Readaptation from simulated microgravity as a stimulus for improved orthostatic tolerance: role of the renal, cardioendocrine, and cardiovascular systems.

Background: Microgravity and simulated microgravity (SM) lead to important changes in orthostatic tolerance (OT), the autonomic nervous system (ANS), and the volume-regulating systems. After one is exposed to microgravity or SM, a period of readaptation to gravity is known to take place, but it is not certain if orthostatic function returns to baseline within the initial recovery and what mechanisms are involved. We hypothesized that after a period of recovery, OT, ANS, and volume-regulating systems would return to pre-SM levels.

Methods: To test this hypothesis, 24 healthy men were placed on a constant diet for 3 to 5 days, after which a tilt-stand test (pre-TST) was performed. The TST was repeated after 14 to 16 days of head-down tilt bed rest (HDTB) (post-TST) and a 3-day period of recovery (rec-TST), at which times measurements of renal, cardioendocrine, and cardiovascular systems were conducted.

Results: Presyncope occurred in 46% of subjects pre-TST, in 72% post-TST, and in 23% during rec-TST. OT was significantly better during the recovery period than at baseline (p = .03). There was a significant decrease in urinary sodium and potassium excretion, along with a decrease in plasma renin activity and serum and urine aldosterone compared with baseline. Serum norepinephrine and sympathetic responsiveness remained below baseline values.

Conclusion: In summary, OT improved compared with baseline after a period of readaptation. Retention of electrolytes (sodium, potassium) could be involved. These findings indicate that recovery after SM is not simply a gradual return to baseline values but is instead a dynamic process reflecting interaction of multiple regulatory systems.