Rapid Decrease of Phosphatidylinositol 4,5-bisphosphate in Thrombin-stimulated Platelets

The addition of thrombin to horse platelets prelabeled with 32P induces a rapid decrease of the radioactivity from phosphatidylinositol 4,5-bisphosphate. Maximum loss of the radioactivity from phosphatidylinositol 4,5-bisphosphate occurs with 10 s of stimulation and is followed by an increased incorporation of 32P into this lipid. The stimulation of phosphatidylinositol 4,5-bisphosphate loss by thrombin is concentration-dependent. The ionophore A23187, which mobilizes Ca2+, is ineffective in inducing the degradation of phosphatidylinositol 4,5-bisphosphate. Measurements of polyphosphoinositides by phosphorus estimation show that, 10 s after thrombin stimulation, there is a decrease of 15-20% of the total phosphatidylinositol 4,5-bisphosphate without any significant change in phosphatidylinositol 4-monophosphate. It appears that thrombin causes a rapid and transient degradation of phosphatidylinositol 4,5-bisphosphate and that this effect might be related to the initiation of platelet activation.