Clonidine‐induced nitric oxide‐dependent vasorelaxation mediated by endothelial α 2 ‐adrenoceptor activation

To assess the involvement of endothelial α 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. In phenylephrine‐precontracted mesenteries, clonidine elicited concentration‐dependent vasodilatations associated to a rise in luminal NO. One hundred n M rauwolscine or 100 μ M L ω ‐nitro‐ L ‐arginine antagonized the clonidine‐induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimicked the clonidine‐induced vasorelaxation. In non‐contracted mesenteries, 100 n M clonidine elicited a maximal rise of NO (123±13 pmol); associated to a peak in tissue cyclic GMP. Endothelium removal, L ω ‐nitro‐ L ‐arginine, or rauwolscine ablated the rise in NO. One hundred n M aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine‐induced surge of NO. Ten μ M ODQ obliterated the clonidine‐induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10 – 100 n M sildenafil increased tissue cyclic GMP accumulation without altering the clonidine‐induced NO release. α 2 ‐Adrenergic blockers antagonized the clonidine‐induced rise in NO. Consistent with a preferential α 2D ‐adrenoceptor activation, the K B s for yohimbine, rauwolscine, phentolamine, WB‐4101, and prazosin were: 6.8, 24, 19, 165, and 1489 n M , respectively. Rat pretreatment with 100 mg kg −1 6‐hydroxydopamine reduced 95% tissue noradrenaline and 60% neuropeptide Y. In these preparations, 100 n M clonidine elicited a rise of 91.9±15.5 pmol NO. Perfusion with 1 μ M guanethidine or 1 μ M guanethidine plus 1 μ M atropine did not modify the NO surge evoked by 100 n M clonidine. Clonidine and congeners activate endothelial α 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. British Journal of Pharmacology (2001) 134 , 957–968; doi: 10.1038/sj.bjp.0704320