title: Adrenoceptor mechanisms in epinephrine-induced anterior choroidal vasoconstriction in cats.
abstract: These studies were undertaken to determine the role of epinephrine in the anterior choroidal circulation and to define the relative contribution of adrenoceptor subtypes in this response. Intra-arterial administration of l-epinephrine (0.03-1 microgram) produced vasoconstrictor responses in the anterior choroid of anesthetized cats as measured using laser-Doppler flowmetry. Ipsilateral nictitating membrane contractions were simultaneously recorded. Responses of both organs were blocked by intravenous treatment with the non-selective alpha-adrenoceptor antagonist phentolamine; however, only nictitating membrane contractions were significantly antagonized with the selective alpha 1-adrenoceptor antagonist, prazosin. In contrast, alpha 2-adrenoceptor blockade with rauwolscine had no depressant effect on the nictitating membrane but was a potent antagonist for epinephrine-induced ocular vasoconstriction. This differential ocular receptor type activation was confirmed with the use of more selective alpha-adrenoceptor agonists. Both the alpha 1-adrenoceptor agonist, methoxamine, and the selective alpha 2-adrenoceptor stimulant, B-HT 933, produced choroidal vasoconstriction when given intra-arterially. B-HT 933 was as potent as methoxamine in producing choroidal vasoconstriction which suggests a high ratio of alpha 2-adrenoceptors in this vascular bed. B-HT 933 was much less potent than methoxamine in producing contraction of the nictitating membrane. These results demonstrate the usefulness of laser-Doppler flowmetry in studies of the choroidal circulation and suggest that, unlike the nictitating membrane, epinephrine-induced anterior segment vasoconstriction is mediated by both postjunctional alpha 1- and alpha 2-adrenoceptors with alpha 2-adrenoceptors being predominant.
authors.affiliation: Department of Pharmacology, University of Oklahoma College of Medicine, Oklahoma City 73190, USA.
authors.forename: M C
authors.initials: MC
authors.lastname: Koss
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000818:Animals; D001794:Blood Pressure / Q000187:drug effects; D002415:Cats; D002829:Choroid / Q000098:blood supply*; D004305:Dose-Response Relationship, Drug; D004837:Epinephrine / Q000494:pharmacology*; D009119:Muscle Contraction / Q000187:drug effects; D009541:Nictitating Membrane / Q000187:drug effects; D010646:Phentolamine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000502:physiology*; D014661:Vasoconstriction / Q000187:drug effects*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D011942:Receptors, Adrenergic, alpha; D015016:Yohimbine; D011433:Propranolol; D011224:Prazosin; D004837:Epinephrine; D010646:Phentolamine