The expression of both A 1 -and A 2a -adenosine receptors occurs in human foreskin and lung fibroblasts (Ahmed et al., 1995, Biochem. Biophys. Res. Commun. 208:871-878). Studies with highly specific A 1 -and A 2a -adenosine receptor agonists provide indirect evidence that binding of adenosine activates G s and G i , after which G sa interacts with bg subunits released from G i . The interaction of G sa with bg augments cyclic adenosine monophosphate (cAMP) accumulation, more than does G sa alone. In the present study, we have provided direct evidence for a role of the bg complex in the augmentation of cAMP accumulation by using a recombinant His 6 fusion protein containing the carboxyl third of bARK1. This portion of bARK1 contains G bg binding sequences and acts as a specific bg scavenger (Koch et al.,1994, Proc. Natl. Acad. Sci. USA 91:12706-12710). In permeabilized fibroblasts, the His 6 fusion protein inhibited the augmentation of cAMP accumulation resulting from adenosine binding to both A 1 and A 2a receptors. In addition, the specific G bg scavenger inhibited the further rise in cellular cAMP levels caused by pretreating cells with pertussis toxin before incubation with adenosine. Finally, we observed that specific A 1 -adenosine receptor agonists augmented the cAMP accumulation stimulated by A 2a -receptor agonists, and this cAMP augmentation was also suppressed by the G bg scavenger. Similar results were obtained when the cells were treated with extracellular ATP and lysophosphatidic acid (LPA) to stimulate G s and release G bg subunits, respectively.