CHARACTERIZATION OF A(2A) ADENOSINE RECEPTORS IN HUMAN LYMPHOCYTE MEMBRANES BY [H-3] SCH-58261 BINDING

1 The present study describes for the first time the characterization of the adenosine A(2A) receptor in human lymphocyte membranes with the new potent and selective antagonist radioligand, -3]-5-amino-7-(2-phe nylethyl)-2-(2-furyl)-pyrazolo [4,3-e]-1,2,4 triazolo [1,5-c] pyrimidi ne, ([H-3]-SCH 58261). In addition, both receptor affinity and potency of reference adenosine receptor agonists and antagonists were determi ned in binding and adenylyl cyclase studies. 2 Saturation experiments revealed a single class of binding sites with K-d and B-max values of 0.85 nM and 35 fmol mg(-1) protein, respectively. A series of adenosin e receptor ligands were found to compete for the binding of 0.8 nM [H- 3]-SCH 58261 to human lymphocyte membranes with a rank order of potenc y consistent with that typically found for interactions with the A(2A) -adenosine receptor. In the adenylyl cyclase assay the same compounds exhibited a rank order of potency similar to that observed in binding experiments. 3 Thermodynamic data indicate that [H-3]-SCH 58261 bindin g to human lymphocytes is entropy and enthalpy-driven, a finding in ag reement with the thermodynamic behaviour of antagonists for rat striat al A(2A)-adenosine receptors. 4 It is concluded that in human lymphocy te membranes [H-3]-SCH 58261 directly labels binding sites showing the characteristic properties of the adenosine A(2A)-receptor. The presen ce of A(2A)-receptors in peripheral tissue such as human lymphocytes s trongly suggests an important role for adenosine in modulating immune and inflammatory responses.