External detection of β-adrenoreceptors with 125I-hydroxybenzylpindolol in isolated perfused hearts

To assess the feasibility of characterizing β-adrenoreceptors externally with ligands labeled with γ-emitting radionuclides, we infused 125I-hydroxybenzylpindolol (125I-HYP) a potent, high affinity, β-adrenoreceptor ligand into isolated perfused hearts and monitored uptake and release of the label externally with a gamma probe. Perfusion with specific β-adrenergic displacing significantly increased the turnover rate constant of 125I-HYP in comparison to that obtained with non-specific displacing agents. After modifying β-adrenoreceptor density by pretreatment of animals with thyroid hormone, an increase in 125I-HYP uptake was observed. Thus, β-adrenoreceptor occupancy and increases in β-adrenoreceptor density can be detected externally with an approach which may offer promise for their characterization in vivo with radiolabeled adrenergic ligands. (author)