Interpretation of the mechanism of cardiovascular deconditioning induced by the long-term exposure to microgravity.

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On return to the earth sfter spaceflight, cardiovascular deconditioning, including fall in blood pressure and sometimes syncope, is known to occur. Variable autonomic symptons were reported during spaceflight. However, the effect of microgravity on autonomic function is almostly unclear. We performed 3 days of dry immersion (DI) as the stimulation of short duration microgravity and 120 days of 6-degree head-down bed rest (BR) as that of long duration weightlessness.After 3 days of DI,resting muscle sympathetic nerve activity (MSNA) recorded with use of microneurography, and resting cardiac sympathetic nerve activity indicated by heart rate variability were higher than those before DI,and resting cardiac vagal activity was lower than that before DI.Systolic and diastolic blood pressure werre similar to that before DI.Increasing response of MSNA,cardiac sympathetic nerve activity and heart rate to orthostatic stress, and decreasing response of cardiac vagal activity to it were maintained. After 12 _o days of BR,resting MSNA and heart rate were higher than those before BR.Systolic and diastolic blood pressure werre similar to that before BR.Rises in MSNA and heart rate in response to orthostatic stress were maintained. Systolic and diastolic blood pressure resposes to it did not change. However, increasing response in MSNA to static leg exercise (30% maximal voluntary contraction) was declined and rise in mean blood pressure to it disappeared. 3 days of the exposure to microgravity and 120 days of it increased MSNA to almost same degree. From the assessment of changes in MSNA,heart rate (variability) and blood pressure, both of them did not effect autonomic responses to orthostatic stress, but the latter declined the MSNA response.