Mechanism of pressor effects by angiotensin in the nucleus tractus solitarius of rats.

We recently reported that microinjection of angiotensin II (ANG II) into the nucleus tractus solitarius (NTS) results in an increase in mean arterial pressure (MAP) in urethan-anesthetized rats in a dose range of 50-500 ng. To investigate the mechanism of this response, hexamethonium (20 mg/kg iv) was used to inhibit sympathetic activation. There was a highly significant (P less than 0.001) reduction in the magnitude of the pressor response (4.7 +/- 1.1 mmHg) compared with preblockade ANG II (500 ng) responses (15.5 +/- 1.6 mmHg). A vasopressin antagonist and hypophysectomized rats were used to study the contribution of pituitary vasopressin. Injection of 500 ng ANG II in hypophysectomized rats produced a pressor response (14.8 +/- 3.2 mmHg) indistinguishable from that in intact controls (15.5 +/- 1.6 mmHg). Pretreatment with the vasopressin antagonist d(CH2)5Tyr(Me)AVP (1 microgram iv) in intact rats also had no effect on the magnitude of the pressor response (15.7 +/- 1.7 mmHg). Microinjection of ANG I and II produces an increase in arterial pressure. It is concluded that the angiotensin pressor response in the NTS is mediated by activation of descending sympathetic fibers and is not dependent on release of blood-borne pressor agents from the pituitary.