Pharmacological evaluation of ocular β-adrenoceptors in rabbit by tissue segment binding method

Abstract

Aims This study evaluates ocular (iris, ciliary body and ciliary process) and nonocular (atria and lung) β-adrenoceptors in rabbit to characterize the plasma membrane β-adrenoceptors and binding affinities of β-adrenoceptor antagonists. Main methods The tissue segment binding method with a hydrophilic radioligand (−)-4-[3- t-butylamino-2-hydroxypropoxy]-[5,7- 3H]benzimidazol-2-one ([ 3H]-CGP12177) was employed. Key findings Specific and saturable binding of [ 3H]-CGP12177 to intact tissue segments was detected by using (±)-propranolol to define nonspecific binding, showing a single population of plasma membrane binding sites with high affinity. Competition experiments with selective β 1- and β 2-adrenoceptor antagonists revealed a single population of β 2-adrenoceptors in ocular tissues and of β 1-adrenoceptors in atria, but mixed populations of β 1- and β 2-adrenoceptors in 70% and 30%, respectively, in lung. A competition curve for timolol was biphasic in lung and its binding affinity for β 2-adrenoceptors was approximately 158-fold higher than for β 1-adrenoceptors, indicating the β 2-selectivity of timolol. In contrast, competition curves for stereoisomers of befunolol, carteolol, and propranolol were monophasic in all tissues. The (−)-enantiomers of these antagonists were more potent than corresponding (+)-enantiomers in displacing from [ 3H]-CGP12177 binding, and the isomeric potency ratios of befunolol and carteolol were less than those of propranolol. Significance This study with tissue segment binding method suggests that the binding affinity of (−)-enantiomers of β-adrenoceptor antagonists for plasma membrane β-adrenoceptors (β 1-adrenoceptors of atria, β 2-adrenoceptors of ocular tissues, and mixed β 1-/β 2-adrenoceptors of lung) is higher than that of corresponding (+)-enantiomers and their stereoselectivity is different between β-adrenoceptor antagonists.