Phosphorylation of phosphoinositides in human platelets

32P-labelling of phosphatidylinositol (PI), PI-4-monophosphate (PIP), PI-4,5-bisphosphate (PIP2) and phosphatidic acid (PA) in 32P-labelled intact human platelets was investigated in the presence of various agents which alter intracellular level of cAMP or Ca2+. Addition of dibutyryl cAMP to intact platelets pre- or pulse labelled with 32P resulted in increased 32P-labelling of PIP and in concomitant decreased 32P-labelling of PI without affecting that of PIP2 or PA. Similar changes were observed in intact platelets treated by prostaglandin I2 (PGI2) or a new low Km phosphodiesterase inhibitor (DN-9693). When intracellular Ca2+ was chelated by loading quin 2-AM to intact platelets, 32P-labelling of PIP was significantly increased in a dose related manner. From these observations it was concluded that PI kinase is activated by elevation of cAMP or chelation of Ca2+ in intact platelets.