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

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Abstract
The binding affinities of racemic derivatives of propranolol to .beta.-adrenoceptors were examined by means of radioligand binding assay to determine whether racemic propranolol shows biphasic binding to .beta.-adrenoceptors in a displacement experiment. Rat brain was used and the .beta.-adrenoceptor binding ssay 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 .beta.-adrenoceptors of rat brain may bind the d- and l-isomers at distinct sites.