Adenosine A 1 Agonists and the Ca 2+ Channel Agonist Bay K 8644 Produce a Synergistic Stimulation of the GTPase Activity of G o in Rat Frontal Cortical Membranes

The identity and role of G proteins in coupling adenosine receptors to effectors have been studied to a limited degree. We have identified the G proteins whose GTPase activity is stimulated by adenosine receptor agonists in neuronal membranes. ( R )‐Phenylisopropyladenosine, 2‐chloroadenosine, and N ‐ethylcarboxamideadenosine produced a concentration‐dependent stimulation of GTPase. At 10 −5 M , the increase above basal GTPase in frontal cortex was 25 ± 4, 20 ± 3, and 8 ± 1%, respectively, and in the cerebellum 55 ± 2, 41 ± 4, and 22 ± 2%, respectively. The effects of ( R )‐phenylisopropyladenosine and 2‐chloroadenosine were inhibited by (1) A 1 antagonists (76–96% reduction), (2) pretreatment with pertussis toxin (90–100% reduction), and (3) antibodies raised against the α‐subunit of G i and G o (55–57% reduction by each), suggesting that A 1 receptors interact equally with G i and G o . ( R )‐Phenylisopropyladenosine increased the binding of a nonhydrolyzable analogue of GTP to membranes in a pertussis toxin‐sensitive manner, indicative of activation of G i or G o . Previously, (±)‐Bay K 8644 enhanced GTP hydrolysis by G o but not G i . Now we report a profound synergistic stimulation of GTPase in the presence of ( R )‐phenylisopropyladenosine and (±)‐Bay K 8644 (10 −7 to 10 −5 M ). (±)‐Bay K 8644 had no effect on nucleotide exchange and, thus, cannot activate G o . It appears that a positive cooperative stimulation of G o occurs when it is first activated by A 1 receptors and subsequently interacts with the L‐type Ca 2+ channel.