Quantitative analysis of the selectivity of radioligands for subtypes of beta adrenergic receptors

In tissues with two classes of binding sites for a drug, it is common to estimate the proportion of each class of binding site by inhibiting the binding of a radioligand with a selective unlabeled ligand. Accurate estimates of the density of each class of binding site, however, will be obtained only if the radioligand is nonselective or used at a concentration that saturates both classes of binding sites. A method of simultaneous regression analysis of multiple inhibition curves, using the program MLAB on the PROPHET system, was used to quantify the selectivity of radioligands for beta-1 or beta-2 adrenergic receptors. The selectivity of [125I]iodopindolol, [125I]iodocyanopindolol, [125I]iodohydroxybenzylpindolol and [3H]dihydroalprenolol for beta-1 and beta-2 adrenergic receptors was assessed by inhibiting the binding of each radioligand with the beta-1-selective unlabeled ligand ICI 89,406 at increasing concentrations of the radioligand, using membranes prepared from C6 glioma cells, which have both beta-1 and beta-2 adrenergic receptors. Scatchard plots for all four radioligands were linear, with correlation coefficients greater than 0.95. [125I]Iodopindolol and [125I]iodocyanopindolol were 3.2- and 2-fold selective, respectively, and [125I]iodohydroxybenzylpindolol and [3H]dihydroalprenolol were 5.8- and 2.3-fold selective, respectively, for beta-2 adrenergic receptors. Values obtained for the densities of beta-1 and beta-2 adrenergic receptors and the affinities of the receptors for ICI 89,406 were independent of the radioligand used.(ABSTRACT TRUNCATED AT 250 WORDS)