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 Ca 2+  ([Ca 2+ ] i ) mobilization in human eosinophils in response to PAF. Methods [Ca 2+ ] 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 (Ca 2+ ) i , which consisted of an initial rapid rise followed by a sustained elevation. This PAF‐induced (Ca 2+ ) i rise was inhibited by WEB 2086, a specific PAF receptor antagonist. The addition of 5 m m EGTA or 1 m m Ni 2+  to a nominally Ca 2+ ‐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 Ca 2+ ATPase inhibitor, induced no appreciable change in a nominally Ca 2+ ‐free solution but induced a marked increase in (Ca 2+ ) i when changed to a Ca 2+ ‐containing solution. Conclusions The initial rapid rise and the following sustained rise in (Ca 2+ ) i by PAF depends on Ca 2+ release from the intracellular Ca 2+ stores and Ca 2+ influx, respectively, which are regulated by protein kinase C in human eosinophils. Furthermore, the so called Ca 2+ ‐capacitative entry is possibly involved in the Ca 2+ influx from the extracellular solution in human eosinophils.