Microneurographic analysis of sympathetic nerve activity under weightlessness simulated by water immersion

点击翻译按钮获取中文摘要

The purpose of this research project was to clarify the neural controlling mechanisms of human body under weihjtlessness simulated by head-out water immersion. Subjects were 30 healthy volunteers ranging in age between 18 and 62. Effects of graded thermo-neutral water innersion up to the level of the neck of standing subject on sympathetic nerve activity, leg volume, cardiovascular and hormonal functions were analyzed. Muscle and skin sympathetic nerve activities (MSNA and SSNA) were recorded microneurographically from the tibial nerve. Leg volume was measured by rubber strain gauge plethysomgraphy. Heart rate, stroke volume, cardiac output, blood pressure and total peripheral resistance were measured as cardiovascular functions. Plasma catechoamines (epinephrine and norepinephrine), antidiuretic hormone(ADH), aldosterone, renin activity, angiotensin I・ II, atrial natriuretic polypeptide (ANP) were measured as homonal functions. 1. MSNA was strongly suppressed by head-out water immersion. However, this suppressive response of MSNA was less prominent in loder subjects than in younger ones. 2. SSNA was also suppressed by head-out water immersion. But this response was much weaker than that of MSNA. 3. Leg volume, heart rate and total peripheral resistance were reduced, while stroke volume and cardiac output were increased by head-out water immersion. Blood pressure was not significanty changed under the same condition. 4. Plasma catecholamines, ADH, aldosterone, renin activity, angiotensin I・ II were reduced, while ANP was markedly increased by head-out water immersion. Based on these findings, it is concluded that the suppressive response of MSNA plays an important role in cooperation with the hormonal responses to compensate cephalad fluid shift under weightlessness simulated by water immersion and to maintain hemodynamic homeostasis. This suppressive response of MSNA seems to be influenced by aging, becoming presumably less important in older subjects.