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. Copyright © 1990, Wiley Blackwell. All rights reserved