cardiovascular system in healthy subjects’ urine during “dry” immersion

We analyzed the urine proteome in 14 healthy volunteers who were subjected to 5-day dry immersion using proteomic analysis methods and bioinformatics approach. We identified nine proteins related to the cardiovascular system. It was shown that 5-day dry immersion modifies the urine proteomic profile, indicating renal, endocrine, circulatory, and metabolic changes. Most of these changes are characterized by both a very rapid development and very rapid restoration within return to normal conditions.     

Craniomandibular System and Postural Balance after 3-Day Dry Immersion

The objective of the study was to determine the influence of simulated microgravity by exposure to dry immersion on the craniomandibular system. Twelve healthy male volunteers participated in a 3-day dry immersion study. Before and immediately after exposure we measured maximal bite force using piezoresistive sensors. The mechanical properties of the jaw and cervical muscles were evaluated before, during, and after dry immersion using MyotonPRO. Because recent studies reported the effects of jaw motor activity on the postural stability of humans, stabilometric measurements of center of pressure were performed before and after dry immersion in two mandibular positions: rest position without jaw clenching, and intercuspidal position during voluntary teeth clenching. Results revealed no significant changes of maximal bite force after dry immersion. All postural parameters were significantly altered by dry immersion. There were however no significant differences in stabilometric data according to mandibular position. Moreover the masseter tonicity increased immediately after the end of dry immersion period. Dry immersion could be used as a valid model for studying the effects of microgravity on human subjects. However, 3 days appear insufficient in duration to evaluate the effects of weightlessness on maximal bite force. Our research suggests a link between postural disturbance after dry immersion and masseter tonicity.     

Predicting shifts in the functional composition of tropical forests under increased drought and CO2 from trade‐offs among plant hydraulic traits

Tropical forest responses are an important feedback on global change, but changes in forest composition with projected increases in CO₂ and drought are highly uncertain. Here we determine shifts in the most competitive plant hydraulic strategy (the evolutionary stable strategy or ESS) from changes in CO₂ and drought frequency and intensity. Hydraulic strategies were defined along a spectrum from drought avoidance to tolerance by physiology traits. Drought impacted competition more than CO₂, with elevated CO₂ reducing but not reversing drought‐induced shifts in the ESS towards more tolerant strategies. Trait plasticity and/or adaptation intensified these shifts by increasing the competitive ability of the drought tolerant relative to the avoidant strategies. These findings predict losses of drought avoidant evergreens from tropical forests under global change, and point to the importance of changes in precipitation during the dry season and constraints on plasticity and adaptation in xylem traits to forest responses.     

Lipid peroxidation rate and the antioxidant protection system during a three-day dry immersion experiment

The content of lipid peroxidation products (diene conjugates, malondialdehyde, Schiff bases) and antioxidant defense system indices (the main lipid antioxidant tocopherol and the level of general antioxidant activity) were measured in the blood serum of five male volunteers aged 25?C40 years in a three-day dry immersion experiment. During the immersion test, no deviations of indices from the background values were found. An increase in the tocopherol concentration within 2 h after the beginning of the experiment was the only exception. A significant increase in the concentration of lipid peroxidation products, particularly, diene conjugates, was observed 2 h after immersion completion during the reconditioning period. However, the tocopherol content was significantly lower than the background values. It is concluded that the subjects?? adaptation to simulated microgravity conditions displays no pronounced stress component, whereas bringing back to normal vital functions after exposure to immersion induces a pronounced stress reaction illustrated by a significant increase in the lipid peroxidation product levels against a background of a decrease in the functional activity of the antioxidant defense system.     

The dynamics of nutrient utilization and growth of apple root stock ‘M9 EMLA’ in temporary versus continuous immersion bioreactors

The present study investigated the dynamics of nutrient utilization and various growth and physiological parameters during in vitro proliferation of apple root stock ‘M9 EMLA’ in two different bioreactor systems, i.e. temporary and continuous immersions. Individual shoots obtained from temporary immersion system had higher dry mass and were of better quality than those obtained from continuous immersion. In continuous immersion bioreactor, apple shoots appeared to utilize more nutrients from liquid culture medium than that from temporary immersion. The shoot growth was limited by the availability of phosphate and nitrogen in continuous immersion system. The shoots produced in temporary immersion bioreactor showed higher photosynthetic rate, maximum quantum yield of photosystem-II and slow but steady rate of nutrient absorption, indicating the occurrence of higher photomixotrophic metabolism. The study also showed that high level of antioxidant scavenging enzymes in shoots grown in continuous immersion system induced physiological changes to foster adaptation to stresses.     

Noninvasive beat-to-beat stroke volume computation during acute hydrostatic pressure changes in parabolic flight

A three-element model of the cardiovascular system was used to monitor stroke volume (SV) changes during parabolic flight. Aortic blood flow was estimated from continuous arterial finger pressure and SV computed by integrating simulated aortic flow during each systole. SV was significantly higher in microgravity (microgravity) compared to 1 G whereas in hypergravity (hG), SV was significantly lower. Exponential SV transients were observed after the transitions to and from microgravity and the succeeding or preceeding hG phases. These SV transients present different time constants, which reflect two different mechanisms of cardiovascular adaptation to sudden gravitational changes. These results show that beat-to-beat computation of SV provides noninvasive information on circulatory adaptation to acute hydrostatic pressure changes.     

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.     

Immune system parameters of healthy humans in a chamber experiment with hyperbaric oxygen—nitrogen—argon air

Data on the immune system state during nine-day isolation in a pressurized compartment with normoxic or hypoxic oxygen-nitrogen-argon air (ONAA) are presented. The results showed that adaptation of the immune system of healthy humans to isolation effects is accompanied by changes in a number of parameters of innate and adaptive immunity. The time course and intensity of shifts caused by the set of experimental factors are likely to be dependent on the hereditary mechanisms of the immunological reactivity. To prevent adaptation failure of the immune system, monitoring of immune homeostasis is necessary.     

The effect of repeated mild cold water immersions on the adaptation of the vasomotor responses

There are several types of cold adaptation based on the alteration of thermoregulatory response. It has been thought that the temperature of repeated cold exposures during the adaptation period is one of the factors affecting the type of cold adaptation developed. This study tested the hypothesis that repeated mild cold immersions would induce an insulative cold adaptation but would not alter the metabolic response. Seven healthy male participants were immersed to their xiphoid process level repeatedly in 26°C water for 60 min, 3 days every week, for 4 weeks. During the first and last exposure of this cold acclimation period, the participants underwent body immersion tests measuring their thermoregulatory responses to cold. Separately, they conducted finger immersion into 5°C water for 30 min to assess their cold-induced vasodilation (CIVD) response before and after cold acclimation. During the immersion to xiphoid process, participants showed significantly lower mean skin temperature and skin blood flow in the forearm post-acclimation, while no adaptation was observed in the metabolic response. Additionally, blunted CIVD responses were observed after cold acclimation. From these results, it was considered that the participants showed an insulative-type of cold acclimation after the repeated mild cold immersions. The major finding of this study was the acceptance of the hypothesis that repeated mild cold immersion was sufficient to induce insulative cold adaptation but did not alter the metabolic response. It is suggested that the adaptation in the thermoregulatory response is specific to the response which is repeatedly stimulated during the adaptation process.