Abstract
Adenylate cyclase activity in platelet membrane preparations was measured in the presence of prostaglandin E1 (PGE1), GTP and a non-hydrolyzable analog of GDP, guanosine 5''-[.beta.-thio]diphosphate (GDP[.beta.S]). A dose-dependent inhibition of adenylate cyclase by GDP[.beta.S] was observed that could be reversed either by adding increased amounts of GTP or of PGE1.