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

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Abstract
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 .mu.M 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 .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 of .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.

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