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Readaptation from Simulated Microgravity as a Stimulus for Improved Orthostatic Tolerance: Role of the Renal, Cardioendocrine, and Cardiovascular Systems
  1. Marlene S. Grenon,
  2. Shelley Hurwitz,
  3. Xinshu Xiao,
  4. Natalie Sheynberg,
  5. Craig D. Ramsdell,
  6. Christine Kim,
  7. Richard J. Cohen,
  8. Gordon H. Williams
  1. From the Division of Endocrinology, Hypertension and Diabetes (S.M.G., S.H., N.S., C.D.R., C.K., G.H.W.), Brigham and Women's Hospital, Boston, MA; NASA Center for Quantitative Cardiovascular Physiology, Modeling and Data Analysis (S.M.G., X.X., R.J.C.), Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology, Cambridge, MA; Department of Cardio-Thoracic Surgery (S.M.G.), McGill University, Montreal, QC. Supported by National Space and Biomedical Research Institute (NASA) Cooperative Agreement NCC 9-58 and the Heart and Stroke Foundation of Canada. The studies were conducted on the General Clinical Research Center of the Brigham and Women's Hospital, supported by a grant from the National Center for Research Resources (5M01RR02635). Dr. Ramsdell is presently affiliated with the Department of Anesthesiology and Perioperative Medicine, Williams Beaumont Hospital, Royal Oak, MI
  1. Address correspondence to: Dr. Gordon H. Williams, Division of Endocrinology, Hypertension and Diabetes, Brigham and Women's Hospital, 221 Longwood Avenue, Boston, MA 02115; tel: 617-732-5661; fax: 617-582-6126; e-mail: gwilliams{at}partners.org.

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 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.

  • recovery
  • simulated microgravity
  • renin-angiotensin-aldosterone system

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