Urinary adrenaline and noradrenaline response to simulated weightless state

Sixteen normal subjects were placed in a simulated weightless state, i.e., water immersion. After 6 hr of water immersion, urine samples were collected and bio-assayed for adrenaline and noradrenaline. The excretion of adrenaline was moderately increased ( P < 0.15 > 0.10), possibly related to the anxiety associated with the immersion. The excretion of noradrenaline was significantly ( P < 0.01) reduced during immersion. Six subjects were also studied during passive vertical tilt following the immersion. The increase in pulse rate and decrease in pulse pressure were significantly greater than those observed during a control tilt. The results of these experiments indicate that the decrease in orthostatic tolerance following a simulated weightless state is probably related to a decrease in sympathetic nerve activity, which in turn is reflected by a decline in the urinary output of the sympathetic neurohormone noradrenaline.