ADENOSINE-A(1) AGONISTS AND THE CA2-8644 PRODUCE A SYNERGISTIC STIMULATION OF THE GTPASE ACTIVITY OF G(O) IN RAT FRONTAL CORTICAL MEMBRANES( CHANNEL AGONIST BAY K)

The identity and role of G proteins in coupling adenosine receptors to effecters have been studied to a limited degree. We have identified t he G proteins whose GTPase activity is stimulated by adenosine recepto r agonists in neuronal membranes. (R)-Phenylisopropyladenosine, 2-chlo roadenosine, 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%, respec tively, and in the cerebellum 55 +/- 2, 41 +/- 4, and 22 +/- 2%, respe ctively. The effects of (R)-phenylisopropyladenosine and 2-chloroadeno sine were inhibited by (1) A, antagonists (76-96% reduction), (2) pret reatment with pertussis toxin (90-100% reduction), and (3) antibodies raised against the alpha-subunit of G(i) and G(o) (55-57% reduction by each), suggesting that A(i) receptors interact equally with G(i) and G(o). (R)-Phenylisopropyladenosine increased the binding of a nonhydro lyzable analogue of GIP to membranes in a pertussis toxin-sensitive ma nner, 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 p rofound synergistic stimulation of GTPase in the presence of(R) -pheny lisopropyladenosine and (+/-)-Bay K 8644 (10(-7) to 10(-5) M). (+/-)-B ay K 8644 had no effect on nucleotide exchange and, thus, cannot activ ate G(o). It appears that a positive cooperative stimulation of G(o) o ccurs when it is first activated by A(1) receptors and subsequently in teracts with the L-type Ca2+ channel.