Binding Characteristics of ^^3 H-CGP12177 to β-adrenoceptors in rat myocardial membranes

The present study was designed to examine the selectivity of ^^3 H-CGP12177 for β_1 - and β_2 -adrenergic receptors by the Scatchard and the displacement analysis. The plots of specific binding obtained from the Scatchard analysis using ^^3 H-CGP12177 for the myocardium membrane were uniphasic when the non-specific binding was detemined by the use of 10 μM 1-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 1-metoprolol was used as a competitor. The existence of these two binding sites implied the selectivity of ^^3 H-CGP12177 to β-adrenoceptors because ^^3 H-CGP12177 was 1.8-fold more selective towards β_1 -adrenoceptors than β_2 -adrenoceptors. These two binding sites could be regarded as β_1 - and β_2 -adrenergic receptors from the evaluation of the binding characteristics of β-adrenoceptors by the displacement analysis using β-selective antagonists. The data suggests that ^^3 H-CGP12177, a hydrophilic radioligand, is useful for the binding assay of β-adrenoceptors in rat myocardial membranes.