Pharmacological evaluation of ocular beta-adrenoceptors in rabbit by tissue segment binding method.
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.
Horinouchi
Takahiro
T
Division of Pharmacology, Department of Biochemistry and Bioinformative Sciences, School of Medicine, University of Fukui, Eiheiji, Fukui 910-1193, Japan.
Morishima
Shigeru
S
Tanaka
Yoshio
Y
Koike
Katsuo
K
Miwa
Soichi
S
Muramatsu
Ikunobu
I
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
D000319:Adrenergic beta-Antagonists; D011943:Receptors, Adrenergic, beta