A Comparison of the Binding Characteristics of the β‐Adrenoceptor Antagonists 3H‐Dihydroalprenolol and 125I‐Iodocyanopindolol in Rat Liver

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Abstract
The binding characteristics of 3H-dihydroalprenolol and 125I-iodocyanopindolol were compared in a particulate fraction from regenerating rat liver. When total 3H-dihydroalprenolol binding and inhibition of total 3H-dihydroalprenolol binding by (-)isoprenaline, (-)alprenolol and (.+-.)cyanopindolol was investigated, it was found that all agents were bound to 2 classes of saturable binding sites. In the inhibition studies, the presence of 2 binding components was not obvious until the data were transformed into Hofstee plots and these were decomposed, except in the case of (.+-.)cyanopindolol. Only (.+-.)cyanopindolol distinguished clearly between the 2 saturable binding sites identified by 3H-dihydroalprenolol, as indicated by a broad plateau in the inhibition curve. When 125I-iodocyanopindolol was used as radioligand, only 1 saturable binding site was identified, even in the presence of less selective inhibiting ligands. The lower affinity component of 3H-dihydroalprenolol binding was inhibited by 10 .mu.M phentolamine. Binding experiments with 3H-prazosin indicated that the lower affinity component was not identical with the .alpha.-adrenoceptor. Phentolamine did not influence 125I-iodocyanopindolol binding. Due to its higher specific activity and a high degree of selectivity, 125I-iodocyanopindolol appeared to be the ligand of choice.