Characterization of adenosine A2 receptors in bovine retinal pigment epithelial membranes

The pharmacological characteristics of adenosine A2 receptors are described for membranes prepared from bovine retinal pigmented epithelial (RPE) cells. RPE cells were isolated after removal of retina, lysed by freeze-thawing, and membranes separated from cytoplasmic components. A single population of adenosine binding sites is present in RPE membranes, as determined from saturation analysis and competition binding assays. From Scatchard plots, this single class of binding sites exhibited low affinity for adenosine receptor agonists. These low affinity sites were labeled by [3H]-N-ethylcarboxamido-adenosine (NECA) or [3H]-CGS 21680 and Kds of 423 and 5.3 microM were determined for each radioligand, respectively. NECA-mediated stimulation of adenylate cyclase demonstrated that these binding sites represent adenosine receptors. No high affinity A2a binding sites were detected in RPE membranes by either saturation studies, or by competition with adenosine A1-selective agonists which only displaced radioligand binding at high micromolecular concentrations. The low affinity A2 receptor on RPE differs from the high affinity A2a receptor characterized in bovine retinal membranes, but may be similar or identical to the lower affinity A2b receptor detected in retinal membranes as well as other tissues.