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

We have compared the ability of three radioligands, [(125)I]-cyanopindolol, [(3)H]-CGP 12,177 and [(3)H]-dihydroalprenolol, to label the three human beta-adrenoceptor subtypes. Saturation and competition binding experiments were performed using membrane preparations from Chinese hamster ovary cells stably transfected with the three subtypes. While [(3)H]-CGP 12,177 had very similar affinity for beta(1)- and beta(2)-adrenoceptors (about 40 pM), [(125)I]-cyanopindolol and [(3)H]-dihydroalprenolol had 4- to 6-fold higher affinity for beta(2)- as compared to beta(1)-adrenoceptors (10 vs 45 and 187 vs 1,021 pM, respectively). The affinity of [(125)I]-cyanopindolol at beta(3)-adrenoceptors was considerably lower (440 pM) than at the other two subtypes. The beta(3)-adrenoceptor affinity of [(3)H]-CGP 12,177 and [(3)H]-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 [(3)H]-CGP 12,177 and [(3)H]-dihydroalprenolol, respectively). We conclude that all three radioligands are ill-suited to label beta(3)-adrenoceptors, particularly in preparations co-expressing multiple subtypes. In the absence of alternatives, [(125)I]-cyanopindolol appears the least unsuitable to label beta(3)-adrenoceptors. There is a need for high-affinity radioligands which are either selective for beta(3)-adrenoceptors or reasonably non-selective among all three beta-adrenoceptor subtypes.