Comparison of three radioligands for the labelling of human β-adrenoceptor subtypes

We have compared the ability of three radioligands, [125I]-cyanopindolol, [3H]-CGP 12,177 and [3H]-dihydroalprenolol, to label the three human β-adrenoceptor subtypes. Saturation and competition binding experiments were performed using membrane preparations from Chinese hamster ovary cells stably transfected with the three subtypes. While [3H]-CGP 12,177 had very similar affinity for β1- and β2-adrenoceptors (about 40 pM), [125I]-cyanopindolol and [3H]-dihydroalprenolol had 4- to 6-fold higher affinity for β2- as compared to β1-adrenoceptors (10 vs 45 and 187 vs 1,021 pM, respectively). The affinity of [125I]-cyanopindolol at β3-adrenoceptors was considerably lower (440 pM) than at the other two subtypes. The β3-adrenoceptor affinity of [3H]-CGP 12,177 and [3H]-dihydroalprenolol was so low that it could not be estimated within the tested range of radioligand concentrations (up to 4,000 pM and 30,000 pM for [3H]-CGP 12,177 and [3H]-dihydroalprenolol, respectively). We conclude that all three radioligands are ill-suited to label β3-adrenoceptors, particularly in preparations co-expressing multiple subtypes. In the absence of alternatives, [125I]-cyanopindolol appears the least unsuitable to label β3-adrenoceptors. There is a need for high-affinity radioligands which are either selective for β3-adrenoceptors or reasonably non-selective among all three β-adrenoceptor subtypes.