Autonomic control of cardiovascular dynamics during weightlessness

High-Intensity Exercise Mitigates Cardiovascular Deconditioning During Long-Duration Bed Rest

Head-down-tilt bed rest (HDT) mimics the changes in hemodynamics and autonomic cardiovascular control induced by weightlessness. However, the time course and reciprocal interplay of these adaptations, and the effective exercise protocol as a countermeasure need further clarification. The overarching aim of this work (as part of a European Space Agency sponsored long-term bed rest study) was therefore to evaluate the time course of cardiovascular hemodynamics and autonomic control during prolonged HDT and to assess whether high-intensity, short-duration exercise could mitigate these effects. A total of n = 23 healthy, young, male participants were randomly allocated to two groups: training (TRAIN, n = 12) and non-training (CTRL, n = 11) before undergoing a 60-day HDT. The TRAIN group underwent a resistance training protocol using reactive jumps (5–6 times per week), whereas the CTRL group did not perform countermeasures. Finger blood pressure (BP), heart rate (HR), and stroke volume were collected beat-by-beat for 10 min in both sitting and supine positions 7 days before HDT (BDC−7) and 10 days after HDT (R+10), as well as on the 2nd (HDT2), 28th (HDT28), and 56th (HDT56) day of HDT. We investigated (1) the isolated effects of long-term HDT by comparing all the supine positions (including BDC−7 and R+10 at 0 degrees), and (2) the reactivity of the autonomic response before and after long-term HDT using a specific postural stimulus (i.e., supine vs. sitting). Two-factorial linear mixed models were used to assess the time course of HDT and the effect of the countermeasure. Starting from HDT28 onwards, HR increased (p < 0.02) and parasympathetic tone decreased exclusively in the CTRL group (p < 0.0001). Moreover, after 60-day HDT, CTRL participants showed significant impairments in increasing cardiac sympathovagal balance and controlling BP levels during postural shift (supine to sitting), whereas TRAIN participants did not. Results show that a 10-day recovery did not compensate for the cardiovascular and autonomic deconditioning following 60-day HDT. This has to be considered when designing rehabilitation programs—not only for astronauts but also in general public healthcare. High-intensity, short-duration exercise training effectively minimized these impairments and should therefore deserve consideration as a cardiovascular deconditioning countermeasure for spaceflight.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Effectiveness of daily short-duration standing in preventing post-suspension cardiovascular dysfunction in rats assessed by cardiovascular signal analysis

The aim of the present study was to clarify whether simulated microgravity-induced post-suspension cardiovascular deconditioning in rats could be prevented by daily short-duration standing (STD). Three groups of rats were used as subjects to perform the experiments. Compared to a control group of male Sprague-Dawley rats (CON), a group of rats with tail-suspension (SUS) for 28 d was used to simulate cardiovascular deconditioning due to microgravity. Another group of tailed-suspended rats with daily STD for 1 h was used to provide -GX (dorso-ventral) gravitational loading as countermeasure. In addition to hemodynamic changes to head-up tilt, blood pressure variability (BPV) signals were also analyzed by spectral and nonlinear analysis. The results showed 1) Blood pressure immediately decreased after head-up tilt in all three groups. After several minutes, blood pressure could restore to the initial condition in both CON and SUS+STD group, but the recovery process was slower in the latter group. In contrast, the recovery process was the slowest in the SUS group and could not restore to the initial level completely. 2) In frequency domain, power spectra has similar pattern in CON and SUS+STD group. However, they are quite different in the SUS group in that spectra peak is obvious increased in very low frequency and spectra is narrower and higher in high frequency.

Neurobehavioral coping to altered gravity: endogenous responses of neurotrophins

An altered gravitational environment represents a unique challenge for biological systems that have evolved against gravitational background. Ground-based and space research indicates that the developing nervous system is potentially affected by exposure to hyper/microgravity. With the construction of the orbiting International Space Station long-term research on the nervous system will be possible. With this perspective, we started ground-based studies to characterize mouse behavioral responses to rotation-induced 2 g hypergravity, using a custom-made centrifuge device. Brain levels of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) as well as NGF and BDNF expression and mast cell distribution in heart and lung, were evaluated and correlated with the changes in mouse behavior upon hypergravity exposure. Hypergravity strongly affected the spontaneous activity of the animals, selectively modifying mouse behavioral repertoire. Such changes were mainly related to variations in brain levels of NGF, while BDNF was slightly affected, thus confirming a role for these neurotrophins in neuronal plasticity underlying experience-induced neurobehavioral changes. Moreover, gender differences were observed in both behavioral and neurobiological responses to hypergravity. These results indicate that changes in the gravitational environment might represent a useful tool to investigate the neurobiological and behavioral responses to stressors and may provide insights into the mechanisms underlying development and plasticity of nervous system in brain, heart, and lung.

Increase in epinephrine-induced responsiveness during microgravity simulated by head-down bed rest in humans

The epinephrine (Epi)-induced effects on the sympathetic nervous system (SNS) and metabolic functions were studied in men before and during a decrease in SNS activity achieved through simulated microgravity. Epi was infused at 3 graded rates (0.01, 0.02, and 0. 03 microg. kg(-1). min(-1) for 40 min each) before and on the fifth day of head-down bed rest (HDBR). The effects of Epi on the SNS (assessed by plasma norepinephrine levels and spectral analysis of systolic blood pressure and heart rate variability), on plasma levels of glycerol, nonesterified fatty acids (NEFA), glucose and insulin, and on energy expenditure were evaluated. HDBR decreased urinary norepinephrine excretion (28.1 +/- 4.2 vs. 51.5 +/- 9.1 microg/24 h) and spectral variability of systolic blood pressure in the midfrequency range (16.3 +/- 1.9 vs. 24.5 +/- 0.9 normalized units). Epi increased norepinephrine plasma levels (P < 0.01) and spectral variability of systolic blood pressure (P < 0.009) during, but not before, HDBR. No modification of Epi-induced changes in heart rate and systolic and diastolic blood pressures were observed during HDBR. Epi increased plasma glucose, insulin, and NEFA levels before and during HDBR. During HDBR, the Epi-induced increase in plasma glycerol and lactate levels was more pronounced than before HDBR (P < 0.005 and P < 0.001, respectively). Epi-induced energy expenditure was higher during HDBR (P < 0.02). Our data suggest that the increased effects of Epi during simulated microgravity could be related to both the increased SNS response to Epi infusion and/or to the beta-adrenergic receptor sensitization of end organs, particularly in adipose tissue and skeletal muscle.