Effects of atrial natriuretic factor on baroreceptor reflexes.

It is now evident that atrial natriuretic factor (ANF) exerts neural effects in addition to vasorelaxant, depressor, and renal effects. The study by Ferrari et al examined the effects of intravenous ANF at doses within the pathophysiological range on arterial baroreceptor reflex control of heart rate and vascular resistance in conscious rats. Their results indicate that ANF markedly potentiated reflex bradycardia in response to phenylephrine but attenuated reflex tachycardia in response to nitroprusside and did not alter reflex increases in vascular resistance caused by carotid occlusion. What might be the mechanisms underlying these complex effects of ANF on arterial baroreceptor reflex function? Previous investigators have shown that ANF markedly affects reflex control of sympathetic nerve activity.Thoren et al and Imaizumi et al demonstrated that, in anesthetized rats with intact baroreceptors, ANF inhibited sympathetic nerve activity or prevented expected reflex increases in sympathetic nerve activity during hypotension.* The sympathoinhibitory effect of ANF was abolished by bilateral vagotomy, which suggested that this effect was mediated by activation of afferent vagal activity.. These results could account for the finding in the earlier studies that bilateral vagotomy attenuated hypotension induced by the administration of crude atrial extracts. ANF decreases venous return and thus cardiac filling pressure, which would deactivate cardiopulmonary receptors and induce reflex increases in sympathetic nerve activity. Therefore, the findings of Thoren et al and Imaizumi et al strongly suggest that ANF sensitizes cardiopulmonary receptors as activation of afferent vagal activity by ANF occurs despite decreased cardiac filling pressure. Other studies have reported increased sympathetic nerve activity during intravenous infusion of ANF in animals with intact baroreceptors. However, the latter results do not necessarily contradict the suggestion made by Thoren et al and Imaizumi et al. The reflex changes in sympathetic nerve activity in vivo are a