Species Differences in Ligand Affinity at Central A-Adenosine Receptors.

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