Role of the vestibular system in sudden shutdown of renal sympathetic nerve activity during microgravity in rats.

The purpose of this study was to examine the effect of microgravity (muG) on renal sympathetic nerve activity (RNA) in rats. Additionally, we estimated the participation of the vestibular system in the response of RNA to muG. Eight normal Sprague-Dawley (SD) rats and five chemically and bilaterally labyrinthectomied SD rats were used to measure RNA during free-drop examination (4.5-s duration of muG); arterial pressure (AP) and aortic flow velocity (AFV) were additionally monitored. Although AFV showed no particular change, AP tended to decrease during muG in the later phase. Prior to this AP fall-off, RNA was immediately and markedly attenuated by muG. This attenuation was transient and RNA returned to 1G level within the mu;muG condition. Interestingly, this phenomenon remained even in labyrinthectomied rats. In conclusion, cephalad shift of the body fluid by loading of muG may cause cardiopulmonary low-pressure receptor activation and consequent RNA attenuation, but the participation of the vestibulosympathetic reflex in this phenomenon is not obvious.