Sympathectomy exaggerates antihypertensive effect of vasopressin withdrawal

The role of sympathetic function in the mechanism of the decrease in arterial pressure that follows withdrawal of an intravenous infusion of arginine vasopressin (AVP) in spontaneously hypertensive rats (SHR) was studied by comparing this withdrawal-induced antihypertensive phenomenon (WAP) in rats with intact sympathetic function to those subjected to sympathectomy. Sympathectomy with guanethidine did not lower blood pressure significantly in either SHR or normotensive Wistar-Kyoto (WKY) rats despite a marked impairment of sympathetic function as judged by a dramatic attenuation of blood pressure responses to tyramine and by evidence of denervation supersensitivity to phenylephrine. Cessation of a 3-h intravenous infusion of AVP (20 ng.kg-1.min-1) was associated with large and prolonged decrease in pressure below preinfusion levels in SHR with intact sympathetic function: 5 h after stopping the infusion, pressure was 27 +/- 3 mmHg below preinfusion levels. In sympathectomized SHR, the decrease in pressure after cessation of the AVP infusion was much larger: 5 h after the infusion, pressure was 44 +/- 2 mmHg below preinfusion levels. In contrast to SHR, pressure returned to control levels in WKY with intact sympathetic function after withdrawal of AVP. A small but significant decrease in pressure occurred after withdrawal of AVP in sympathectomized WKY. The results are consistent with the hypothesis that withdrawal of sympathetic activity is a contributing factor or a prerequisite condition for development of a WAP.