Characterization of platelet-activating factor-induced cytosolic calcium mobilization in human eosinophils

Background: Activated eosinophils play an important role in the pathogenesis of bronchial asthma and other allergic diseases, and platelet- activating factor (PAF) is a potent activator of eosinophils. Objective: To characterize the cytosolic Ca²⁺ [Ca²⁺](i) mobilization in human eosinophils in response to PAF. Methods: [Ca²⁺](i) responses to PAF were examined in human eosinophils using a microscopic fura-2 fluorescence-ratio imaging system. Results: PAF caused a significant and dose-dependent increase in (Ca²⁺)(i), which consisted of an initial rapid rise followed by a sustained elevation. This PAF-induced (Ca²⁺)(i) rise was inhibited by WEB 2086, a specific PAF receptor antagonist. The addition of 5 mM EGTA or 1 mM Ni²⁺ to a nominally Ca²⁺-free solution did not appreciably reduce the initial rise but significantly inhibited the sustained rise. The application of a protein kinase C inhibitor, Ro31-8220, augmented the sustained increase by PAF. Thapsigargin, a microsomal Ca²⁺ ATPase inhibitor, induced no appreciable change in a nominally Ca²⁺-free solution but induced a marked increase in (Ca²⁺)(i) when changed to a Ca²⁺-containing solution. Conclusions: The initial rapid rise and the following sustained rise in (Ca²⁺)(i) by PAF depends on Ca²⁺ release from the intracellular Ca²⁺ stores and Ca²⁺ influx, respectively, which are regulated by protein kinase C in human eosinophils. Furthermore, the so called Ca²⁺-capacitative entry is possibly involved in the Ca²⁺ influx from the extracellular solution in human eosinophils.