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 α2-adrenoceptors. In the presence of induced tone, UK-14,304 produced concentration-related vasodilatation that was abolished by rauwolscine, Nω-nitro-l-arginine methyl ester (l-NAME), or endothelium removal, indicating that endothelial α2-adrenoceptors can release nitric oxide. In the α2A-adrenoceptor knockout mouse and the D79N mouse, a functional knockout of the α2A-adrenoceptor, these relaxant effects of UK-14,304 were lost, indicating the involvement of the α2A-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 α1D-adrenoceptor knockout mouse, indicating activation of the α1D-adrenoceptor. There was no evidence for a contractile α2-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 α2-adrenoceptors on the endothelium. Norepinephrine reduced tone in the α1D-adrenoceptor knockout and controls, an effect blocked by rauwolscine and l-NAME but not by prazosin. This suggests that norepinephrine activates endothelial α2-adrenoceptors. In conclusion, the endothelium of mouse aorta has an α2A-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.
Footnotes
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This work was supported by the EC project VASCAN-2000 (QLG-CT-1999-00084); Iranian Government Ministry of Education (to M.M.S.); British Heart Foundation, Junior Fellowship (to C.D.) and Postgraduate Scholarship (to M.M.); The Anne B. McNaught Bequest; and The Muirhead Trust of Glasgow University.
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Article, publication date, and citation information can be found at http://jpet.aspetjournals.org.
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doi:10.1124/jpet.105.085944.
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ABBREVIATIONS: QAPB, quinazoline piperazine bodipy; UK-14,304, 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine; U-46619, 9,11-dideoxy-9α,11α-methanoepoxy-prostaglandin F2α; l-NAME, Nω-nitro-l-arginine methyl ester; 5MeU, 5-methylurapidil; BMY7378, 8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4.5]decane-7,9-dione; DMSO, dimethyl sulfoxide.
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↵1 These authors contributed equally to this work.
- Received March 7, 2005.
- Accepted May 3, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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