Abstract
In saponin-permeabilized platelets, GTPγS (1-100μM) induced serotonin secretion and enhanced thrombin-induced secretion. In [3H] arachidonic acid (AA)-labeled platelets, GTPγS stimulated [3H] AA release and [3H] diacylglycerol (DG) formation. RHC 80267, an inhibitor of DG lipase, was not effective to reduce [3H] AA release, suggesting hydrolysis of phospholipids by phospholipase A2. NaF, an activator of GTP-binding proteins, also stimulated platelet responses. Thrombin-induced AA liberation was almost completely inhibited with pretreatment of pertussis toxin (PT) (10μg/ml), whereas DG formation was decreased by only 20-40% in the toxin-treated platelets. Although GDPβS suppressed AA release and DG formation in a dose-dependent manner, the half maximal inhibition required less than 10μM for AA release but more than 100μM for DG production. These results indicate that GTP-binding protein (s) plays an important role in signal transduction during platelet activation. Since AA release and DG formation are differently modulated by PT and GDPβS, the distinct GTP-binding proteins may regulate the activity of phospholipase C and phospholipase A2.
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