Contrasting reflex effects of chemosensitive and mechanosensitive vagal afferents.

Previous studies have identified two distinct types of cardiac vagal afferents, mechanosensitive and chemosensitive. We tested the hypothesis that these two types of vagal afferents mediate different reflex sympathetic nerve responses. We compared effects of stimulation of chemosensitive and mechanosensitive vagal afferents on renal and adrenal sympathetic nerve activity in rats. In anesthetized, sinoaortic baroreceptor-denervated rats, we measured arterial pressure, heart rate, and renal and adrenal sympathetic nerve activity during intrapericardial administration of phenyl diguanide as a stimulus for chemosensitive afferents and during volume expansion and acute occlusion of the ascending aorta as stimuli for mechanosensitive afferents. Stimulation of chemosensitive afferents with phenyl diguanide (3, 10, and 30 micrograms/kg intrapericardially) caused decreases in renal sympathetic nerve activity (-36 +/- 5, -52 +/- 11, and -71 +/- 5%; p less than 0.01) and in arterial pressure and heart rate but increased adrenal sympathetic nerve activity (+14 +/- 27, +63 +/- 21, and +83 +/- 28%; p less than 0.05). These responses were abolished by vagotomy. Intrapericardial injection of saline vehicle did not change renal and adrenal sympathetic nerve activity. In contrast to the effects of phenyl diguanide, activation of mechanosensitive afferents by volume expansion with intravenous infusion of 6% dextran in 0.9% saline (Dextran 75) decreased both renal and adrenal sympathetic nerve activity. Stimulation of mechanosensitive afferents by acute occlusion of the ascending aorta also decreased both renal and adrenal sympathetic nerve activity. These results indicate that chemosensitive and mechanosensitive cardiac vagal afferents produce different reflex responses: Chemosensitive afferents increase and mechanosensitive afferents decrease adrenal sympathetic outflow.