SYMPATHECTOMY EXAGGERATES ANTIHYPERTENSIVE EFFECT OF VASOPRESSIN WITHDRAWAL

The role of sympathetic function in the mechanism of the decrease in a rterial pressure that follows withdrawal of an intravenous infusion of arginine vasopressin (AVP) in spontaneously hypertensive rats (SHR) w as studied by comparing this withdrawal-induced antihypertensive pheno menon (WAP) in rats with intact sympathetic function to those subjecte d to sympathectomy. Sympathectomy with guanethidine did not lower bloo d pressure significantly in either SHR or normotensive Wistar-Kyoto (W KY) 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. Cessati on of a 3-h intravenous infusion of AVP (20 ng.kg(-1).min(-1)) was ass ociated with large and prolonged decrease in pressure below preinfusio n levels in SHR with intact sympathetic function: 5 h after stopping t he infusion, pressure was 27 +/- 3 mmHg below preinfusion levels. In s ympathectomized SHR, the decrease in pressure after cessation of the A VP infusion was much larger: 5 h after the infusion, pressure was 44 /- 2 mmHg below preinfusion levels. In contrast to SHR, pressure retur ned to control levels in WKY with intact sympathetic function after wi thdrawal of AVP. A small but significant decrease in pressure occurred after withdrawal of AVP in sympathectomized WKY. The results are cons istent with the hypothesis that withdrawal of sympathetic activity is a contributing factor or a prerequisite condition for development of a WAP.