Calcium‐ionophore‐induced formation of platelet‐activating factor and leukotrienes by horse eosinophils: a comparative study

Horse eosinophils preincubated with ³H‐labelled acetate and stimulated with the Ca²⁺ ionophores ionomycin or A23187 form a radioactive compound, which we have shown to be 1‐O‐alkyl‐2‐[³H]acetyl‐sn‐glycero‐3‐phosphocholine (platelet‐activating factor). We could detect no 1‐O‐acyl‐2‐[³H]acetyl‐glycero‐3‐phosphocholine in the radioactive fraction. The formation of platelet‐activating factor was strongly correlated to the generation of leukotriene C₄, the main arachidonate metabolite in horse eosinophils, suggesting that platelet‐activating factor and leukotriene C₄ have a common precursor pool (1‐O‐alkyl‐2‐arachidonyl‐glycero‐3‐phosphocholine) and a common regulation of synthesis. Even though both ionomycin and A23187 are described as Ca²⁺ ionophores, they have a series of significantly different effects on the eosinophils with respect to formation of platelet‐activating factor and leukotriene C₄. While A23187 induces an asymptotic maximum of mediator formation at concentrations higher than 20 μM, ionomycin expressed a narrow optimum at 2μM. The effects of exogenous pH on the release of mediators also differ strongly between the two ionophores: for A23187 the effects are the same for both mediators but when ionomycin is used as stimulant, generation of platelet‐activating factor and leukotriene C₄ are affected differently.