| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
1Human Adaptation and Countermeasures Office, Wyle Laboratories, Inc.; 2Division of Space Life Sciences, Universities Space Research Association; and 3Astronaut Office and 4Human Adaptation and Countermeasures Office, National Aeronautics and Space Administration Johnson Space Center, Houston, Texas
Submitted 5 March 2004 ; accepted in final form 6 October 2004
Head-down bed rest changes the values of many cardiovascular and endocrine variables and also elicits significant hypovolemia. Because previous studies had not controlled for hypovolemia, it is unknown whether the reported changes were primary effects of bed rest or secondary effects of bed rest-induced hypovolemia. We hypothesized that restoring plasma volume with salt tablets and water after 12 days of head-down bed rest would result in an absence of hemodynamic and endocrine changes and a reduced incidence of orthostatic hypotension. In 10 men, we measured changes from pre-bed-rest to post-bed-rest in venous and arterial pressures; heart rate; stroke volume; cardiac output; vascular resistance; plasma norepinephrine, epinephrine, vasopressin, renin activity (PRA), and aldosterone responses to different tilt levels (0°, –10°, 20°, 30°, and 70°); and plasma volume and platelet 
2- and lymphocyte 
2-adrenoreceptor densities and affinities (0° tilt only). Fluid loading at the end of bed rest restored plasma volume and resulted in the absence of post-bed-rest orthostatic hypotension and changes in supine hemodynamic and endocrine variables. Fluid loading did not prevent post-bed-rest increases in 2-adrenoreceptor density or decreases in the aldosterone-to-PRA ratio (P = 0.05 for each). Heart rate, epinephrine, and PRA responses to upright tilt after bed rest were increased (P < 0.05), despite the fluid load. These results suggest that incidents of orthostatic hypotension and many of the changes in supine hemodynamic and endocrine variables in volume-depleted bed-rested subjects occur secondarily to the hypovolemia. Despite normovolemia after bed rest, 2-adrenoreceptors were upregulated, and heart rate, epinephrine, and PRA responses to tilt were augmented, indicating that these changes are independent of volume depletion.
simulated microgravity; cardiovascular; hypovolemia; cardiopulmonary-arterial baroreceptor reflex interaction; adrenergic receptors
This article has been cited by other articles:
|
|
 |
|
  D Gupta and M D Nair Neurogenic orthostatic hypotension: chasing "the fall" Postgrad. Med. J., January 1, 2008; 84(987): 6 - 14. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. Fischer, P. Arbeille, J. K. Shoemaker, D. D. O'Leary, and R. L. Hughson Altered hormonal regulation and blood flow distribution with cardiovascular deconditioning after short-duration head down bed rest J Appl Physiol, December 1, 2007; 103(6): 2018 - 2025. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. Aardenburg, M. E. Spaanderman, H. W. van Eijndhoven, P. W. de Leeuw, and L. L. Peeters A Low Plasma Volume in Formerly Preeclamptic Women Predisposes to the Recurrence of Hypertensive Complications in the Next Pregnancy Reproductive Sciences, December 1, 2006; 13(8): 598 - 603. [Abstract] [PDF]
|
|
|
|
 |
|
 C. M. Foley, P. J. Mueller, E. M. Hasser, and C. M. Heesch Hindlimb unloading and female gender attenuate baroreflex-mediated sympathoexcitation Am J Physiol Regulatory Integrative Comp Physiol, November 1, 2005; 289(5): R1440 - R1447. [Abstract] [Full Text] [PDF]
|
|
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
