The PKCs involved in platelet secretion require JNK1. Dense granule secretion (measured by ATP release, A) and TXA₂ synthesis (measured by production of TXB₂, B) were determined for WT and JNK1^−/− washed platelets (10⁸) after stimulation with collagen (1 μg/mL), with stirring, over 3 minutes. When indicated, platelets were first incubated for 10 minutes at 37°C with the cyclooxygenase inhibitor indomethacin (5μM) or the antagonists of ADP receptors, MRS2179 (100μM) and AR-C69931MX (10μM). Data are expressed as the relative percentage of ATP released (A) or of TXB₂ produced (B). Each bar represents the mean plus or minus SEM of 3 experiments. ***P < .001 (unpaired Student t test). (C) The PKC activity of WT and JNK1^−/− platelets (10⁸) induced by collagen (1 μg/mL), with stirring, over a period of 3 minutes. Stimulated platelets were lysed and samples analyzed by Western blotting with an antibody against phosphorylated PKC substrates. The levels of phosphorylation of PKC substrates were evaluated as described in “Methods,” for WT and JNK1^−/− platelets. The Western blot shown is representative of at least 3 independent experiments.