The three members of the α1-adrenoceptor family (α1A α1B α1D) are present and functional on both vascular endothelium and smooth muscle.

Background and Purpose: α 1-adrenoceptor agonists can cause endothelium-dependent arterial dilatation; no direct evidence of an endothelial α 1-adrenoceptor receptor location exists. Are any/all the three α 1-adrenoceptors (α 1A α 1B α 1D) located on arterial endothelium? Could this explain potentiation of catecholamine-mediated vasoconstriction by NOS blockade and/or the putative vasodilator effect of α 1D-adrenoceptor agonism? Experimental Approach: α 1-adrenoceptors were located by a fluorescent ligand, Bodipy-FL prazosin (QAPB) employing confocal microscopy. Receptors were identified in two ways: using α 1-adrenoceptor knockout mice (single, double and triple KOs) and competition with non-fluorescent antagonists. Contractile effects were analysed pharmacologically using wire myography. Key results: Prazosin-sensitive fluorescent ligand binding was found in endothelial cells of all mouse strains possessing one, two or three α 1-AR subtypes. Selective antagonists RS100-329 and BMY7378 reduced binding when the mice possessed the gene for α 1A-AR or α 1D-AR; together they abolished fluorescence except when the α 1B-AR gene was present. Thus, all three subtypes are present on endothelium whenever the mouse possesses the appropriate gene. Sub-nanomolar concentrations of Phenylephrine could cause dilatation of preconstricted arteries in “α 1D-AR-only” mice but not WT. L-NAME potentiated vasoconstriction to phenylephrine and A-61603 (relatively selective for α 1D-AR and α 1A-AR, respectively) but not to 5-HT, in all strains possessing the relevant α 1-AR genes. Conclusions and Implications - The three α 1-adrenoceptor types are present on carotid arterial endothelium and smooth muscle. Endothelial α 1D- or α 1A-adrenoceptor agonism produces a NO-mediated counter-contractile effect that opposes vasoconstriction within the α 1-adrenoceptor agonists’ concentration/contraction ranges. In a more limited study, α 1B-adrenoceptor localisation in endothelium and vascular smooth muscle; its mediation of vasoconstriction and vasodilatation were revealed directly for the first time. A reassessment of the balance between vasoconstriction and vasodilatation of catecholamines via the α 1-adrenoceptor family, and of the consequences of their non-smooth muscle location, is indicated.