抄録
Head-down tilt (HDT) has been used to simulate microgravity environment. HDT causes a headward fluid shift as real microgravity does, but HDT also induces a change in vestibular input which is absent during real microgravity. Some studies have shown that the vestibulo-sympathetic reflex (VSR) has an additive influence to cardiovascular reflexes (i.e., baro reflexes) on sympathetic nerve activity. Thus HDT, using animals without VSR, may provide a better model for microgravity. Little information is, however, available for relationship between the otolith inputs and cardiovascular responses during HDT. In the present experiments, we examined changes in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) during 45°HDT using anesthetized rabbits. Chemical labyrinthectomy was performed bilaterally under anesthesia 2 days before experiments in order to make vestibular lesions (VL group). The animal's head was mounted on a tilting table during experiments. In control rabbits, RSNA decreased markedly during HDT, which was followed by a MAP decrease. After 10 seconds from the onset of HDT, the RSNA went up to the pre-HDT baseline activity, and then MAP recovered to the pre-HDT baseline level. In VL group, RSNA did not change significantly and the MAP was very variable and tended to increase during HDT. These results suggest that vestibular apparatus is involved in arterial pressure control during HDT, which is possibly mediated by the sympathetic nerve outflow. [J Physiol Sci. 2006;56 Suppl:S237]
