Angiotensin II- and IV-induced changes in cerebral blood flow: Roles of AT 1 AT 2, and AT 4 receptor subtypes

Abstract

Our laboratory has previously reported the discovery of a unique angiotensin binding site (termed AT 4) specific for angiotensin IV (AngIV) in cultured vascular endothelial and smooth muscle cells. The present investigation employed laser-Doppler flowmetry to examine the effect of angiotensin II (AngII) and AngIV stimulation of these receptors on cerebral microcirculation in anesthetized Sprague–Dawley rats. Internal carotid artery infusion of AngII at a low dose (0.1 pmol min −1 ) revealed a 23% reduction in cerebral blood flow (CBF), while the infusion of AngIV increased CBF in a dose-dependent fashion with the highest dose (100 pmol min −1 ) resulting in an elevation of 30%. In a second experiment separate groups of rats were pre-treated with the AT 1 receptor subtype antagonist DuP 753 (Losartan), the AT 2 receptor subtype antagonist PD123177, or a newly synthesized AT 4 receptor subtype antagonist Divalinal-AngIV (Divalinal), followed by AngII or AngIV for the purpose of determining which angiotensin receptor subtype is responsible for mediating these AngII- and AngIV-induced responses. Pre-treatment with Losartan completely blocked subsequent AngII-induced reductions in CBF, while both PD123177 and Divalinal failed to inhibit this response. In contrast, significant increases in CBF were measured due to AngIV stimulation following pre-treatment with Losartan and PD 123177, while Divalinal abolished this AngIV-induced response. These results suggest that AngII and IV play opposite roles in cerebral microcirculation, i.e., the AT 1 receptor subtype mediates AngII-induced reductions in CBF, while the AT 4 receptor subtype regulates increases in CBF.