Baroreflex modulation of central angiotensin II pressor responses in conscious rabbits

We examined the cardiovascular responses to intracisternal administration of angiotensin II (Ang II) in conscious rabbits and determined the effects of removing baroreflex feedback inhibition. Administration of intracisternal bolus doses of Ang II produced short-duration dose-dependent increases in blood pressure (maximum response 30 mmHg) accompanied by bradycardia, with the dose required to produce the half maximal response averaging 5 pg. Dose-response curves to intracisternal Ang II spanned four log(dose) units from the threshold to the maximum doses (1-5 ng). After sino-aortic baroreceptor denervation the threshold for pressor responses was reduced by a factor of 1000, with the ED50 in these animals as low as 5 fg. No bradycardia was observed and the dose-pressor response curves were markedly steeper. The results suggest that in conscious rabbits Ang II receptors, presumably located in the brainstem, respond to minute intracisternal doses of Ang II. Furthermore, this site is normally inhibited by the input from arterial baroreceptors.