Differential regulation of leukotriene and platelet-activating factor synthesis in rat alveolar macrophages

Leukotrienes (LT) and platelet-activating factor (PAF) are synthesized by several lung cells, including alveolar macrophages (AM), and may contribute to the airway inflammation that characterizes asthma. Phospholipases A2 (PLA2) can release arachidonic acid and lysophosphatidylcholine (lysoPC), precursors for leukotriene and PAF synthesis, respectively, from membrane phospholipids. The present study sought to determine the extent to which this common initial step contributes to leukotriene and PAF synthesis in rat AM. AM were obtained by lung lavage, cultured, and exposed to one or more of the following: PAF, zileuton (5-lipoxygenase inhibitor), Ro 25-4331 (dual PLA2 inhibitor), and either A23187 (Cal) or zymosan. The following measurements were made: LTB4 synthesis, PAF synthesis, and PLA2 activity. CaI stimulation increased PAF synthesis 3-fold (P < 0.001), LTB4 synthesis 20-fold (P < 0.001), and PLA2 activity 52% (P < 0.001). Incubation with PAF (2.5 microM) for 10 min decreased basal LTB4 synthesis 33% (P < 0.001), but it had no effect on basal PAF synthesis or PLA2 activity. The same dose of PAF (2.5 microM) decreased CaI-stimulated PAF synthesis 40% (P < 0.02). After CaI stimulation of PAF-pretreated cells, a relationship was found between PAF-induced changes in PLA2 activity and LTB4 synthesis (r = 0.68; P < 0.001) but not between changes in PLA2 activity and PAF synthesis. Zileuton (1 microM) decreased basal and CaI-stimulated LTB4 synthesis 50% (P < 0.02), and 80% (P < 0.002) respectively, but it did not alter PAF synthesis.