Hormonal responses to head-out water immersion diminish after exposure to head-down tilt

It has been postulated that gravitational change from 1 g to microgravity may cause cephalad fluid shift, resulting in suppression of antidiuretic hormone (ADH) secretion and diuresis (Gauer-Henry's reflex). However, results obtained in space flights did not confirm this. Since astronauts are confined at supine position for hours before launch, this posture may abolish the reflex in space flight. To investigate this possibility, effects of head-out water immersion (WI) after 2-hour head-down tilt (HDT) on hormonal and metabolic responses were examined and compared with those after 1-hour upright posture (UP). Hematocrit decreased by WI after UP, indicating hemodilution, but it did not change by WI after HDT. Plasma ADH, renin activity and aldosterone fell and atrial natriuretic peptide (ANP) rose by WI after UP, resulting in increased urine flow. On the other hand, appreciable hormonal changes were not elicited by WI after HDT, and urine flow stayed unchanged. These results indicate that fluid shift and hormonal and metabolic responses to WI are strongly attenuated by the prior exposure to HDT. Pre-launch posture of astronauts may at least partly explain why either suppression of ADH or diuresis was not observed on arrival at space.