BINDING CHARACTERISTICS OF THE ADENOSINE-A(2) RECEPTOR-LIGAND [H-3] CGS-21680 TO HUMAN PLATELET MEMBRANES

The binding characteristics of the selective adenosine A(2) agonist [H -3]CGS 21680 )-phenethyl-amino]-5'-N-ethylcarboxamidoadenosine) were d etermined in human platelet membranes. Specific binding was saturable, reversible and dependent upon protein concentration. Saturation exper iments revealed a single class of binding sites with K-d and B-max val ues of 1.4 mu M and 5.9 pmol/mg of protein, respectively. Adenosine re ceptor agonists and antagonists competed for the binding of [H-3]CGS 2 1680 (50 or 200 nM) to human platelet membranes showing a rank order o f potency consistent with that typically found for interactions at the adenosine Al receptor. Adenylate cyclase stimulation and platelet agg regation inhibition induced by adenosine agonists exhibited a rank ord er of potency close to that observed in binding experiments. However, the adenosine A(1) receptor agonists, R- and S-N-6-(2-phenylisopropyl) adenosine, (R-PIA) and (S-PIA), N-6-cyclohexyladenosine (CHA) and 2-ch loro-N-6-cyclopentyladenosine (CCPA), which stimulate adenylate cyclas e and inhibit platelet aggregation in the low mu M range, displaced [H -3]CGS 21680 only in the high mu M range. In conclusion, we have found that [3H]CGS 21680, which is widely used as a specific A(2) agonist i n binding studies on brain tissues, is not appropriate for the charact erization of the human platelet adenosine Az receptor.