Ionophore A23187-induced leukotriene biosynthesis in equine granulocytes-neutrophils, but not eosinophils require exogenous arachidonic acid.

Equine granulocyte suspensions, mainly consisting of neutrophils, failed to produce detectable amounts of leukotrienes when stimulated with calcium ionophore A23187 alone, whereas leukotrienes were dose-dependently formed in control incubations with human granulocytes. In contrast, ionophore A23187 initiated synthesis of leukotrienes B4 and C4 in equine granulocytes when added in combination with low concentrations of exogenous arachidonic acid. Similarly, ionophore A23187 provoked leukotriene biosynthesis when added alone to human whole blood, whereas addition of exogenous arachidonic acid was a prerequisite for ionophore A23187-induced leukotriene formation in equine whole blood. Leukotriene biosynthesis was provoked by A23187 alone after addition of homologous platelets to equine granulocyte suspensions. After separation of equine neutrophils and eosinophils, purified eosinophil suspensions produced LTC4 after stimulation with ionophore A23187 alone, whereas exogenous arachidonic acid was required for ionophore-induced LTB4 formation in purified neutrophil suspensions. Leukotriene synthesis in both eosinophils and neutrophils was suppressed by the 5-lipoxygenase activating protein (FLAP) inhibitor, MK-886. Exogenous arachidonic acid was needed for ionophore-induced leukotriene synthesis also in bovine granulocytes, but was not a prerequisite for the production of leukotrienes in porcine granulocytes or in rat and rabbit white blood cell suspensions. The results indicate differences in the mechanisms regulating leukotriene synthesis in equine neutrophils, as compared to human granulocytes or equine eosinophils, and suggest that elevation of intracellular calcium is an insufficient stimulus to provoke utilisation of endogenous arachidonic acid for leukotriene synthesis in equine neutrophils.