β-Adrenergic receptor subtypes in iris-ciliary body of rabbits

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Abstract
Direct comparison of drug interaction with intraocular relative to extraocular β-adrenergic receptors was used to test the hypothesis that the receptors in ocular tissues are of the β2-subtype. Potencies for the displacement of3H-dihydroalprenolol radioligand binding to membranes obtained from rabbit-iris ciliary bodies by β-adrenoceptor agonists was in the order: isoproterenol ≧ epinephrine > salbutamol > terbutaline > tazolol > norepinephrine. Similar radioligand displacement by β-adrenoceptor antagonists was in the order of potency: alprenolol ≧ bupranolol = oxprenolol = timolol = propranolol > ICI 118,551 = IPS 339 ≫ betaxolol > metoprolol. Finally, for the above β-adrenoceptor antagonists, a highly significant correlation was obtained for displacement of radioligand to ocular tissues (pKi values) relative to antagonism (pA2 values) of isoproterenol-induced guinea-pig tracheal relaxation but not guinea-pig atrial chronotropic effects. Consequently, the above three observations reinforce the notion that rabbit-iris ciliary bodies contain β-adrenergic receptors predominantly of the β2-subtype.

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