title: Alpha 2-adrenoceptor agonists enhance responses to certain other vasoconstrictor agonists in the rat tail artery.
abstract: 1. The effects of the alpha2-adrenoceptor agonists clonidine, rilmenidine, TL99 and UK14304 on the vasoconstrictor response to sympathetic nerve stimulation and on the concentration-response curves to noradrenaline and phenylephrine were compared in two isolated, perfused vascular tissues: the rat tail artery (which has both postjunctional alpha 1- and alpha 2-adrenoceptors), and the rabbit ear artery (in which only alpha 1-adrenoceptors are present postjunctionally). 2. In the rabbit ear artery, the first observable effect of alpha 2-adrenoceptor agonists was inhibition of vasoconstrictor responses to sympathetic nerve stimulation. This occurred with concentrations of the alpha 2-adrenoceptor agonists which were far below those producing vasoconstriction. Responses to noradrenaline were not affected. 3. In contrast, in the rat isolated perfused tail artery, alpha 2-adrenoceptor agonists, in concentrations that produced no other observable effects, enhanced the vasoconstrictor responses to sympathetic nerve stimulation and to noradrenaline. Much higher concentrations of alpha 2-adrenoceptor agonists produced vasoconstriction in most preparations and only then reduced the response to sympathetic nerve stimulation. The enhancing effect of alpha 2-adrenoceptor agonists was blocked by idazoxan, but not by prazosin. 4. Vasoconstrictor responses in the rat tail artery to the relatively selective alpha 1-adrenoceptor agonist phenylephrine were enhanced by alpha 2-adrenoceptor agonists. The enhancement of the response to phenylephrine was greater than that to the mixed alpha 1- and alpha 2-adrenoceptor agonist noradrenaline. 5. Vasoconstrictor responses in the rat tail artery to vasopressin, ATP and KCl, like those to alpha 1-adrenoceptor agonists, were enhanced by alpha 2-adrenoceptor agonists.2+owever, vasoconstrictor responses to
authors.affiliation: Department of Pharmacology, University of Melbourne, Victoria, Australia.
authors.forename: X H
authors.initials: XH
authors.lastname: Xiao
authors.forename: M J
authors.initials: MJ
authors.lastname: Rand
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D004357:Drug Synergism; D004558:Electric Stimulation; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011817:Rabbits; D051381:Rats; D011919:Rats, Inbred Strains; D012039:Regional Blood Flow / Q000187:drug effects; D013564:Sympathetic Nervous System / Q000502:physiology; D013623:Tail / Q000098:blood supply; D014662:Vasoconstrictor Agents / Q000494:pharmacology*
chemical_list: D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D014662:Vasoconstrictor Agents; D010656:Phenylephrine; D009638:Norepinephrine