Binding affinities of ocular hypotensive β-blockers levobetaxolol, levobunolol, and timolol at endogenous guinea pig β-adrenoceptors

The current study determined the relative affinities and selectivities of numerous beta-adrenoceptor antagonists at the endogenous beta(1)- and beta(2)-adrenoceptors in guinea pig heart and lung, respectively, using [(3)H]-CGP12177. Specific binding of [(3)H]-CGP12177 comprised 80 +/- 0.2% (n = 11) and 94 +/- 0.2% (n = 16) of the total binding in washed heart and lung homogenates, respectively. Concentration-dependent displacement of [(3)H]-CGP12177 binding from beta-adrenoceptors in both preparations was observed with nine different beta-adrenoceptor antagonists. Levobetaxolol, betaxolol, CGP-20712A, levobunolol, and timolol yielded bi-phasic (two-site-fit) competition curves in the heart, while CGP-20712A, ICI-118551 and levobunolol produced bi-phasic curves in the lung preparation. The high-affinity component of [(3)H]-CGP12177 binding in the heart and lung reflected binding to beta(1)-receptors and beta(2)-receptors, respectively. The binding inhibition parameters (IC(50)s) for displacement of [(3)H]-CGP12177 from these predominantly high-affinity sites were: levobetaxolol (24.9 +/- 1.6 nM heart, 4810 +/- 367 nM lung), racemic betaxolol (37.9 +/- 8.7 mM heart; 8840 +/- 424 mM lung), CGP-20712A (4.6 +/- 0.9 nM heart; 171,000 +/- 109,000 nM lung), ICI-118551 (9230 +/- 3240 nM heart; 2.9 +/- 0.6 nM lung), levobunolol (42 +/- 15 nM heart, 0.3 +/- 0.2 nM lung), (l)-timolol (3.1 nM heart, 2.9 +/- 1.5 nM lung), ICI-215001 (5840 +/- 114 nM heart; 26100 +/- 3200 nM lung), BRL-37344 (83,300 +/- 2660 nM heart; 13,200 +/- 1250 lung). These data indicated that while levobetaxolol and betaxolol possessed a 193-233-fold selectivity for beta(1)-receptors, levobunolol exhibited a 140-fold beta(2)-receptor selectivity and (l)-timolol was essentially nonselective.