Afferent mechanisms of acute responses of renal sympathetic nerve activity to microgravity

Exposure to microgravity is thought to induce an alteration of autonomic function through several afferent pathways. First, a removal of all hydrostatic gradients, which results in a large cephalad fluid shift. The fluid shift may induce an increase in transmural pressure in the cardiopulmonary region, which stimulates cardiopulmonary mechanoreceptors and baroreceptors, and elicits neurogenic responses. Secondly, an alteration of the vestibular input may modify sympathetic output via the vestibulosympathetic reflex. However, there is a lack of direct evidence for the role of cardiopulmonary mechanoreceptors, baroreceptors, and vestibular system in autonomic responses to microgravity. Accordingly, responses of renal sympathetic nerve activity (RNA) to microgravity, produced by free drop, were examined in anesthetized rats. To examine the afferent pathways of the RNA responses, the same experiment was performed in rats that had undergone labyrinthectomy, sinoaortic baroreceptor denervation (SAD), vagotomy, or SAD plus vagotomy.