Multi-System Deconditioning in 3-Day Dry Immersion without Daily Raise

Abstract

Dry immersion (DI) is a Russian-developed, ground-based model to study the physiological effects of microgravity. It accurately reproduces environmental conditions of weightlessness, such as enhanced physical inactivity, suppression of hydrostatic pressure and supportlessness. We aimed to study the integrative physiological responses to a 3-day strict DI protocol in 12 healthy men, and to assess the extent of multi-system deconditioning. We recorded general clinical data, biological data and evaluated body fluid changes. Cardiovascular deconditioning was evaluated using orthostatic tolerance tests (Lower Body Negative Pressure + tilt and progressive tilt). Metabolic state was tested with oral glucose tolerance test. Muscular deconditioning was assessed via muscle tone measurement. Results: Orthostatic tolerance time dropped from 27 ± 1 to 9 ± 2 min after DI. Significant impairment in glucose tolerance was observed. Net insulin response increased by 72 ± 23% on the third day of DI compared to baseline. Global leg muscle tone was approximately 10% reduced under immersion. Day-night changes in temperature, heart rate and blood pressure were preserved on the third day of DI. Day-night variations of urinary K⁺ diminished, beginning at the second day of immersion, while 24-h K⁺ excretion remained stable throughout. Urinary cortisol and melatonin metabolite increased with DI, although within normal limits. A positive correlation was observed between lumbar pain intensity, estimated on the second day of DI, and mean 24-h urinary cortisol under DI. In conclusion, DI represents an accurate and rapid model of gravitational deconditioning. The extent of glucose tolerance impairment may be linked to constant enhanced muscle inactivity. Muscle tone reduction may reflect the reaction of postural muscles to withdrawal of support. Relatively modest increases in cortisol suggest that DI induces a moderate stress effect. In prospect, this advanced ground-based model is extremely suited to test countermeasures for microgravity-induced deconditioning and physical inactivity-related pathologies.

Keywords: cardiovascular deconditioning; day-night variations; glucose intolerance; kaliuresis; modeled weightlessness; muscle tone; physical inactivity; supportlessness.

Figure 1

Global protocol timeline. Thick arrows stand for blood sampling. OGTT, Oral Glucose Tolerance Test; LBNP, Lower Body Negative Pressure; B-3, B-2, B-1, days before dry immersion; DI1, DI2, DI3, first, second and third days of dry immersion. R0, R+1, R+2–days after dry immersion.

Figure 2

Morning and evening heart rate (A), body temperature (B), systolic (C), and diastolic (D) blood pressure before DI and on the 3rd day of DI. Data are mean ± SEM. No significant difference on DI3 vs. B-1. #p ≤ 0.05 vs. Morning.

Figure 3

Number of finishers for different stages of LBNP-tilt test before and immediately after 3-day DI.

Figure 4

Cardiovascular changes during orthostatic tests for systolic (A) and diastolic (B) blood pressure and heart rate (C). Data are mean ± SEM. ^*p ≤ 0.05 vs. Supine; #p ≤ 0.05 vs. Before.

Figure 5

Cardiovascular changes during orthostatic tests for total peripheral resistance (A), stroke volume (B), spontaneous baroreflex sensitivity (C) and sympathetic index (D). Data are mean ± SEM. ^*p ≤ 0.05 vs. Supine; #p ≤ 0.05 vs. Before.

Figure 6

The effect of 48-h dry immersion on the glucose (A) and insulin (B) response to 75-g oral glucose tolerance test in 12 healthy subjects. Data are mean ± SEM. ^*p ≤ 0.05 vs. B-1.

Figure 7

Percent change in muscle tone of leg muscles (A), superficial neck and upper trunk muscles (B) and deep back muscles (C) 6 h following the onset (D1), on day 3 of immersion (D3), in recovery period on R0 (6 h following the end of DI) and on R+1. Data are mean ± SEM. ^*p ≤ 0.05 vs. B-1.

Figure 8

Day-night difference in renal excretion for sodium (A) and potassium (B). ^*p ≤ 0.05 vs. baseline.

Figure 9

12-h renal excretion for cortisol (A) and melatonin metabolite (B). ^*p ≤ 0.05 vs. baseline.