Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial α2-adrenoceptor activation

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 um 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 rauwo1scine ablated the rise in NO. One hundred nm aminoclonidine, guanfacine, guanabenz, UK14,304 and oxymetazoline mimicked the clonidine-induced surge of NO. Ten muM 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, WB4101, 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 muM guanethidine or I MM guanethidine plus 1 muM 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.