Cloning of a chick A3 adenosine receptor: characterization of ligand binding and receptor-effector coupling of chick A1 and A3 adenosine receptors.

We have reported previously the cloning and partial characterization of a chick A1 adenosine receptor expressed in the heart. We report herein the cloning of a chick A3 adenosine receptor and a comprehensive characterization of both the A1 and A3 receptors expressed in human embryonic kidney 293 cells. [125I]N6-(p-aminobenzyl)adenosine bound to both receptors with similar affinities and was used in competition studies. Although the selectivities of both agonists and antagonists were less than in other species, two antagonists, 3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynal-(+/-)-dihydropyridine-3,5-dicarboxylate and 3,6-dichloro-2'-(isopropyloxy)-4'-methylflavone), were at least partially selective for A3 receptors while one antagonist [C8-(N-methylisopropyl)amine-N6-(5'endohydroxy)endonorboman-2-yl-9-methyladenine] was selective for A1 receptors. While both receptors coupled to the inhibition of adenylyl cyclase, we were unable to detect coupling of either receptor to phospholipase C or D.