title: Two "knockout" mouse models demonstrate that aortic vasodilatation is mediated via alpha2a-adrenoceptors located on the endothelium.
abstract: UK-14,304 [5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine]-mediated vasodilator responses were studied on wire myograph-mounted mouse aorta to determine the cells involved, mechanisms of action, and subtypes of alpha(2)-adrenoceptors. In the presence of induced tone, UK-14,304 produced concentration-related vasodilatation that was abolished by rauwolscine, N(omega)-nitro-L-arginine methyl ester (L-NAME), or endothelium removal, indicating that endothelial alpha(2)-adrenoceptors can release nitric oxide. In the alpha(2A)-adrenoceptor knockout mouse and the D79N mouse, a functional knockout of the alpha(2A)-adrenoceptor, these relaxant effects of UK-14,304 were lost, indicating the involvement of the alpha(2A)-adrenoceptor. UK-14,304 could also contract aorta: a small contraction occurred at high concentrations, was enhanced by L-NAME, and was absent in the alpha(1D)-adrenoceptor knockout mouse, indicating activation of the alpha(1D)-adrenoceptor. There was no evidence for a contractile alpha(2)-adrenoceptor-mediated response. A fluorescent ligand, quinazoline piperazine bodipy, antagonized the relaxant action of UK-14,304. This compound could be visualized on aortic endothelial cells, and its binding could be prevented by rauwolscine, providing direct evidence for the presence of alpha(2)-adrenoceptors on the endothelium. Norepinephrine reduced tone in the alpha(1D)-adrenoceptor knockout and controls, an effect blocked by rauwolscine and L-NAME but not by prazosin. This suggests that norepinephrine activates endothelial alpha(2)-adrenoceptors. In conclusion, the endothelium of mouse aorta has an alpha(2A)-adrenoceptor that responds to norepinephrine; promotes the release of nitric oxide, causing smooth muscle relaxation; and that can be directly visualized. Knockout or genetic malfunction of this receptor should increase arterial stiffness, exacerbated by raised catecholamines, and contribute to heart failure.
authors.affiliation: Autonomic Physiology Unit, Institute of Biomedical and Life Sciences, West Medical Bldg., University of Glasgow, Glasgow G12 8QQ, Scotland, UK.
authors.forename: Majid Malekzadeh
authors.initials: MM
authors.lastname: Shafaroudi
authors.forename: Melissa
authors.initials: M
authors.lastname: McBride
authors.forename: Clare
authors.initials: C
authors.lastname: Deighan
authors.forename: Alexis
authors.initials: A
authors.lastname: Wokoma
authors.forename: Joyce
authors.initials: J
authors.lastname: Macmillan
authors.forename: Craig J
authors.initials: CJ
authors.lastname: Daly
authors.forename: John C
authors.initials: JC
authors.lastname: McGrath
mesh_terms: D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects*; D000068438:Brimonidine Tartrate; D004305:Dose-Response Relationship, Drug; D042783:Endothelial Cells / Q000187:drug effects / Q000378:metabolism; D004730:Endothelium, Vascular / Q000187:drug effects* / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D007091:Image Processing, Computer-Assisted; D066298:In Vitro Techniques; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D018613:Microscopy, Confocal; D009213:Myography; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009569:Nitric Oxide / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects* / Q000235:genetics*; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D014664:Vasodilation / Q000187:drug effects*
chemical_list: C547769:Adra2a protein, mouse; D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D014662:Vasoconstrictor Agents; D009569:Nitric Oxide; D000068438:Brimonidine Tartrate; D019331:NG-Nitroarginine Methyl Ester; D009638:Norepinephrine