(-)[I-125]PINDOLOL BINDING TO HUMAN PERIPHERAL LUNG BETA-RECEPTORS

Beta-adrenergic receptors in human peripheral lung were characterized by biochemical and radioligand assays employing binding of the beta-antagonist (-)[125I]pindolol to plasma membrane preparations. The specific binding of (-)[125I]pindolol reached equilibrium by 45 min with an initial rate constant of 0.0282 min-1. Binding was reversible with a kinetic dissociation rate constant of 0.0146 min-1. The calculated kinetic Kd (dissociation constant) was 430 pM which agreed very well with the Kd of 394 pM obtained by Scatchard analyses of equilibrium binding data. Computer analyses of equilibrium binding experiments revealed a similar Kd of 336 ± 24 pM. The binding capacities calculated by computer analyses (155 ± 7 fmol/mg protein) and Scatchard analyses (1013 fmol/mg protein) were also in close agreement. By all three methods (kinetic, Scatchard, and computer analyses), the data were most compatible with a single (-)[125I]pindolol binding site. Analyses of equilibrium binding data from ten different human lungs revealed values for the Kd ranging from 79 to 360 pM (mean, 136 pM), and for the receptor concentration ranging from 58 to 196 fmol/mg protein (mean, 118 fmol/mg protein). The displacement of (-)[125I]pindolol binding by various agents exhibited stereoselectivity and the expected rank order of potency predicted for interactions with beta-receptors. Isoproterenol induced a rapid and dose-related increase in cyclic AMP that was prevented by specific beta-antagonists. Approximately 70% of the beta-receptors were found to be of the beta2-subtype by both radioligand binding and biochemical assays. Thus, (-)[125I]pindolol appears to be an excellent ligand for characterizing human lung beta-receptors since accurate and reproducible results can be obtained with this radioligand using limited tissue sample quantities. © 1987.