Guanosine 5′-O-thiotriphosphate (GTPγS) and thrombin stimulate the activity of phospholipase C in platelets that have been permeabilized with saponin and whose inositol phospholipids have been prelabeled with [3H]inositol. Ca2+has opposite effects on the formation of [3H]inositol phosphates induced by thrombin or GTPγS. While the action of GTPγS on the formation of [3H]inositol phosphates is inhibited by Ca2+, action of thrombin is stimulated by Ca2+. Guanosine 5′-O-(2-thiodiphosphate) (GDPβS), which inhibits the function of GTP-binding proteins, also inhibits the effect of GTPγS on phospholipase C stimulation but, surprisingly, increases the effect of thrombin. Ca2+increases the inhibitory effect of GDPβS on GTPγS activation of phospholipase C, but Ca2+further enhances the stimulatory effect of GDPγS on the thrombin activation of phospholipase C. This indicates that two mechanisms are responsible for the activation of phospholipase C in platelets. A GTP-binding protein is responsible for regulation of phospholipase C induced by GTPγS, while the effect of thrombin on the stimulation of phospholipase C is independent of GTP-binding proteins. However, the effect of thrombin may be modulated by the action of an inhibitory GTP-binding protein. © 1987.
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