Structures of a: agonists. and b: antagonists at adenosine receptors APEC, 2-[(2-aminoethylamino)carbonylethylphenylethylamino]-5′-N-ethylcarboxamidoadenosine; CPT, 8-cyclopentyltheophylline; CPX. 8-cyclopentyl-1,3-dipropylxanthine; CSC, 8-(3-chlorostyryl)caffeine; DPMA, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl]adenosine: IB-MECA, N6-(3-iodobenzyl)-5′-N-methylcarboxamidoadenosine; and XAC. 8-(4-[({[(2-aminoethyl)amino]carbonyl}methyl)oxy]phenyl}-1,3-dipropylxanthine.