Regulation of leukotriene biosynthesis in human polymorphonuclear leukocytes

Since the formation of leukotrienes in human polymorphonuclear leukocytes is Ca-dependent we have determined the Ca-pools involved and the enzymes affected. With PAF, fMLP and LTB4 as agonists the Ca-transients measured with Fura-2 showed the same height of the intracellular Ca-signal but an external Ca-component in decreasing magnitude. In parallel the activity of 5-lipoxygenase was also decreasing. 5-lipoxygenase was linearly dependent on Ca with saturation of around 350 nMol/l already provided by the release of intracellular Ca. Phospholipase A2, however, started to activate at about this concentration. Hence arachidonate is a limiting factor in stimulated leukocytes due to a limited influx of extracellular Ca.