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

AimsThis 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 methodsThe 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 findingsSpecific 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 β1- and β2-adrenoceptor antagonists revealed a single population of β2-adrenoceptors in ocular tissues and of β1-adrenoceptors in atria, but mixed populations of β1- and β2-adrenoceptors in 70% and 30%, respectively, in lung. A competition curve for timolol was biphasic in lung and its binding affinity for β2-adrenoceptors was approximately 158-fold higher than for β1-adrenoceptors, indicating the β2-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.SignificanceThis study with tissue segment binding method suggests that the binding affinity of (−)-enantiomers of β-adrenoceptor antagonists for plasma membrane β-adrenoceptors (β1-adrenoceptors of atria, β2-adrenoceptors of ocular tissues, and mixed β1-/β2-adrenoceptors of lung) is higher than that of corresponding (+)-enantiomers and their stereoselectivity is different between β-adrenoceptor antagonists.