Pasteurella haemolytica Leukotoxin-Induced Increase in Phospholipase A(2) Activity in Bovine Neutrophils

Exposure of bovine neutrophils to Pasteurella haemolytica leukotoxin (LKT) stimulates the production of leukotriene B(4) (LTB(4)), which is believed to be an important chemotactic agent in the development of acute fibrinopurulent pneumonic infection in cattle. The involvement of phospholipase A(2) (PLA(2)) in LKT-induced synthesis of LTB(4) was studied by using bovine neutrophils labeled with (3)H-arachidonate ([(3)H]AA). Incubation of isolated neutrophils with [(3)H]AA resulted in incorporation of radioactivity in the PLA(2) substrates phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine. Exposure of radiolabeled neutrophils to LKT caused concentration- and time-dependent release of radioactivity and redistribution of radioactivity in neutrophil membranes consistent with utilization of phosphoglyceride substrate and release of free fatty acid and eicosanoid products. These LKT-induced effects could be inhibited by pretreatment with arachidonyl trifluoromethyl ketone, an inhibitor of type IV cytoplasmic PLA(2), and were dependent on extracellular calcium. These results support the conclusion that LKT-induced synthesis of LTB(4) involves a calcium-mediated increase in PLA(2) activity.