title: Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation.
abstract: 1. To assess the involvement of endothelial alpha(2)-adrenoceptors in the clonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley rats was cannulated and perfused with Tyrode solution (2 ml min(-1)). We measured perfusion pressure, nitric oxide (NO) in the perfusate using chemiluminescence, and tissue cyclic GMP by RIA. 2. In phenylephrine-precontracted mesenteries, clonidine elicited concentration-dependent vasodilatations associated to a rise in luminal NO. One hundred nM rauwolscine or 100 microM L(omega)-nitro-L-arginine antagonized the clonidine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine-induced vasorelaxation. 3. In non-contracted mesenteries, 100 nM clonidine elicited a maximal rise of NO (123+/-13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L(omega)-nitro-L-arginine, or rauwolscine ablated the rise in NO. One hundred nM aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten microM ODQ obliterated the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 - 100 nM sildenafil increased tissue cyclic GMP accumulation without altering the clonidine-induced NO release. 4. alpha(2)-Adrenergic blockers antagonized the clonidine-induced rise in NO. Consistent with a preferential alpha(2D)-adrenoceptor activation, the K(B)s for yohimbine, rauwolscine, phentolamine, WB-4101, and prazosin were: 6.8, 24, 19, 165, and 1489 nM, respectively. 5. Rat pretreatment with 100 mg kg(-1) 6-hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 nM clonidine elicited a rise of 91.9+/-15.5 pmol NO. Perfusion with 1 microM guanethidine or 1 microM guanethidine plus 1 microM atropine did not modify the NO surge evoked by 100 nM clonidine. 6. Clonidine and congeners activate endothelial alpha(2D)-adrenoceptors coupled to the L-arginine pathway, suggesting that the antihypertensive action of clonidine involves an endothelial vasorelaxation mediated by NO release, in addition to presynaptic mechanisms.
authors.affiliation: Unidad de Regulación Neurohumoral, Departamento de Ciencias Fisiológicas, Facultad de Ciencias Biológicas, Pontificia Universidad Católica de Chile, Santiago, Chile.
authors.forename: X F
authors.initials: XF
authors.lastname: Figueroa
authors.forename: M I
authors.initials: MI
authors.lastname: Poblete
authors.forename: M P
authors.initials: MP
authors.lastname: Boric
authors.forename: V E
authors.initials: VE
authors.lastname: Mendizábal
authors.forename: E
authors.initials: E
authors.lastname: Adler-Graschinsky
authors.forename: J P
authors.initials: JP
authors.lastname: Huidobro-Toro
mesh_terms: D015106:3',5'-Cyclic-GMP Phosphodiesterases; D000109:Acetylcholine / Q000494:pharmacology; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D003000:Clonidine / Q000494:pharmacology*; D006152:Cyclic GMP / Q000378:metabolism; D054706:Cyclic Nucleotide Phosphodiesterases, Type 5; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000187:drug effects / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006162:Guanylate Cyclase / Q000037:antagonists & inhibitors / Q000378:metabolism; D066298:In Vitro Techniques; D008638:Mesenteric Arteries / Q000187:drug effects / Q000378:metabolism / Q000502:physiology; D009569:Nitric Oxide / Q000378:metabolism / Q000502:physiology*; D019335:Nitroarginine / Q000494:pharmacology; D010069:Oxadiazoles / Q000494:pharmacology; D016627:Oxidopamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology; D010727:Phosphoric Diester Hydrolases / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology; D011687:Purines; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects* / Q000378:metabolism; D012503:Saponins / Q000494:pharmacology; D000068677:Sildenafil Citrate; D012995:Solubility; D013450:Sulfones; D013565:Sympatholytics / Q000494:pharmacology; D013997:Time Factors; D014655:Vascular Resistance; D014664:Vasodilation / Q000187:drug effects*; D014665:Vasodilator Agents / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: C095284:1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004791:Enzyme Inhibitors; D010069:Oxadiazoles; D010726:Phosphodiesterase Inhibitors; D010879:Piperazines; D011687:Purines; D011810:Quinoxalines; D018341:Receptors, Adrenergic, alpha-2; D012503:Saponins; D013450:Sulfones; D013565:Sympatholytics; D014665:Vasodilator Agents; D010656:Phenylephrine; D019335:Nitroarginine; D015016:Yohimbine; D009569:Nitric Oxide; D016627:Oxidopamine; D000068677:Sildenafil Citrate; D010727:Phosphoric Diester Hydrolases; D015106:3',5'-Cyclic-GMP Phosphodiesterases; D054706:Cyclic Nucleotide Phosphodiesterases, Type 5; C517384:Pde5a protein, rat; D006162:Guanylate Cyclase; D006152:Cyclic GMP; D003000:Clonidine; D000109:Acetylcholine