Binding Characteristics of 3H-CGP12177 to β-Adrenoceptors in Rat Myocardial Membranes

Abstract

The present study was designed to examine the selectivity of 3H-CGP-12177 (4-(3-t-butylamino-2-hydroxyproppxy)-[5,7-3H]benzimidazole-2-one hydrochloride) for β1- and β2-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 detemined by the use of 10 μM l-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 l-metoprolol was used as a competitor. The existence of these two binding sites implied the selectivity of 3H-CGP12177 to β-adrenoceptors because 3H-CGP12177 was 1.8-fold more selective towards β1-adrenoceptors than β2-adrenoceptors. In addition, these two binding sites could be regarded as β1 and β2-adrenergic receptors from the evaluation of the binding characteristics of β-adrenoceptors by the displacement analysis using β-selective antagonists. Thus, 3H-CGP12177, a hydrophilic radioligand, was useful for the binding assay of β-adrenoceptors in rat myocardial membranes.