Hypertension and thirst outlasting renal vasoconstriction as effects of a brief elevation of systemic angiotensin II in sheep

The influence of 10 min intracarotid (i.e.) and intravenous (i.v.) infusions of angiotensin II (Ang II; 20 pmol kg^-1 min^-1) on carotid blood pressure (cBP) and renal blood flow (RBF) was studied in unanaesthetized ewes without and with pre-treatment with the α₁-and β-adrenoceptor blocker labetalol. RBF was also monitored during 30 min intracerebroventricular (ICV) infusions of Ang II at 2 pmol kg^-1 min^-1. The i.e. infusions of Ang II induced about 50 mmHg rise in cBP. A steep decline occurred during 5 min post-infusion, followed by a much slower reduction with the cBP remaining above control level at 40 min post-infusion. The pressure elevation induced by i.v. Ang II was less pronounced but exhibited a similar pattern. Labetalol significantly reduced the pressor response to i.e. as well as i.v. Ang II. The i.e. and i.v. infusions of Ang II conspicuously reduced the RBF regardless of whether the ewes were labetalol-treated or not. At 5 min after the infusions RBF had returned to control level. The ICV infusions did not influence the RBF. Ang II i.e. elicited thirst in 50% of the ewes with the urge to drink remaining at 40 min post-infusion. The dipsogenic response was not reduced by labetalol pre-treatment. The results imply that no cerebral component contributes to the reduction in RBF induced by systemic Ang II. However, a centrally mediated action seems to be the cause of the long-lasting post-infusion cBP elevation and dipsogenic response. It suggests that, once bound at brain sites, Ang II may have a sustained action, alternatively may initiate cerebral processes of long duration.