Differential Uncoupling of A₁ Adenosine and D₂ Dopamine Receptors by Suramin and Didemethylated Suramin (NF037)

Abstract

Suramin analogues uncouple two G_i/G_o-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 GTPγS 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 α subunit (i.e., after reconstituting pertussis toxin-treated membranes with exogenous rG_iα-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 rG_iα-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.