RESUMO
[Table: see text] Binding affinities of purine derivatives at A3 adenosine receptors in different species were compared. Binding was carried out using the novel high affinity agonist ligand [125I]AB-MECA (3-iodo-4-aminobenzyladenosine-5'-N-methyluronamide) in the presence of 1.0 µM XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]phenyl]-1,3-dipropylxanthine), an A1- and A2a-adenosine antagonist. XAC was added to eliminate binding to non-A3 receptors. In rat brain membranes [125I]AB-MECA exhibited saturable, specific binding with a Kd of 2.28 nM and a Bmax of 43 fmol/mg protein. The affinity of [125I]AB-MECA at the gerbil and rabbit brain A3-receptors was similar to the rat, suggesting that the affinity of this agonist is not highly species dependent. The affinity of various xanthine derivatives was measured in [125I]AB-MECA competition binding assays. Gerbil and rabbit brain A3-receptors were similar in the affinity of antagonists whose potency order in both species was: BWA522 ≥ CPX > XCC, XAC, SPX, BWA1433 > theophylline. The affinities of 8-arylxanthines at the rat, rabbit, and gerbil brain A3 receptors were considerably less than the previously reported affinities at cloned sheep and human A3 receptors. Species differences in agonist affinity were assessed by comparing Ki values at cloned rat brain A3 receptors expressed in CHO cells with cloned sheep and human A3 receptors. Human and rat brain A3 receptors were highly similar in the relative affinities of agonists, and sheep brain A3 receptors were unlike either human or rat A3 receptors in agonist affinity.