Role of the renin-angiotensin system in isoprenaline-induced vasopressin release.

We examined whether the activation of the renin-angiotensin system after intramuscular isoprenaline injection contributes to the simultaneous increase in vasopressin release. Plasma concentrations of vasopressin and angiotensin II were measured in conscious rats using specific radioimmunoassays. Intravenous infusions of angiotensin II caused a dose-dependent increase in vasopressin release. Intravenous infusions of the angiotensin II antagonist saralasin did not diminish the isoprenaline-induced vasopressin release. However, the curve relating the isoprenaline-induced decrease in blood pressure and the concomitant increase in vasopressin levels was shifted to the right and no longer linear in saralasin-treated rats. Nephrectomy diminished the vasopressin release caused by isoprenaline when compared to sham-operation. The correlation between the decrease in blood pressure and the simultaneous vasopressin release was changed in a strikingly similar manner by nephrectomy and by saralasin infusions. It may be concluded that small doses of isoprenaline, which cause only minor decreases in blood pressure, induce vasopressin release via the renin-angiotensin system. However, the contribution of this system to vasopressin release declines as hypotension becomes more severe.