Abstract
Two major subtypes of adenosine receptors occur in different tissues which have been distinguished by pharmacological and biochemical criteria. The A~1~ adenosine receptor has a high‐affinity for adenosine and mediates inhibition of adenylyl cyclase, whereas the A~2~ adenosine receptor usually has a lower affinity and mediates stimulation of the enzyme. Furthermore, evidence has been obtained that A~1~ receptors increase the conductance of receptor‐regulated potassium channels, induce inactivation of calcium channels, and modulate the breakdown of phosphoinositides by phospholipase C. Selective agonists and antagonists have been developed for both receptor subtypes. In addition, both adenosine receptors have extensively been characterized by radioligand binding studies. Suitable radioligands for the A~1~ receptor are the agonist [^3^H]2‐chloro‐N^6^‐cyclopentyladenoisine (CCPA) and the antagonist [^3^H]8‐cyclopentyl‐1,3‐dipropylxanthine (DPCPX)and for the A~2a~ receptor [^3^H]2‐[p‐(carboxyethyl)phenethylamino]‐ 5′‐N‐carboxamidoadenosine (CGS 21860). Furthermore, photoaffinity ligands were developed from adenosine derivatives, which can be covalently incorporated into the binding unit of both receptor subtypes. With this approach, it has been shown that the A~1~ receptor has an apparent molecular weight of approximately 36 kDa and the A~2a~ receptor of 45 kDa. A second approach to elucidate the structure of adenosine receptors involves the purification of receptor protein by affinity chromatography. With this procedure, cerebral A~1~ receptors have been purified to apparent homogeneity. More recently, the structure of receptor subtypes has been elucidated by cloning the receptors from a cDNA library. Furthermore, a novel adenosine binding with [^3^H] 5′ ‐N‐ethylcarboxamidoadenosine ([^3^H]NECA). The pharmacological profile of this NECA‐binding protein has been determined in competition experiments with adenosine receptor ligands. It can be distinguished from that of A~2a~ adenosine receptors and other adenosine binding proteins. We propose the name A~x~ for this unique adenosine binding protein. © 1993 Wiley‐Liss, Inc.