Role of arachidonic acid in leukotriene B 4-induced guinea-pig eosinophil homotypic aggregation

Abstract

The activation of eosinophils with the lipid mediator, leukotriene B 4, induces their homotypic aggregation. Upon activation with leukotriene B 4, eosinophils release a significant amount of arachidonic acid, a process dependent on the activation of phospholipase A 2. Here, we have evaluated whether arachidonic acid could induce aggregation of eosinophils and whether the release of arachidonic acid mediated the aggregation induced by leukotriene B 4. The exogenous administration of arachidonic acid induced a concentration-dependent eosinophil homotypic aggregation. Pretreatment of eosinophils with a 5-lipoxygenase inhibitor or a leukotriene B 4 receptor antagonist abrogated arachidonic-acid-induced aggregation. Arachidonic acid induced a significant increase in leukotriene B 4 levels and desensitised leukotriene B 4-induced aggregation in a dose-dependent manner. Moreover, this desensitisation was effectively reversed by a 5-lipoxygenase inhibitor. However, arachidonic acid failed to induce a rise in intracellular Ca 2+ in eosinophils and failed to desensitise these cells to rises in intracellular Ca 2+ induced by leukotriene B 4. Pretreatment of eosinophils with the phospholipase A 2 inhibitor, mepacrine, inhibited the aggregation responses induced by 1 nM leukotriene B 4 by approximately 50% but had no significant effect on the other concentrations of leukotriene B 4 tested (0.1 to 100 nM). In conclusion, arachidonic acid stimulates eosinophil aggregation indirectly via the release of leukotriene B 4. Although a significant amount of arachidonic acid is released in response to activation of eosinophils with leukotriene B 4, the arachidonic acid released does appear to play a major role in mediating leukotriene B 4-induced eosinophil aggregation.