ROLES OF PHOSPHOLIPASE A(2), 5-LIPOXYGENASE, AND CYCLOOXYGENASE IN ACTIVATION OF RESPIRATORY BURST IN HUMAN NEUTROPHILS - MODULATORY EFFECTS OF OSMOLARITY OF THE MEDIUM

The roles of phospholipase A(2), 5-lipoxygenase, and cyclooxygenase in respiratory burst were studied in human neutrophils suspended in isosmotic (320 mOsM), hypoosmotic (200 mOsM), and hyperosmotic (420 mOsM) media. Respiratory burst was induced by phorbol-12-myristate-13-acetate (PMA), the calcium ionophore A-23187, or opsonized zymosan and recorded as luminol-dependent chemiluminescence (CL). There was no difference in the concentration dependences of CL inhibition at 320 and 200 mOsM in the presence of p-bromophenacyl bromide, an inhibitor of phospholipase A(2). The sensitivity to the inhibitor decreased at 420 mOsM when PMA or A-23187 was used to induce respiratory burst, but the sensitivity of respiratory burst induced by opsonized zymosan to inhibition increased. The concentration dependence of the inhibitory effect of quercetin, an inhibitor of 5-lipoxygenase, on CL induced by any agent was not changed with the osmolarity of the medium. There was a decrease in sensitivity of respiratory burst induced by PMA or A-23187 to the inhibitory effect of meclofenamic acid, an inhibitor of cyclooxygenase, at 200 mOsM, whereas the sensitivity increased at 420 mOsM in comparison with isosmotic conditions. The data suggest that phospholipase A(2) and cyclooxygenase are involved in the mechanisms underlying the modulatory effects of osmolarity on neutrophil respiratory burst; 5-lipoxygenase plays no substantial role in this process.