Effects of chronic peripheral sympathectomy on plasma levels of, and the pressor response to, vasopressin.

The purpose of the present study was to assess the effect of chronic peripheral sympathectomy in rats on plasma vasopressin (basal and dehydrated) and on pressor sensitivity to vasopressin. Sympathectomy was produced in male Sprague-Dawley rats by daily injection of guanethidine (45 mg/kg) for 9 days. Control rats received saline over the same period. Plasma vasopressin was determined by radio-immunoassay, and pressor sensitivity was determined by monitoring mean arterial pressure response to graded injections of vasopressin (0.1-20 mU) in conscious rats. Sympathectomized rats showed ptosis and supersensitivity to norepinephrine, and had significantly greater basal and dehydrated plasma vasopressin levels than controls (10.3 +/- 1.5 versus 6.2 +/- 0.7, and 12.4 +/- 0.9 versus 8.6 +/- 1.0 pg/ml, s.e.m, respectively, P less than 0.05 for both). Sympathectomized rats also had an increased pressor sensitivity to vasopressin (dose response curve shifted to left, lower threshold, greater slope, P less than 0.001). Injection of a vasopressin pressor-antagonist, d (CH2)5 Me Tyr AVP, had no effect on blood pressure in control rats but caused a significant decrease of blood pressure in sympathectomized rats (15 +/- 1.0 mmHg, P less than 0.001). These results suggest that chronic peripheral sympathectomy in rats is associated with increased basal and dehydrated plasma vasopressin, and increased pressor sensitivity to vasopressin. The effect of the vasopressin antagonist suggests that vasopressin may play a role in blood pressure maintenance in sympathectomized rats.