Characterization of adenosine A2 receptors in bovine retinal membranes.

Two classes of extracellular receptors for adenosine, A1 and A2, have been demonstrated in the mammalian retina. Our laboratory has previously reported the pharmacological characteristics of the mammalian retinal A1 receptors. We now report our characterization of retinal A2 receptors based on data obtained from both adenylate cyclase assays and radioligand binding studies. [3H]-5'-N-ethylcarboxamidoadenosine (NECA) in the presence of 10 nM cyclopentyladenosine (CPA, which selectively binds to A1 receptors) or [3H]-CGS 21680 were used to label the A2 binding sites. Using [3H]-NECA (plus CPA), two populations of binding sites, having Kds of 106 nM and 9.4 microM, were determined. [3H]-CGS 21680, a derivative of NECA which has been demonstrated to be highly selective for A2 receptors in brain synaptic membrane preparations was more potent than NECA at the higher affinity population of A2 sites, and saturation analysis revealed the presence of both a high affinity site, Kd of 18 nM, and a lower affinity site having a Kd of 4.3 microM. The high affinity site labeled by [3H]-CGS 21680 corresponds to the A2a receptor. Using either radioligand, guanosine triphosphate-dependent shifts to a single population of binding sites were observed. Despite the differences in affinities revealed by the two radioligands for the high affinity A2 site, both [3H]-CGS 21680 and [3H]-NECA were competitively displaced by increasing concentrations of a variety of adenosine receptor agonists and antagonists, and exhibited an identical rank order of potency that is consistent with that reported for high affinity A2a receptors.(ABSTRACT TRUNCATED AT 250 WORDS)