The time-dependent alteration of anti-diuretic hormone system in hindlimb unloaded rats

It is important to understand the mechanism on the fluid shift and volume regulation occurring in astronauts after spaceflight for future life in space. In the present study, we examined the time-dependent alteration of anti-diuretic hormone (ADH) concentrating on the water reabsorption system in hindlimb unloaded rats. Male Sprague-Dawley rats were hindlimb unloaded for 1 (HU1), 7(HU7), 14 days (HU14) or rested in the ground for 3 days after HU14 (HU14+3). The plasma ADH and angiotensin II level showed peak value at HU7, and the alterations were restored at HU14. However, several serum electrolytes (Na, K, Cl) were not changed regardless of HU period. In the immunohistochemical study, we examined that ADH and c-Fos immunoreactivities (IR) were maximized at HU7 in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). Aquaporin 2 (AQP2) IR also was increased in the renal collecting duct for water re-absorption at HU7 showing a similar pattern with ADH. These results present a series of physiological ADH system alteration following to period of hindlimb unloading stimulus, indicating that ADH system is activated significantly at HU7. In addition, our results suggest that ADH system activation may be involved in anti-diuretic phenomenon in early spaceflight period. Furthermore, it is speculated that ADH system may require 14 days for adaptation to microgravity.