IONOPHORE A23187-INDUCED LEUKOTRIENE BIOSYNTHESIS IN EQUINE GRANULOCYTES - NEUTROPHILS, BUT NOT EOSINOPHILS REQUIRE EXOGENOUS ARACHIDONIC-ACID

Equine granulocyte suspensions, mainly consisting of neutrophils, fail ed to produce detectable amounts of leukotrienes when stimulated with calcium ionophore A23187 alone, whereas leukotrienes were dose-depende ntly formed in control incubations with human granulocytes. In contras t, ionophore A23187 initiated synthesis of leukotrienes B-4 and C-4 in equine granulocytes when added in combination with low concentrations of exogenous arachidonic acid. Similarly, ionophore A23187 provoked l eukotriene biosynthesis when added alone to human whole blood, whereas addition of exogenous arachidonic acid was a prerequisite for ionopho re A23187-induced leukotriene formation in equine whole blood. Leukotr iene biosynthesis was provoked by A23187 alone after addition of homol ogous platelets to equine granulocyte suspensions. After separation of equine neutrophils and eosinophils, purified eosinophil suspensions p roduced LTC4 after stimulation with ionophore A23187 alone, whereas ex ogenous arachidonic acid was required for ionophore-induced LTB4 forma tion in purified neutrophil suspensions. Leukotriene synthesis in both eosinophils and neutrophils was suppressed by the 5-lipoxygenase acti vating protein (FLAP) inhibitor, MK-886. Exogenous arachidonic acid wa s needed for ionophore-induced leukotriene synthesis also in bovine gr anulocytes, but was not a prerequisite for the production of leukotrie nes in porcine granulocytes or in rat and rabbit white blood cell susp ensions. The results indicate differences in the mechanisms regulating leukotriene synthesis in equine neutrophils, as compared to human gra nulocytes or equine eosinophils, and suggest that elevation of intrace llular calcium is an insufficient stimulus to provoke utilisation of e ndogenous arachidonic acid for leukotriene synthesis in equine neutrop hils. (C) 1998 Elsevier Science B.V.