Clonidine-induced nitric oxide-dependent vasorelaxation mediated by endothelial alpha(2)-adrenoceptor activation

1 To assess the involvement of endothelial alpha (2)-adrenoceptors in the c lonidine-induced vasodilatation, the mesenteric artery of Sprague Dawley ra ts was cannulated and perfused with Tyrode solution (2 ml min(-1)). We meas ured perfusion pressure, nitric oxide (NO) in the perfusate using chemilumi nescence, and tissue cyclic GMP by RIA. 2 In phenylephrine-precontracted mesenteries, clonidine elicited concentrat ion-dependent vasodilatations associated to a rise in luminal NO. One hundr ed nM rauwolscine or 100 um L-omega-nitro-L-arginine antagonized the clonid ine-induced vasodilatation. Guanabenz, guanfacine, and oxymetazoline mimick ed the clonidine-induced vasorelaxation. 3 In non-contracted mesenteries, 100 nm clonidine elicited a maximal rise o f NO (123 +/- 13 pmol); associated to a peak in tissue cyclic GMP. Endothel ium removal, L-omega-nitro-L-arginine, or rauwo1scine ablated the rise in N O. One hundred nm aminoclonidine, guanfacine, guanabenz, UK14,304 and oxyme tazoline mimicked the clonidine-induced surge of NO. Ten muM ODQ obliterate d the clonidine-induced vasorelaxation and the associated tissue cyclic GMP accumulation; 10-100 nm sildenafil increased tissue cyclic GMP accumulatio n without altering the clonidine-induced NO release. 4 alpha (2)-Adrenergic blockers antagonized the clonidine-induced rise in N O. 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 clonidi ne elicited a rise of 91.9 +/- 15.5 pmol NO. Perfusion with 1 muM guanethid ine or I MM guanethidine plus 1 muM atropine did not modify the NO surge ev oked by 100 nm clonidine. 6 Clonidine and congeners activate endothelial alpha (2D)-adrenoceptors cou pled 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.