THE AFFINITY OF BETAXOLOL, A BETA-1-ADRENOCEPTOR-SELECTIVE BLOCKING-AGENT, FOR BETA-ADRENOCEPTORS IN THE BOVINE TRACHEA AND HEART

1 The specificity of betaxolol, a beta-adrenoceptor antagonist, for beta1- and beta2-adrenoceptors was compared with that of other beta-antagonists, atenolol, ICI-118551, butoxamine and (+/-)-propranolol, in the bovine trachea and heart by competitive interaction with [H-3]-CGP12177 as a radioligand. 2 The radioligand K(d) values were 0.75 +/- 0.12 and 1.60 +/- 0.11 nm in the trachea and heart, respectively, and the B(max) values were 34.00 +/- 4.41 and 21.54 +/- 2.94 fmol mg-1 protein, respectively. 3 Using ICI-118551, we determined the ratio of beta1:beta2-adrenoceptors in the trachea and heart to be approximately 29:71 and 56:44, respectively. 4 In the trachea, a beta2-predominant tissue, betaxolol and atenolol were more selective for beta1-adrenoceptor binding sites than beta2-adrenoceptor binding sites, whereas ICI-118551 and butoxamine were more selective for beta2-adrenoceptor binding sites. 5 The beta1-selectivity of betaxolol was 2.2 and 2 7 fold higher than that of atenolol in the bovine trachea and heart. These findings suggest that betaxolol may be useful in the treatment of hypertension, cardiac arrhythmia and angina pectoris.