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Renal, Endocrine, and Cardiovascular Responses to Bed Rest in Male Subjects on a Constant Diet
  1. Marlene S. Grenon,
  2. Natalie Sheynberg,
  3. Shelley Hurwitz,
  4. Grace Xiao,
  5. Craig D. Ramsdell,
  6. Michael D. Ehrman,
  7. Lan C. Mai,
  8. Siri Rostoft Kristjansson,
  9. Grete H. Sundby,
  10. Richard J. Cohen,
  11. Gordon H. Williams
  1. From the Division of Endocrinology (S.M.G., N.S., S.H., C.D.R., M.D.E., C.L.M., R.K., G.H.S., G.H.W.), Diabetes and Hypertension, Brigham and Women's Hospital, Boston, MA; Harvard-MIT Division of Health Sciences and Technology (G.X., R.J.C.), Massachusetts Institute of Technology, Cambridge, MA; Department of Cardio-Thoracic Surgery (S.M.G.), McGill University, Montreal, PQ.
  1. Address correspondence to: Gordon H. Williams, Division of Endocrinology, Hypertension and Diabetes, Brigham and Women's Hospital, 221, Longwood Avenue, Boston, MA 02115; e-mail: gwilliams{at}partners.org.
  2. Presented at the Clinical Research Meeting2003, Baltimore, MD, March 2003.
  3. This work was supported by grants from the National Space and Biomedical Research Institute (NASA) (5M01RR02635) and the Heart and Stroke Foundation of Canada.

Abstract

Background Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems—the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally.

Methods Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB).

Results Sodium excretion increased initially (pre-HDTB = 182.8 ± 10.4 mEq/total volume; early HDTB = 236.4 ± 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 ± 2.4/total volume; mid- to late HDTB = 89.4 ± 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 ± 0.21 ng/mL/h; end HDTB = 1.69 ± 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed.

Conclusions We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Key Words
  • renin-angiotensin-aldosterone system
  • autonomic function
  • simulated microgravity

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