title: A comparison of the actions of noradrenaline and UK-14,304 in the longitudinal smooth muscle of the rat isolated portal vein--no evidence for a population of post-junctional alpha 2-adrenoceptors.
abstract: The pharmacological characteristics of post-junctional alpha-adrenoceptors mediating contractions of the longitudinal smooth muscle of the rat isolated portal vein have been examined. Responses to the noncumulative addition of either noradrenaline, or the selective alpha 2-adrenoceptor agonist UK-14,304, were equally sensitive to a low concentration (0.005 mumol/l) of prazosin. Idazoxan (0.1 mumol/l-0.5 mumol/l), a selective alpha 2-adrenoceptor antagonist, was less potent than prazosin against both agonists. The combination of 0.1 mumol/l idazoxan and 0.125 mumol/l prazosin failed to produce a greater inhibition of responses to UK-14,304 than 0.125 mumol/l prazosin alone. A study involving the effect of various antagonists against a single concentration producing a submaximal response to UK-14,304, provided evidence for a prazosin-resistant component of responses which was sensitive to phentolamine. This component could, therefore, be attributed to an alpha-adrenoceptor. However, this particular response could not be ascribed to stimulation of an alpha 2-subtype since the selective alpha 1-adrenoceptor antagonist, corynanthine, produced greater inhibition than the selective alpha 2-adrenoceptor diastereoisomer rauwolscine.
authors.affiliation: Department of Pharmaceutical Sciences, University of Aston, Birmingham, England.
authors.forename: O A
authors.initials: OA
authors.lastname: Downing
authors.forename: W
authors.initials: W
authors.lastname: Stein
authors.forename: K A
authors.initials: KA
authors.lastname: Wilson
authors.forename: V G
authors.initials: VG
authors.lastname: Wilson
mesh_terms: D000818:Animals; D000068438:Brimonidine Tartrate; D004146:Dioxanes / Q000494:pharmacology; D019329:Idazoxan; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000502:physiology; D009638:Norepinephrine / Q000494:pharmacology*; D011169:Portal Vein / Q000187:drug effects / Q000502:physiology; D011224:Prazosin / Q000494:pharmacology; D011810:Quinoxalines / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*
chemical_list: D004146:Dioxanes; D011810:Quinoxalines; D011942:Receptors, Adrenergic, alpha; D000068438:Brimonidine Tartrate; D009638:Norepinephrine; D011224:Prazosin; D019329:Idazoxan