PHARMACOLOGICAL PROPERTIES OF ADENOSINE RECEPTORS AND ADENOSINE BINDING-PROTEINS

Two major subtypes of adenosine receptors occur in different tissues which have been distinguished by pharmacological and biochemical criteria. The A1 adenosine receptor has a high-affinity for adenosine and mediates inhibition of adenylyl cyclase, whereas the A2 adenosine receptor usually has a lower affinity and mediates stimulation of the enzyme. Furthermore, evidence has been obtained that A1 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 A1 receptor are the agonist [H-3]2-chloro-N6-cyclopentyladenosine (CCPA) and the antagonist [H-3]8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and for the A2a receptor [H-3]2-[p-(carboxyethyl)phenethylamino]-5'-N-carboxamido-adenosine (CGS 21680). 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 Al receptor has an apparent molecular weight of approximately 36 kDa and the A2a 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 A1 receptors have been purified to apparent homogeneity. More recently, the structure of both receptor subtypes has been elucidated by cloning the receptors from a cDNA library. Furthermore, a novel adenosine binding protein was identified in bovine brain by radioligand binding with [H-3]5'-N-ethylcarboxamidoadenosine ([H-3]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 A2a adenosine receptors and other adenosine binding proteins. We propose the name A(x) for this unique adenosine binding protein.