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

在太空飞行后返回地球时，已知会发生心血管疾病，包括血压下降，有时还会晕厥。据报道，在航天飞行期间出现了可变的自主神经症状。然而，微重力对自主神经功能的影响几乎还不清楚。在短时间微重力刺激下干浸泡(DI)3d，长期失重状态下6度头低位卧床(BR)120d。DI后3d，显微神经造影记录的静息肌交感神经活动(MSNA)和心率变异性反映的静息心交感神经活动均高于DI前，静息心迷走神经活动低于DI前。收缩压和舒张压与DI前相似，MSNA、心交感神经活动和心率对立位应激的反应增强，而心迷走神经活动对此的反应降低。BR后12天，静息MSNA和心率均高于BR前，收缩压和舒张压与BR前相似，MSNA和心率对立位应激的反应保持上升。收缩压和舒张压反应到它没有变化。然而，MSNA对静态腿部运动(30%最大自主收缩)的增加反应下降，平均血压上升消失。微重力暴露3天和120天使MSNA增加到几乎相同的程度。从MSNA、心率(变异性)和血压的变化来看，它们都不影响对立位应激的自主神经反应，但后者降低了MSNA反应。

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.