The Phospholipase C Inhibitor U73122 Inhibits Phorbol Ester-Induced Platelet Activation

Activation of phospholipase C (PLC) is a central component of the signal transduction process in numerous cells, including platelets. U73122 has been widely used as a selective PLC inhibitor. In the present study, the effects of U73122 on platelet function have been further examined. Platelets were stimulated with collagen (via PLC-γ), the stable thromboxane mimetic U46619 (via PLC-β), or phorbol myristate acetate (PMA) via protein kinase C (PKC). Consistent with inhibition of PLC, U73122 inhibited platelet aggregation and [ 3 H]-serotonin release in response to collagen and U46619 in a concentration-dependent manner. Similarly, U73122 blocked collagen-induced release of thromboxane A 2 . U73122 also inhibited U46619-induced [ 32 P]phosphatidic acid production and phosphorylation of the major PKC substrate, pleckstrin. U73122 had no effect on PMA-induced pleckstrin phosphorylation, [ 3 H]-serotonin release, or intracellular vacuole formation. However, U73122 did inhibit PMA-induced platelet aggregation and fibrinogen binding. Overall, these results suggest that U73122, in addition to its inhibition of PLC, also affects PKC-independent events that interfere with platelet aggregation.