Evidence against α: α2-agonists relaxation depends on interaction with α1-adrenoceptors2: α2-agonists relaxation depends on interaction with α1-adrenoceptors-adrenoceptors mediating relaxation in rat thoracic aortae: α2-agonists relaxation depends on interaction with α1-adrenoceptors

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Abstract

In rat aorta, the presence of functional α2-adrenoceptors (α2-AR) was investigated in ring preparations preconstricted with α1-adrenergic and non- α1-adrenergic agonists. Particularly, the hypothetical interference of α2-AR agonists with α1-AR-mediated vasoconstriction was evaluated. Relaxant and contractile responses to α2-AR agonists were obtained. In endothelium-intact and endothelium-denuded aortic rings preconstricted with phenylephrine (1 × 10−6 M), the imidazoline derivatives, clonidine and UK14304, induced relaxations with similar order of potencies (−log EC50) and maxima relaxant effects respectively. Pretreatment with the NO synthase inhibitor, NG-nitro-L-arginine methyl ester (L-NAME) had no effect on the relaxant responses to clonidine and UK14304. In phenylephrine-constricted rings with endothelium, relaxations to clonidine and UK 14304 were not antagonized by the selective α2-AR antagonist, rauwolscine (≤1 × 10−6 M). Clonidine and UK 14304 induced only contractions on endothelium-intact and endothelium-denuded aortic rings contracted with prostaglandin F2α (3 × 10−7 M). Moreover, clonidine and UK 14304-induced relaxation of endothelium-denuded arteries precontracted with methoxamine but not with serotonin. Finally, the concentration–contraction curves to clonidine and UK 14304 in endothelium-denuded aortic rings were significantly shifted to the right by the α1D-AR selective antagonist, BMY 7378, and rauwolscine. The pA2 and pKB values for BMY 7378 and rauwolscine, respectively, against endothelium-independent actions of clonidine and UK 14304 were characteristic of an effect on the α1D-AR. The other selective α2-AR agonist tested BHT 933 (an azepine derivative), lacks considerable relaxant and contractile effects in rat aorta. The results provide no evidence for the presence of functional α2-AR in rat aorta. Respectively, the relaxant and contractile effects of the imidazoline derivatives, clonidine and UK 14304, may be due to an adjustable (in relation to the agonist-dependent active state of the α1-AR), inhibitory and excitatory, interaction with α1-ARs.

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