Characterization of beta-adrenoreceptors in vivo with iodine-131 pindolol and gamma scintigraphy.

The aim of this study to was to assess the feasibility of using iodopindolol to delineate myocardial beta-adrenoreceptors in vivo. Preliminary biodistribution studies indicated that binding of 131I-d,I-pindolol in the heart was stereospecific, saturable, and displaceable by I-propranolol but not by phenoxybenzamine. However, considerable nonspecific binding was encountered. Subsequently, the stereoisomer, 131I-I-pindolol, was shown to be a high affinity beta-adrenoreceptor antagonist (Kd approximately 0.37 nM) as assessed by Scatchard analysis, and one exhibiting marked specific uptake in lung and heart in rabbits. In contrast, 131I-d-pindolol exhibited no specific binding in rabbit left ventricular membrane preparations nor specific organ uptake. Gamma camera scintigraphy with both isomers demonstrated that the I-isomer accumulated in lung and heart, and that its accumulation was blocked by I-propranolol. In contrast, d-isomer uptake was nonspecific and diffuse. The results indicate that it should be possible to externally visualize receptors by differentiating specific and nonspecific binding components of a ligand in vivo with the use of radiolabeled stereoisomers.