Influence of head-down bed rest on the circadian rhythms of hormones and electrolytes involved in hydroelectrolytic regulation

Effect of 60 days of head down tilt bed rest on amplitude and phase of rhythms in physiology and sleep in men

Twenty-four-hour rhythms in physiology and behaviour are shaped by circadian clocks, environmental rhythms, and feedback of behavioural rhythms onto physiology. In space, 24 h signals such as those associated with the light-dark cycle and changes in posture, are weaker, potentially reducing the robustness of rhythms. Head down tilt (HDT) bed rest is commonly used to simulate effects of microgravity but how HDT affects rhythms in physiology has not been extensively investigated. Here we report effects of -6° HDT during a 90-day protocol on 24 h rhythmicity in 20 men. During HDT, amplitude of light, motor activity, and wrist-temperature rhythms were reduced, evening melatonin was elevated, while cortisol was not affected during HDT, but was higher in the morning during recovery when compared to last session of HDT. During recovery from HDT, time in Slow-Wave Sleep increased. EEG activity in alpha and beta frequencies increased during NREM and REM sleep. These results highlight the profound effects of head-down-tilt-bed-rest on 24 h rhythmicity.

Individual Variation Exists Within the Psychological Response to Hypoxic Bed Rest: A Retrospective Analysis

Individual variation is of interest to Space Agency's, which cannot be explored with astronauts due to anonymity. We retrospectively analysed data collected throughout three projects (LunHab: 10-day male, PlanHab: 21-day male, and FemHab: 10-day female) to elucidate the potentially masked individual variation in the psychological responses to bed rest. The Profile of Mood State (POMS) and Positive and Negative Affect Schedule (PANAS) - instruments used to asses psychological state - and Lake Louise Mountain Sickness (LLMS) scores were collected prior to, following and throughout three interventions: 1: normoxic bed rest 2: hypoxic bed rest and 3: hypoxic ambulatory confinement. Total Mood Disturbance (TMD) was calculated from the POMS results, positive affect (PA), and negative affect (NA) from PANAS. The three instruments were included in a latent class mixed model. TMD, NA, and LLMS were included in a four-class model, with each class representing a specific type of response (Class 1: descending, Class 2: flat, Class 3: somewhat flat, Class 4: ascending). Responses for PA were assigned to only two classes (Classes 1 and 2). 54.55% or 24 participants were included in Class 2 (TMD, NA, and LLMS), where the responses did not change and neither hypoxia or activity level had a significant effect on emotional state. The remaining participants were allotted to Class 1, 3, or 4, where hypoxia was a significant covariate, while activity (bed rest) was significant only for class 3. For PA, 84.09% or 37 participants were assigned to class 2 indicating a significant effect of hypoxia on the participants responses with no effect of physical activity. Class 1 participants (n = 7) were not affected by hypoxia, however, physical activity improved their PA. Participants undergoing confinement, hypoxia and bed rest do not exhibit a uniform emotional response and may be categorised into 2-4 distinct classes. These results indicate significant individual emotional responses, that may be masked and underreported by traditional statistical approaches like means ± SD. The emotional state of our participants is a complex construct likely influenced by past experiences and different coping mechanisms which allowed some to adapt to the experimental environment more readily.

Diurnal variation of urinary sodium-to-potassium ratio in free-living Japanese individuals

High sodium-to-potassium ratios are associated with elevated blood pressure levels and an increased risk of cardiovascular diseases. We aimed to determine whether urinary sodium-to-potassium ratios fluctuate diurnally during the day to understand measured values of casual urinary sodium-to-potassium ratios. A total of 13,277 casual urine specimens were collected under free-living conditions from 122 Japanese normotensive and hypertensive individuals. Participants collected all casual urine samples in aliquot tubes, reported urine volumes and the time at each voiding for 10-22 days. Then, specimens were classified into hourly data. Diurnal patterns of urinary sodium-to-potassium ratios and urinary concentrations of sodium and potassium were evaluated. Overall mean values of hourly urinary sodium-to-potassium ratios were highest (4.1-5.0) in the early morning, lower (3.3-3.8) in the daytime and higher (4.0-4.4) toward evening hours. The mean urinary sodium and potassium concentrations were the lowest (90-110 and 24-32 mmol l^-1, respectively) during the early morning and higher (110-140 and 35-43 mmol l^-1, respectively) after mid-morning. Diurnal variability of potassium concentrations was larger than for sodium concentrations. Diurnal variations in urinary sodium-to-potassium ratios were comparable between normotensive and hypertensive individuals, between hypertensive individuals with and without antihypertensive medications, and among age and gender-specific subgroups. Overall mean hourly urinary sodium-to-potassium ratios fluctuated diurnally under free-living conditions and were higher during the morning and evening and lower during the daytime compared with 24-h urinary sodium-to-potassium ratios. Diurnal variation in urinary sodium-to-potassium ratios should be considered to understand actual daily dietary levels and avoid over- and under-estimation in clinical practice.

The effects of bed-rest and countermeasure exercise on the endocrine system in male adults: evidence for immobilization-induced reduction in sex hormone-binding globulin levels

BACKGROUND AND AIM

There is limited data on the effects of inactivity (prolonged bed-rest) on parameters of endocrine and metabolic function; we therefore aimed to examine changes in these systems during and after prolonged (56- day) bed-rest in male adults.

SUBJECTS AND METHODS

Twenty healthy male subjects underwent 8 weeks of strict bed-rest and 12 months of follow-up as part of the Berlin Bed Rest Study. Subjects were randomized to an inactive group or a group that performed resistive vibration exercise (RVE) during bed-rest. All outcome parameters were measured before, during and after bed-rest. These included body composition (by whole body dual X-ray absorptiometry), SHBG, testosterone (T), estradiol (E2), PRL, cortisol (C), TSH and free T3 (FT3).

RESULTS

Serum SHBG levels decreased in inactive subjects but remained unchanged in the RVE group (p<0.001). Serum T concentrations increased during the first 3 weeks of bed-rest in both groups (p<0.0001), while E2 levels sharply rose with re-mobilization (p<0.0001). Serum PRL decreased in the control group but increased in the RVE group (p=0.021). C levels did not change over time (p≥0.10). TSH increased whilst FT3 decreased during bed-rest (p all ≤0.0013).

CONCLUSIONS

Prolonged bed-rest has significant effects on parameters of endocrine and metabolic function, some of which are related to, or counteracted by physical activity.

Diurnal variation of aldosterone and plasma renin activity: timing relation to melatonin and cortisol and consistency after prolonged bed rest

Exposure to prolonged bed rest is known to induce changes in the renin-angiotensin-aldosterone system (RAAS) by way of posture, sodium and potassium balance, and stress, which may have serious consequences for patients. We focused on the diurnal variation of the RAAS by investigating changes in the levels of plasma renin activity (PRA) and aldosterone; for comparison to markers of the intrinsic pacemaker and to stress, we measured melatonin and cortisol. PRA, aldosterone, melatonin, and cortisol were measured hourly in 10 normal subjects with standardized sleep patterns, posture, and diet at baseline and after 11 days of prolonged bed rest conducted under a light-dark cycle. Circadian characteristics of hormone secretion patterns were estimated by multiple harmonic regression with excellent goodness-of-fit measures. Variability in the melatonin and cortisol patterns across subjects was minimal. Even for pulsatile hormones, this technique successfully estimated the acrophase, which was the salient feature. Baseline hormone peak times started with melatonin near the middle of the sleep period, followed by PRA, then aldosterone, and then cortisol around wake time. Prolonged bed rest did not induce significant changes in any timing characteristic of the secretion patterns. Baseline and prolonged bed rest peak times for melatonin and cortisol and amplitude characteristics for all hormones were highly correlated, indicating consistency within individuals. These data provide strong evidence that prolonged bed rest of 11 days' duration does not disrupt either the timing characteristics of the RAAS or the intrinsic pacemaker.

Hormonal changes during 17 days of head-down bed-rest

We investigated in six men the impact of 17 days of head-down bed rest (HDBR) on the daily rhythms of the hormones involved in hydroelectrolytic regulation. This HDBR study was designed to mimic a real space flight. Urine samples were collected at each voiding before, during and after HDBR. Urinary excretion of Growth Hormone (GH), Cortisol, 6 Sulfatoxymelatonin, Normetadrenaline (NMN) and Metadrenaline (NM) was determined. A decrease in urinary cortisol excretion during the night of HDBR was noted. For GH, a rhythm was found before and during HDBR. The rhythm of melatonin, evaluated with the urine excretion of 6 Sulfatoxymelatonin (aMT6S), the main hepatic metabolite, persisted throughout the experiment without any modification to the level of phase. A decrease during the night was noted for normetadrenaline urinary derivates, but only during the HDBR.