Evidence against alpha-adrenoceptors mediating relaxation in rat thoracic aortae: alpha-agonists relaxation depends on interaction with alpha-adrenoceptors.
In rat aorta, the presence of functional alpha(2)-adrenoceptors (alpha(2)-AR) was investigated in ring preparations preconstricted with alpha(1)-adrenergic and non- alpha(1)-adrenergic agonists. Particularly, the hypothetical interference of alpha(2)-AR agonists with alpha(1)-AR-mediated vasoconstriction was evaluated. Relaxant and contractile responses to alpha(2)-AR agonists were obtained. In endothelium-intact and endothelium-denuded aortic rings preconstricted with phenylephrine (1 x 10(-6) m), the imidazoline derivatives, clonidine and UK14304, induced relaxations with similar order of potencies (-log EC(50)) and maxima relaxant effects respectively. Pretreatment with the NO synthase inhibitor, N(G)-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 alpha(2)-AR antagonist, rauwolscine (< or =1 x 10(-6) m). Clonidine and UK 14304 induced only contractions on endothelium-intact and endothelium-denuded aortic rings contracted with prostaglandin F(2alpha) (3 x 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 alpha(1D)-AR selective antagonist, BMY 7378, and rauwolscine. The pA(2) and pK(B) values for BMY 7378 and rauwolscine, respectively, against endothelium-independent actions of clonidine and UK 14304 were characteristic of an effect on the alpha(1D)-AR. The other selective alpha(2)-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 alpha(2)-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 alpha(1)-AR), inhibitory and excitatory, interaction with alpha(1)-ARs.
Castillo
Enrique F
EF
Sección de Estudios de Posgrado e Investigación, Escuela Superior de Medicina, Instituto Politécnico Nacional, México, DF, México. fech_esm@hotmail.com
Ortíz
Cindy S
CS
López
Ruth M
RM
Ruíz
Antonio
A
Vélez
Juan M
JM
Castillo
Carlos
C
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001013:Aorta, Thoracic / Q000187:drug effects* / Q000378:metabolism; D001381:Azepines / Q000494:pharmacology; D000068438:Brimonidine Tartrate; D003000:Clonidine / Q000494:pharmacology; D015237:Dinoprost / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004730:Endothelium, Vascular / Q000187:drug effects / Q000378:metabolism; D066298:In Vitro Techniques; D008297:Male; D008729:Methoxamine / Q000494:pharmacology; D009569:Nitric Oxide / Q000378:metabolism; D010656:Phenylephrine / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018340:Receptors, Adrenergic, alpha-1 / Q000187:drug effects / Q000378:metabolism; D018341:Receptors, Adrenergic, alpha-2 / Q000187:drug effects* / Q000378:metabolism; D012701:Serotonin / Q000494:pharmacology; D014662:Vasoconstrictor Agents / Q000494:pharmacology; D014664:Vasodilation / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D001381:Azepines; D010879:Piperazines; D011810:Quinoxalines; D018340:Receptors, Adrenergic, alpha-1; D018341:Receptors, Adrenergic, alpha-2; D014662:Vasoconstrictor Agents; D010656:Phenylephrine; D015016:Yohimbine; D009569:Nitric Oxide; D012701:Serotonin; D000068438:Brimonidine Tartrate; D015237:Dinoprost; C015603:azepexole; D008729:Methoxamine; C053428:BMY 7378; D003000:Clonidine