PHOSPHOLIPASE A(2) ACTIVATION IN HUMAN NEUTROPHILS REQUIRES INFLUX OFEXTRACELLULAR CA2-4( AND LEUKOTRIENE B)

We have demonstrated that phospolipase A(2) (PLA(2)) activation in hum an neutrophils requires both the influx of extracellular Ca2+ and leuk otriene B-4 (LTB(4)). Surprisingly, the eicosanoids (LTB(4) and its om ega-oxidation products) formed were quantitatively very similar in bot h thapsigargin (Thap)- and A-23187-stimulated neutrophils. In contrast , Thap had very little effect on the activation of PLA(2) when 5-lipox ygenase (5-LO) was blocked by BW755C or MK-886, whereas A-23187 caused a substantial activation. The lack of PLA(2) activation in Thap-stimu lated neutrophils results from the inhibition of LTB(4) formation in t he presence of 5-LO inhibitors. It appears that A-23187 activates both LTB(4)-dependent and -independent PLA(2), whereas Thap activates LTB( 4)-dependent PLA(2). Experiments with ethylene glycol-bis(beta-aminoet hyl ether)-N,N,N',N'-tetraacetic acid demonstrated that activation of Thap-sensitive PLA(2) and 5-LO requires the influx of Ca2+. Neither th e transient elevation of cytosolic Ca2+ from intracellular stores nor the sustained Ca2+ influx alone without LTB(4) appears sufficient to c ause the activation of LTB(4)-dependent PLA(2). We suggest that the ac tivation of LTB(4)-dependent PLA(2) involves 1) a sustained elevation of cytosolic Ca2+ coupled to the influx of extracellular Ca2+ and 2) a coupling between LTB(4) and its receptor. We conclude that LTB(4)-dep endent PLA(2) plays an important role in the poststimulatory formation of lipid mediators such as prostaglandins, leukotrienes, and platelet -activating factor.