The decrease in phosphatidylinositol 4,5-bisphosphate in ADP-stimulated washed rabbit platelets is not primarily due to phospholipase C activation.
Addition of 10 micron-ADP to washed rabbit platelets caused platelet shape change and aggregation without release of the contents of the amine-storage granules, and caused a transient decrease (8.8% at 10 s) in the amount of phosphatidylinositol 4,5-bisphosphate (PIP2). By 20 s the decrease in PIP2 was no longer apparent, but by 60 s the amount of PIP2 was again decreased. Addition of thrombin (1 unit/ml), which causes platelet shape change, aggregation and the release of the contents of the amine-storage granules, caused a decrease in the amount of PIP2 (8.0% at 10 s); at 60 s the amount of PIP2 was not significantly different from that in controls. In platelets prelabelled with [3H]glycerol, the specific radioactivity of PIP2 was increased at 10 s in ADP-stimulated platelets, and unchanged in thrombin-stimulated platelets. In platelets prelabelled with [3H]inositol and incubated with 20 mM-Li+ to inhibit the degradation of the inositol phosphates to inositol, there was no increase in the labelling of inositol trisphosphate (IP3) upon stimulation with ADP. In contrast, stimulation with thrombin caused a significant increase in the labelling of IP3 at 10 s. These differences in the changes in polyphosphoinositide metabolism in ADP- and thrombin-stimulated platelets are consistent with the hypothesis that the decrease in PIP2 in ADP-stimulated platelets may be due not to degradation of PIP2 by phospholipase C, but rather to a shift in the equilibrium between PIP2 and phosphatidylinositol 4-phosphate (PIP). Increases in the labelling of phosphatidic acid at 10 s and of inositol bisphosphate and inositol phosphate after 20 s are consistent with phospholipase C being stimulated through some other mechanism that leads to the degradation of PIP and phosphatidylinositol; one possibility is that ADP causes an increase in cytoplasmic Ca2+.
D000244:Adenosine Diphosphate / Q000494:pharmacology*; D000818:Animals; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D004789:Enzyme Activation / Q000187:drug effects; D005990:Glycerol / Q000097:blood; D066298:In Vitro Techniques; D007295:Inositol Phosphates / Q000097:blood; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols / Q000097:blood*; D011817:Rabbits; D013917:Thrombin / Q000494:pharmacology; D010738:Type C Phospholipases / Q000097:blood*
D007295:Inositol Phosphates; D019269:Phosphatidylinositol 4,5-Diphosphate; D010716:Phosphatidylinositols; D000244:Adenosine Diphosphate; D010738:Type C Phospholipases; D013917:Thrombin; D005990:Glycerol