title: Existence of two types of postjunctional alpha-adrenoceptors in the isolated canine internal carotid artery.
abstract: The pharmacological characteristics of postjunctional alpha-adrenoceptors in isolated canine internal carotid arteries were investigated by the use of selective agonists and antagonists for alpha 1- and alpha 2-adrenoceptors. Norepinephrine, phenylephrine, and xylazine caused concentration-dependent contractions in the helical strips. The contraction induced by 10(-4)M xylazine was significantly smaller than that produced by 10(-4)M norepinephrine or 10(-4)M phenylephrine. The contraction induced by 10(-4)M phenylephrine was almost the same value as that induced by 10(-4)M norepinephrine. Phentolamine (10(-8) and 10(-7)M) caused a parallel shift to the right of the concentration-response curve to norepinephrine. The contractile responses to low concentrations of norepinephrine were significantly suppressed by pretreatment with an alpha 2-antagonist such as yohimbine (10(-9) and 10(-8)M) or DG5128 (10(-7) and 10(-6)M). On the other hand, the responses to higher concentrations of norepinephrine were mainly reduced by low concentrations of an alpha 1-antagonist, prazosin (3 x 10(-10) and 3 x 10(-9)M). These results suggest that both alpha 1- and alpha 2-adrenoceptors are located on the plasma membrane of smooth muscle cells in canine internal carotid arteries and that the norepinephrine-induced contractions at low and high concentrations are mainly mediated by activation of alpha 2- and alpha 1-adrenoceptors, respectively.
authors.affiliation: Department of Physiology, Shinshu University School of Medicine, Matsumoto, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Kawai
authors.forename: S
authors.initials: S
authors.lastname: Kobayashi
authors.forename: T
authors.initials: T
authors.lastname: Ohhashi
mesh_terms: D000818:Animals; D002343:Carotid Artery, Internal / Q000378:metabolism*; D004285:Dogs; D004305:Dose-Response Relationship, Drug; D005260:Female; D007093:Imidazoles / Q000494:pharmacology; D008297:Male; D009638:Norepinephrine / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000378:metabolism*; D014991:Xylazine / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D007093:Imidazoles; D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D014991:Xylazine; D015016:Yohimbine; C032368:midaglizole; D009638:Norepinephrine; D011224:Prazosin; D010646:Phentolamine