Differential uncoupling of A1 adenosine and D2 dopamine receptors by suramin and didemethylated suramin (NF037).

Suramin analogues uncouple two Gi/Go-coupled receptors, the D2 dopamine receptor in rat striatum and the A1 adenosine receptor in human cerebral cortex, with distinct structure-activity relations. This discrepancy may reflect true differences in the affinity of the analogues for specific receptor/G protein complexes or may be attributable to differences in species or in the tissue source used. We addressed this question by using human embryonic kidney 293 cells that stably express the human A1 and rat A1 receptor and the human D2 receptor. Suramin is 10-fold more potent than its didemethylated analogue NF037 in inhibiting the interaction between G proteins and the rat A1 or human A1 receptor; in contrast, both compounds are equipotent in uncoupling the D2 receptor. These differences are observed regardless of whether (1) inhibition of high affinity agonist binding to the receptors or (2) agonist-stimulated GTPgammaS binding is used as readout, (3) the receptors are allowed to interact with the G protein complement in human embryonic kidney 293 cell membranes, or (4) the receptors are forced to interact with a defined G protein alpha subunit (i.e., after reconstituting pertussis toxin-treated membranes with exogenous rGi alpha-1). The apparent affinity of suramin depends in a linear manner on receptor occupancy, which shows that suramin and the receptor compete for the G protein. Finally, the affinity of the receptors for rGi alpha-1 (human A1 > rat A1 > human D2) is inversely correlated with the potency of suramin in uncoupling ternary complexes formed by these receptors and thus determines the selectivity of the suramin analogues for specific receptor/G protein tandems.