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The adenosine derivative, N6-phenylisopropyladenosine (PIA), which inhibits adenylate cyclase in adipocyte membranes by a GTP-dependent and sodium-amplified process, was studied on GTPase activity in hamster adipocyte ghosts. PIA stimulated a high affinity GTPase without apparent lag phase. Both unstimulated and PIA-stimulated GTPases exhibited very similar Km values of about 0.2 microM GTP. PIA-induced low Km GTPase stimulation was amplified by sodium ions and was half-maximal and maximal at about 0.02 and 0.1 microM PIA, respectively. Stimulations of the low Km GTPase by PIA and PGE1, both inhibiting adipocyte adenylate cyclase, were not additive. Similar to PIA-induced adenylate cyclase inhibition, stimulation of the GTPase by PIA but not by PGE1 was prevented by the adenosine receptor antagonist, 3-isobutyl-1-methylxanthine. The data suggest that PIA-induced stimulation of a high affinity GTPase is an essential mechanism of adenosine receptor-mediated adipocyte adenylate cyclase inhibition.
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