title: Pharmacological evaluation of ocular beta-adrenoceptors in rabbit by tissue segment binding method.
abstract: OBJECTIVE
  This study evaluates ocular (iris, ciliary body and ciliary process) and nonocular (atria and lung) beta-adrenoceptors in rabbit to characterize the plasma membrane beta-adrenoceptors and binding affinities of beta-adrenoceptor antagonists.
  
  
  METHODS
  The tissue segment binding method with a hydrophilic radioligand (-)-4-[3-t-butylamino-2-hydroxypropoxy]-[5,7-(3)H]benzimidazol-2-one ([(3)H]-CGP12177) was employed.
  
  
  RESULTS
  Specific and saturable binding of [(3)H]-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 beta(1)- and beta(2)-adrenoceptor antagonists revealed a single population of beta(2)-adrenoceptors in ocular tissues and of beta(1)-adrenoceptors in atria, but mixed populations of beta(1)- and beta(2)-adrenoceptors in 70% and 30%, respectively, in lung. A competition curve for timolol was biphasic in lung and its binding affinity for beta(2)-adrenoceptors was approximately 158-fold higher than for beta(1)-adrenoceptors, indicating the beta(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 [(3)H]-CGP12177 binding, and the isomeric potency ratios of befunolol and carteolol were less than those of propranolol.
  
  
  CONCLUSIONS
  This study with tissue segment binding method suggests that the binding affinity of (-)-enantiomers of beta-adrenoceptor antagonists for plasma membrane beta-adrenoceptors (beta(1)-adrenoceptors of atria, beta(2)-adrenoceptors of ocular tissues, and mixed beta(1)-/beta(2)-adrenoceptors of lung) is higher than that of corresponding (+)-enantiomers and their stereoselectivity is different between beta-adrenoceptor antagonists.
authors.affiliation: Division of Pharmacology, Department of Biochemistry and Bioinformative Sciences, School of Medicine, University of Fukui, Eiheiji, Fukui 910-1193, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Horinouchi
authors.forename: Shigeru
authors.initials: S
authors.lastname: Morishima
authors.forename: Yoshio
authors.initials: Y
authors.lastname: Tanaka
authors.forename: Katsuo
authors.initials: K
authors.lastname: Koike
authors.forename: Soichi
authors.initials: S
authors.lastname: Miwa
authors.forename: Ikunobu
authors.initials: I
authors.lastname: Muramatsu
mesh_terms: D000319:Adrenergic beta-Antagonists / Q000737:chemistry / Q000494:pharmacology*; D000818:Animals; D002462:Cell Membrane / Q000187:drug effects / Q000378:metabolism; D005123:Eye / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D006321:Heart / Q000187:drug effects; D066298:In Vitro Techniques; D008168:Lung / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D011485:Protein Binding; D011817:Rabbits; D011869:Radioligand Assay; D011943:Receptors, Adrenergic, beta / Q000378:metabolism*; D013237:Stereoisomerism
chemical_list: D000319:Adrenergic beta-Antagonists; D011943:Receptors, Adrenergic, beta