The lipoxygenase products of arachidonic acid (AA) metabolism, 5-hydroxyeicosatetraenoic acid (5-HETE) and leucotriene B4 (LTB4), are considered to have an important pathophysiological role in inflammatory bowel disease by stimulating the inflammatory response and by contributing to the diarrhoea. The present studies were designed to investigate the effect of the physiological stimulants bradykinin (BK) and 5-hydroxytryptamine (5-HT), in addition to the influence of the calcium ionophore A23187, on the source of AA release and 5-lipoxygenation in human neutrophils (PMNs) in vitro. This was done to elucidate the specificity of the mechanism by which PMNs respond to physiological, extracellular Ca2+ dependent agonists. The results of the study indicate that stimulation of 1-14C-AA-prelabelled PMNs with BK liberates AA mainly from phosphatidylinositol, while A23187 causes release of AA from phosphatidylcholine, phosphatidylethanolamine, and possibly phosphatidylserine. Furthermore BK (10(-9)-10(-6)M) dose-dependently stimulated the formation of 5-HETE and LTB4, reaching a maximum at 10(-7)M, while 5-HT (10(-8)-10(-4)M) released only negligible amounts of eicosanoids, similar to those observed in control experiments. Stimulation with A23187 (10(-5)M) caused a high release of both 5-HETE and LTB4. These results offer evidence that BK, but not 5-HT, initiates formation of lipoxygenase products by binding to specific receptors on the external surface of PMNs, whereas A23187 accelerates 5-lipoxygenation through mechanisms which do not involve a cell surface receptor.
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