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  • Title: Adrenoceptor mechanisms in epinephrine-induced anterior choroidal vasoconstriction in cats.
    Author: Koss MC.
    Journal: Exp Eye Res; 1994 Dec; 59(6):715-22. PubMed ID: 7698264.
    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.
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