Re-adaptation from Simulated Microgravity as a Stimulus for Improved Orthostatic Tolerance
- Paper number
IAC-05-A1.4.02
- Author
Dr. Marlene Grenon, McGill University, Canada
- Coauthor
Dr. Richard J. Cohen, Massachussets Institute of Technology (MIT), United States
- Coauthor
Dr. Shelley Hurwitz, United States
- Coauthor
Ms. Christine Kim, United States
- Coauthor
Dr. Gordon H. Williams, United States
- Year
2005
- Abstract
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 re-adaptation 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 3-5 days, after which a tilt-stand test (pre-TST) was performed. The TST was repeated after 14-16 days of head-down tilt bedrest (HDTB) (post-TST) and a 3-day period of recovery (rec-TST), at which times measurements of renal, cardio-endocrine and cardiovascular systems were conducted, which included 24-hour urine collections and hormonal measurements, and cardiovascular system identification (a non-invasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness).
Results: Presyncope occurred in 46 percent of subjects pre-TST; in 72 percent post-TST; and in 23 percent during rec-TST. OT was significantly better during the recovery period than at baseline (p=0.03). 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 to baseline. Serum norepinephrine and sympathetic responsiveness remained below baseline values.
Conclusions: In summary, OT improved compared with baseline after a period of re-adaptation. 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.
- Abstract document