Interactions of vasopressin with the area postrema in arterial baroreflex function in conscious rabbits.

This study compares the effect of arginine-vasopressin with phenylephrine on arterial pressure, heart rate, and renal sympathetic nerve activity in conscious rabbits with and without functional arterial baroreflexes and in rabbits with lesions of the area postrema. In intact rabbits, progressive infusions of arginine-vasopressin result in large decreases in renal sympathetic nerve activity and heart rate for a given increase in blood pressure as compared to progressive infusions of phenylephrine. In sinoaortic-denervated rabbits, the responses of arterial pressure on heart rate and renal sympathetic nerve activity to both arginine-vasopressin and phenylephrine are markedly attenuated, indicating the necessity for afferent baroreceptor activity in this response. This observation indicates that arginine-vasopressin is acting centrally to enhance the baroreflex. A central site of action of circulating vasopressin may be the area postrema, since it is the only circumventricular organ in the hindbrain. Lesioning the region of the area postrema resulted in a normalization of the responses evoked with arginine-vasopressin and phenylephrine. There was no difference in the phenylephrine responses of arterial pressure on renal sympathetic nerve activity or heart rate in area postrema-lesioned animals, compared to control rabbits. Therefore, we conclude that the area postrema or its surrounding tissue is either a site of action of circulating arginine-vasopressin or contains fibers of passage from another site where arginine-vasopressin acts to enhance baroreflex activity.