Characterization of 3H-dihydroalprenolol binding to beta-adrenergic receptors of rat brain: two binding sites of racemic propranolol in displacement experiments.

Abstract

The binding affinities of racemic derivatives of propranolol to β-adrenoceptors were examined by means of radioligand binding assay to determine whether racemic propranolol shows biphasic binding to β-adrenoceptors in a displacement experiment. Rat brain was used and the β-adrenoceptor binding assay was carried out with 3H-dihydroalprenolol as the radioligand. A higher affinity of l-propranolol than of d-propranolol was observed and d-and l-propranolol each gave a uniphasic curve. However, dl-propranolol showed a biphasic curve. Furthermore, a mixture of d-and l-propranolol also showed a biphasic curve. These results imply that β-adrenoceptors of rat brain may bind the d-and l-isomers at distinct sites.