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 [I-125]AB-MECA (3-iodo-4-aminobenzyladenosine-5'-N-methyluronamide) in the presence of 1.0 mu M XAC (8-[4-[[[[(2-aminoethyl)amino]carbonyl]methyl]oxy]phenyl]-1,3-dipropylxanthine), an A(1)- and A(2a)-adenosine antagonist. XAC was added to eliminate binding to non-A(3) receptors. In rat brain membranes [I-125]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 [I-125]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 [I-125]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 greater than or equal to 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. (C) 1994 Wiley-Liss, Inc.