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Related Experiment Videos

Sympathetic vasoconstriction and orthostatic intolerance after simulated microgravity.

A Kamiya1, D Michikami, Q Fu

  • 1Dept. of Autonomic Neuroscience, Nagoya University, Japan.

Journal of Gravitational Physiology : a Journal of the International Society for Gravitational Physiology
|September 7, 2001
PubMed
Summary

Astronauts experience fainting upon return to Earth due to microgravity. This study investigated sympathetic nerve activity and norepinephrine release during head-up tilt tests after bed rest, revealing altered responses in those who fainted.

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Area of Science:

  • Cardiovascular Physiology
  • Space Medicine
  • Neuroscience

Background:

  • Astronauts frequently experience orthostatic intolerance, characterized by presyncope during standing after spaceflight.
  • Microgravity simulation via 6 degrees head-down bed rest (HDBR) is used to study orthostatic hypotension mechanisms.
  • Previous research suggests potential roles for reduced blood volume, vagal tone withdrawal, or impaired baroreflex function.

Purpose of the Study:

  • To investigate the underlying mechanisms of orthostatic hypotension following microgravity exposure.
  • To examine alterations in sympathetic nervous system outflow and norepinephrine release in response to orthostatic stress.
  • To compare responses between individuals who experience presyncope (fainters) and those who do not (non-fainters) after HDBR.

Main Methods:

Related Experiment Videos

  • 14 days of 6 degrees head-down bed rest (HDBR) was conducted.
  • Muscle sympathetic nerve activity (MSNA) was measured using microneurography.
  • 60 degrees head-up tilt (HUT) tests were performed before and after HDBR to assess orthostatic stress responses.

Main Results:

  • Subjects who experienced presyncope after HDBR showed significantly different MSNA responses during HUT tests compared to non-fainters.
  • The indicated norepinephrine release, assessed by the ratio of plasma norepinephrine increase to MSNA increase, also differed between fainters and non-fainters.
  • These findings suggest a hypoadrenergic response contributes to post-HDBR orthostatic intolerance.

Conclusions:

  • Sympathetic nervous system outflow and norepinephrine release are altered following simulated microgravity.
  • A blunted sympathetic response to orthostatic stress may be a key factor in post-flight orthostatic intolerance.
  • Further research is needed to fully elucidate the neurocirculatory adaptations to microgravity and their implications for astronaut health.