Neural and humoral controlling mechanisms of cardiovascular functions in man under weightlessness simulated by water immersion

To clarify how neural and humoral mechanisms operate to control cardiovascular unctions in man under weightlessness, the response of sympathetic nerve activity was observed in healthy human subjects by means of microneurographic technique with the changes of several hemodynamic parameters and hormonal responses during thermoneutral head-out water immersion. Muscle sympathetic nerve activity was markedly suppressed by head-out immersion, concomitantly with a reduction of the leg volume, an increase of the stroke volume and a reduction of total peripheral resistance. At the same time, plasma level of norepinephrine, vasopressive and antidiuretic hormones (ADH, aldosterone, renin activity, angiotensin I·II) were reduced, while vasodepressive and diuretic hormone (ANP) was markedly increased. The systemic blood pressure was maintained almost unchanged during head-out water immersion. The suppressive response of sympathetic nerve activity seemed to be age-dependent. This response was less prominent in the elderly than in young subjects. It is concluded that the suppressive response of muscle sympathetic activity plays an important role to maintain hemodynamic homeostasis under weightlessness to compensate for the cephalad fluid shift and the resultant increase of the stroke volume in cooperation with the hormonal responses.