Neutrophil release of arachidonic acid, oxidants, and proteinases: causally related or independent

Abstract

This investigation examined the concept that arachidonic acid (AA) serves as a second messenger in stimulation of the respiratory burst and degranulation of polymorphonuclear neutrophils (PMN). The main support for this idea is from observations that reagent AA, added to cell suspensions, stimulates the respiratory burst and degranulation and these events are blocked by PLA 2 inhibitors. We verified that exogenously-added AA stimulated release of O 2 −, myeloperoxidase (MPO), and lysozyme (LZ), but this required amounts of AA which approximated the critical micellar concentration. This suggested that such administration of AA might act as an extracellular agonist, similar to particulate stimuli, rather than acting as a second messenger as might occur following mobilization of AA from cellular membranes. To investigate the role of fatty acids released by hydrolysis of cellular phospholipids, exogenously-added group I, II or III PLA 2's were used to mobilize fatty acids from cellular membranes. Mole quantities of cell-associated free fatty acids were measured by negative ion chemical ionization gas chromatography/mass spectrometry. AA mobilization in response to exogenous PLA 2 was dose-(0.1 to 10 U/ml PLA 2) and time-dependent (peak at 1 to 2 min with a reduction by 4 min). Resting neutrophils contained < 10 pmol free AA/10 7 PMN; the receptor-mediated agonist N-formyl-methionyl-leucyl-phenylalanine (fMLP) alone did not increase these values. Exogenously-added PLA 2 generated large quantities of free AA in control and fMLP-treated cells (462 ± 122 and 2097 ± 176 pmol/10 7 PMN, respectively); however, this did not induce O 2 −, nor did it augment the level of O 2 − stimulated by fMLP. Also, PLA 2 caused no degranulation and did not alter degranulation induced by fMLP. PLA 2 also did not alter O 2 − or degranulation responses in primed PMN. The data indicate that mobilization of AA from cellular phospholipids neither stimulates nor modulates the respiratory burst or degranulation of PMN.