Binding characteristics of 3H-CGP12177 to beta-adrenoceptors in rat myocardial membranes

The present study was designed to examine the selectivity of 3H-CGP-12177 (4-(3-t-butylamino-2-hydroxypropoxy)-[5,7-3H]benzimidazole-2-one hydrochloride) for beta 1- and beta 2-adrenergic receptors by the Scatchard and the displacement analysis. The plots of specific binding obtained from the Scatchard analysis using 3H-CGP12177 for the rat myocardium membrane were uniphasic when the non-specific binding was determined by the use of 10 microM I-propranolol, and the Kd and Bmax values were 408.53 +/- 67.20 pM and 12.27 +/- 0.83 fmoles/mg protein, respectively. On the other hand, two binding sites were observed in the displacement curve when I-metoprolol was used as a competitor. The existence of these two binding sites implied the selectivity of 3H-CGP12177 to beta-adrenoceptors because 3H-CGP12177 was 1.8-fold more selective towards beta 1-adrenoceptors than beta 2-adrenoceptors. In addition, these two binding sites could be regarded as beta 1- and beta 2-adrenergic receptors from the evaluation of the binding characteristics beta-adrenoceptors by the displacement analysis using beta-selective antagonists. Thus, 3H-CGP12177, a hydrophilic radioligand, was useful for the binding assay of beta-adrenoceptors in rat myocardial membranes.