Species differences in ligand affinity at central A 3 ‐adenosine receptors

Binding affinities of purine derivatives at A 3 adenosine receptors in different species were compared. Binding was carried out using the novel high affinity agonist ligand [ 125 I]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‐dipropylxanthin), an A 1 ‐ and A 2a ‐adenosine antagonist. XAC was added to eliminate binding to non‐A 3 receptors. In rat brain membranes [ 125 I]AB‐MECA exhibited saturable, specific binding with a K d of 2.28 nM and a B max of 43 fmol/mg protein. The affinity of [ 125 I]AB‐MECA at the gerbil and rabbit brain A 3 ‐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 [ 125 I]AB‐MECA competition binding assays. Gerbil and rabbit brain A 3 ‐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 A 3 receptors were considerably less than the previously reported affinities at cloned sheep and human A 3 receptors. Species differences in agonist affinity were assessed by comparing K i values at cloned rat brain A 3 receptors expressed in CHO cells with cloned sheep and human A 3 receptors. Human and rat brain A 3 receptors were highly similar in the relative affinities of agonists, and sheep brain A 3 receptors were unlike either human or rat A 3 receptors in agonist affinity. © 1994 Wiley‐Liss, Inc.