Polyphosphoinositide synthesis in platelets stimulated with low concentrations of thrombin is enhanced before the activation of phospholipase C.

When platelets, prelabelled with [32P]orthophosphate, were stimulated with thrombin (0.5 U.ml-1) there was an immediate increase in the radioactivity associated with the pools of polyphosphoinositides. Only subsequent to this increase, did the radioactivity of these phospholipid pools decrease as expected from a receptor-mediated activation of phospholipase C (phosphoinositidase). Phosphorylation of diacylglycerol (one of the second messengers formed in the hydrolysis of phosphatidylinositol-bisphosphate) to phosphatidic acid took place with a lag phase of about 3-5 s. Together these experiments suggest that stimulation of kinases phosphorylating phosphatidylinositol and phosphatidylinositol-phosphate may precede or occur in parallel with activation of receptor-linked phosphoinositidase.