Role of arachidonic acid in leukotriene B₄-induced guinea-pig eosinophil homotypic aggregation

The activation of eosinophils with the lipid mediator, leukotriene B₄, induces their homotypic aggregation. Upon activation with leukotriene B₄, eosinophils release a significant amount of arachidonic acid, a process dependent on the activation of phospholipase A₂. 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 B₄. The exogenous administration of arachidonic acid induced a concentration-dependent eosinophil homotypic aggregation. Pretreatment of eosinophils with a 5-lipoxygenase inhibitor or a leukotriene B₄ receptor antagonist abrogated arachidonic-acid-induced aggregation. Arachidonic acid induced a significant increase in leukotriene B₄ levels and desensitised leukotriene B₄-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 Ca²⁺ in eosinophils and failed to desensitise these cells to rises in intracellular Ca²⁺ induced by leukotriene B₄. Pretreatment of eosinophils with the phospholipase A₂ inhibitor, mepacrine, inhibited the aggregation responses induced by 1 nM leukotriene B₄ by approximately 50% but had no significant effect on the other concentrations of leukotriene B₄ tested (0.1 to 100 nM). In conclusion, arachidonic acid stimulates eosinophil aggregation indirectly via the release of leukotriene B₄. Although a significant amount of arachidonic acid is released in response to activation of eosinophils with leukotriene B₄, the arachidonic acid released does appear to play a major role in mediating leukotriene B₄-induced eosinophil aggregation. Copyright (C) 1999 Elsevier Science B.V.