DIFFERENT REQUIREMENT OF INTRACELLULAR CALCIUM AND PROTEIN-KINASE-C FOR ARACHIDONIC-ACID RELEASE AND SEROTONIN SECRETION IN CATHEPSIN G-ACTIVATED PLATELETS

Previous studies have shown that platelet stimulation with cathepsin G rapidly results in cytoplasmic calcium ([Ca2+](i)) increase and activ ation of protein kinase C(PKC). To elucidate the relationship between these two biochemical events and their relative contribution to the re gulation of platelet response to cathepsin G, arachidonic acid (AA) re lease and serotonin (5HT) secretion were studied. Platelets made Ca2+- depleted and -permeable by treatment with A23187 were compared to inta ct platelets to better dissociate calcium changes from other receptor stimulated events. AA release elicited by cathepsin G in intact platel ets was prevented by the Ca2+ chelator BAPTA; in Ca2+-depleted, -perme able platelets AA was released in direct response to added Ca2+ and wa s not increased by simultaneous stimulation with cathepsin G. Tn intac t platelets, PKC inhibition by Po 31-8220 or PKC induction with PMA ei ther enhanced or reduced, respectively, cathepsin G-induced AA release . Both BAPTA and Ro 31-8220 prevented 5HT secretion from intact platel ets; however, in Ca2+-depleted, -permeable platelets, cathepsin G was able to evoke 5HT secretion and p47 phosphorylation independently of [ Ca2+](i) increase, both effects being hampered by Po 31-8220. Ca-2+ an d PKC therefore regulate PLA(2) activity and 5HT secretion in cathepsi n G-stimulated platelets in a different manner: the former is mainly t riggered by [Ca2+](i) increase, while PKC represents the major factor in determining dense granule secretion.