Systemic angiotensin II, blood pressure and supraoptic neuronal activity

Recordings of SON single unit activity and systemic arterial blood pressure (B.P.) were taken from 10 rats while systemic infusions of angiotensin II (AII), 1-1000 ng/kg body weight/min in 7 steps, or phenylephrine, 1-100 ng in 3 steps were administered. The relationship between AII concentrations and neuronal activity was biphasic. Within the physiological range (1 ng to 100 ng) AII excited single units in a dose dependent manner, but it had little effect on B.P. At higher concentrations, B.P. rose and neuronal activity was decreased. Phenylephrine, however, did not excite neuronal activity. With increasing phenylephrine concentrations, B.P. rose and neuronal activity slowed. We conclude that increased B.P. may dampen the SON neuronal output by baroreceptor inhibition. Under physiological conditions, therefore, AII may serve to reinforce tonic vasopressin release while inhibiting vasopressin release at pressor doses. This further suggests a role for plasma AII as an important link of the renal-hypothalamic-hormonal feedback loop.