title: Evidence that the population of postjunctional-adrenoceptors mediating contraction of smooth muscle in the rabbit isolated ear vein is predominantly alpha 2.
abstract: 1. Noradrenaline (NA), phenylephrine and UK-14304 elicited concentration-dependent contractions of the rabbit isolated ear vein of similar maximal magnitude. The rank order of potency, UK-14304 greater than noradrenaline greater than phenylephrine, is consistent with that of an effect mediated through an alpha 2-subtype. 2. The potent and highly selective alpha 1-adrenoceptor antagonists prazosin and YM-12617, at concentrations as high as 1 microM, produced less than a 4 fold rightward displacement of the NA concentration-response curve. 3. The selective alpha 2-adrenoceptor antagonists rauwolscine, Wy-26703 and CH-38083 antagonized responses to noradrenaline in a competitive manner. For all three antagonists, the pA2 values were consistent with an effect at alpha 2-adrenoceptors. However, 0.1 microM YM-12617 increased the potency of rauwolscine 2 fold indicating the presence of a small population of postjunctional alpha 1-adrenoceptors. 4. The relative antagonist potency of the yohimbine diastereoisomers rauwolscine and corynanthine against noradrenaline (rauwolscine 30 fold greater than corynanthine) is also consistent with an effect at alpha 2-adrenoceptors. 5. Contractions elicited by noradrenaline in the rabbit isolated ear vein appear to be mediated predominantly by postjunctional alpha 2-adrenoceptors.
authors.affiliation: Autonomic Physiology Unit, University of Glasgow, Scotland.
authors.forename: C J
authors.initials: CJ
authors.lastname: Daly
authors.forename: J C
authors.initials: JC
authors.lastname: McGrath
authors.forename: V G
authors.initials: VG
authors.lastname: Wilson
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology; D000068438:Brimonidine Tartrate; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000378:metabolism*; D009638:Norepinephrine / Q000037:antagonists & inhibitors / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology; D011817:Rabbits; D011941:Receptors, Adrenergic / Q000502:physiology*; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism / Q000502:physiology*; D014680:Veins / Q000502:physiology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000317:Adrenergic alpha-Antagonists; D000959:Antihypertensive Agents; D011810:Quinoxalines; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D015016:Yohimbine; D000068438:Brimonidine Tartrate; D009638:Norepinephrine; D011224:Prazosin