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To determine the role of protein kinase C (PKC) in the signal transduction in eosinophil pathways, we have assessed the effects of the phorbol ester phorbol 12-acetate 13-myristate (PMA) on guinea pig peritoneal eosinophils challenged with platelet-activating factor (PAF). Pretreatment with PMA completely inhibited the PAF-induced release of eosinophil peroxidase (EPO) and superoxide anions and the rise in intracellular Ca2+ concentration ([Ca2+]i), with IC50s of 2 to 10 nM. This inhibition was reversed when the cells were preincubated with the PKC inhibitor staurosporine for 5 min before the addition of PMA. Staurosporine also inhibited PAF-induced EPO release but not the rise in [Ca2+]i. The inactive ester 4 alpha-phorbol 12,13-didecanoate had no inhibitory effect on eosinophil activation. Finally, PMA inhibited the binding of the PAF antagonist [3H]WEB 2086 to intact eosinophils. Taken together, these data suggest that PKC may have a physiologic role in regulating PAF-induced eosinophil responses through expression of PAF receptors on the cell surface.