The A2a adenosine receptor, a member of the G protein-coupled receptor family, is important in the regulation of dopaminergic pathways of the brain and in platelet and cardiovascular functions. In this study, the role of extracellular loops in ligand binding to the human A2a receptor was explored through site-directed mutagenesis. Four glutamate/aspartate residues (Glu151, Glu161, Glu169, and Asp170) in the second extracellular loop (E2) and a cysteine residue (Cys262) in the third extracellular loop (E3) were individually replaced with alanine and other amino acids. A proline residue (Pro173) in E2 was mutated to arginine, the homologous amino acid in A3 receptors. The binding properties of the resultant mutant receptors were determined in transfected COS-7 cells. The mutant receptors were tagged at their amino terminus with a hemagglutinin epitope, thus allowing their detection in the plasma membrane with immunological techniques. High affinity specific binding of [3H]2-[4-[(2-carboxyethyl)phenyl]ethyl-amino]-5'-N-ethylcarboxamidoad eno sine (15 nM) and [3H]8-[4-[[[[2-aminoethyl)-amino]carbonyl]methyl]oxy]phenyl]-1,3- dipropylxanthine (4nM), an A2a agonist and antagonist, respectively, was not observed with four of the mutant receptors, E151A, E151Q, E151D, and E169A, although they were well expressed at the cell surface. The E151A and E169A mutant receptors showed nearly full stimulation of adenylyl cyclase at approximately 10(3)-fold higher concentrations of 2-[4-[(2-carboxyethyl)phenyl]ethyl-amino]-5'-N-ethylcarboxamidoadenosine . The E161A mutant receptor showed as increase in affinity for the nonxanthine adenosine antagonist 9-chloro-2-(furyl)[1,2,4]triazolo[1,5-c]quinazolin-5-amine(6 fold) but not for other ligands. An E169Q mutant gained affinity (5-22 fold) for adenosine derivatives (agonists) substituted at N6 but not at C2 or C5' positions. Mutant receptors D170K and P173R were similar to wild-type receptors in binding of both agonist and antagonist radioligands. A C262G mutant also resembled the wild-type receptor in radioligand binding, indicating that a potential disulfide bridge with another cysteine residue in proximity is not required for the structural integrity of the receptor. Our data suggest that certain amino acids in the second extracellular loop may be directly or indirectly involved in ligand binding.