Agonist and Antagonist Radioligands and Photoaffinity Labels for the Adenosine A1 Receptor

Experiments illustrating the use of recently developed 125I-labeled agonist and antagonist radioligands and photoaffinity labels have been described. Adenosine A1 receptors of rat cerebral cortex were characterized with arylamine-containing radioligands: an agonist, 125I-aminobenzyladenosine (125I-ABA), and an antagonist, 1-propyl-8-phenylxanthine derivative 125I-BW A827. In optimal binding conditions, both radioligands bind with KD of < 1 nM to receptors in membranes or to receptors solubilized in detergent. Photoaffinity labels have been synthesized from the radioligands by converting the arylamines to the corresponding arylazides. The resulting antagonist compound photoincorporates into a 34 000-dalton polypeptide with greater specificity and 10 times greater efficiency than the agonist.