title: Evidence for two subtypes of alpha adrenergic receptors in canine airway smooth muscle.
abstract: The contractile response of canine tracheal muscle to i.a. phenylephrine, clonidine and norepinephrine was studied isometrically in situ in 32 dogs after beta adrenergic and ganglionic blockade. Intra-arterial phenylephrine caused dose-related tracheal contraction which was not altered by yohimbine (5 microgram/kg i.a.). Prazosin (4 mg/kg i.v.) caused a 77 +/- 3% decrease in tracheal response to i.a. phenylephrine. Clonidine also caused dose-related tracheal contraction, which was not altered by prazosin (4 mg/kg i.v.) but was 95 +/- 2% blocked by 5 microgram/kg i.a. of yohimbine. Norepinephrine caused tracheal muscle contraction which was greater than both phenylephrine (P less than .05) and yohimbine (P less than .001). Prazosin (4 mg/kg i.v.) caused 53 +/- 6% blockade and yohimbine (5 microgram/kg i.a.) caused 76 +/- 2% blockade of the response to i.a. norepinephrine; prazosin plus yohimbine caused greater than 98% blockade of the response to i.a. norepinephrine. The dose-response curve to i.a. acetylcholine was not altered by treatment with prazosin (4 mg/kg i.v.) plus yohimbine (5 microgram/kg i.a.). These results demonstrate that tracheal contraction induced by sympathomimetic amines is mediated by two subtypes of alpha adrenergic receptors on tracheal muscle, alpha-1 for phenylephrine, alpha-2 for clonidine and both alpha-1 and alpha-2 for norepinephrine.
mesh_terms: D000403:Airway Resistance / Q000187:drug effects; D000818:Animals; D003000:Clonidine / Q000494:pharmacology; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D008297:Male; D009130:Muscle, Smooth / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011941:Receptors, Adrenergic / Q000145:classification*; D011942:Receptors, Adrenergic, alpha / Q000145:classification* / Q000187:drug effects; D012137:Respiratory System / Q000187:drug effects*; D014132:Trachea / Q000187:drug effects
chemical_list: D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D003000:Clonidine; D009638:Norepinephrine