Adrenergic vascular control.

The gravity-dependency and age-dependency of noradrenergic vasoconstrictor outflow to skeletal muscle (muscle sympathetic nerve activity; MSNA) in humans was analyzed by applying microneurography technique. The basal level of MSNA in the horizontally supine position increased with aging without significant changes in arterial baroreflex sensitivity. MSNA responded to +Gz load by head-up tilt and to simulated microgravity by thermoneutral head-out immersion with increasing and decreasing the activity, respectively. These gravity related responses of MSNA were age-dependent, being reduced by aging. Simultaneously monitored hemodynamic responses to +Gz load and to simulated microgravity were also age-dependent, being reduced by aging. The gravity-dependent and also age-dependent changes in the noradrenergic vasoconstrictor outflow to muscle seem to be related to the mechanisms controlling the gravity-dependent fluid shift in the human body.