(±)[125Iodo]cyanopindolol, a new ligand for β-adrenoceptors: Identification and quantitation of subclasses of β-adrenoceptors in guinea pig

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Abstract
(±)[125Iodo]cyanopindolol (ICYP) is a radioligand which binds with an extraordinarily high affinity and specificity to β-adrenoceptors. In contrast to (±)[125Iodo]-hydroxybenzylpindolol (IHYP), the new ligand has neither affinity to α- nor to 5-HT-receptors. The dissociation constants of ICYP for β-adrenoceptors in various tissues range from 27 to 40 pM, thereby exceeding the affinity of IHYP by a factor of ∼ 3. ICYP does not discriminate between β1− and β2−. Therefore, the densities of the two receptor subtypes can be determined from competition curves of ICYP by drugs previously found to show in vitro selectivity for β1−adrenoceptors. The guinea pig left ventricle contains only β1− adrenoceptors, whereas in the lung tissue, the ratio of β1− to β2−adrenoceptors is 1 to 4. The calculated affinities of five β1− selective antagonists for β1−adrenoceptors were nearly identical in the ventricle and the lung. Kinetic studies of ICYP binding to guinea pig lung membranes indicated that the dissociation reaction consists of two components, a fast process (t1/2=9 min) and a slower process (t1/2=8.8 h). A mathematical treatment revealed two possibilities of interpretation: 1. Two forms of the receptor exist which are interconvertible. 2. The (+)- and (−)-enantiomers of ICYP dissociate with different rate constants. The low dissociation constant of ICYP in combination with its high specific radioactivity (2175 Ci mmole−1) allows binding studies to be carried out with small protein and ligand concentrations, e.g. 3 μg protein per assay in guinea pig lung membranes.