Labeling of A(2A) adenosine receptors in human platelets by use of the new nonxanthine antagonist radioligand [H-3]SCH 58261

The present study describes the binding to human platelet A(2A) adenosine receptors of the new potent and selective antagonist radioligand [H-3]5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c] pyrimidine ([H-3]SCH 58261). Saturation experiments revealed that [H-3]SCH 58261 labels a single class of recognition sites with high affinity (K-d = 0.85 nM), limited capacity (apparent B-max = 85 fmol/mg of protein) and good specific binding (about 60%). [H-3]SCH 58261 binding was not modulated by either the divalent cation Mg++ or guanine nucleotides. In competition experiments, a series of both adenosine agonists and antagonists inhibited [H-3]SCH 58261 binding to A(2A) platelet receptors with rank order of potency and affinity similar to those observed in rat striatal membranes with the same radioligand. This confirms that the platelet A(2A) receptor is similar to that labeled in the brain striatum. Binding data were also found to be in good agreement with the results from functional studies such as A(2A) agonist-induced stimulation of adenylate cyclase or platelet aggregation inhibition. The present findings indicate that [H-3]SCH 58261 is the first radioligand available for the characterization of the A(2A) receptor subtype in platelets.