PAF-induced synthesis of tetraenoic and pentaenoic leukotrienes in the isolated rabbit lung

Abstract

In an isolated rabbit lung model, we tested the hypothesis that platelet-activating factor (PAF)-induced leukotriene (LT) synthesis is critically dependent on the free precursor fatty acid supply and the possible substitution of arachidonic acid (AA) by eicosapentaenoic acid (EPA). To augment the intravascular polymorphonuclear neutrophils (PMNs) in the isolated lung, human PMNs were infused into the pulmonary artery. LTs and hydroxyeicosatetra(penta)enoic acids were quantified with HPLC techniques. Application of PAF (5 μM) or AA (10 μM) provoked the generation of limited quantities of 4-series LTs and 5-hydroxyeicosatetraenoic acid (total sum of 5-lipoxygenase products ≈7 and ≈27 pmol/ml in lungs both with and without infused PMNs, respectively). Combined administration amplified 5-lipoxygenase product formation, with a predominance of cysteinyl-LT synthesis in lungs both without (total sum ≈ 67 pmol/ml) and, much more strikingly, with (total sum ≈308 pmol/ml) an infusion of neutrophils. EPA (10 μM) elicited exclusive generation of 5-series LTs and 5-hydroxyeicosapentaenoic acid (total sum ≈ 82 pmol/ml). Dual stimulation with PAF and EPA provoked amplification of EPA-derived 5-lipoxygenase product formation, again with predominance of cysteinyl-LTs in lungs without (total sum ≈224 pmol/ml) and, in particular, with (total sum ≈545 pmol/ml) preceding microvascular PMN entrapment. Combined application of PAF, AA, and EPA resulted in the synthesis of LTs derived from both fatty acids, with a predominance of 5-series products. We conclude that the PAF-evoked 5-lipoxygenase product formation in the neutrophil-harboring lung capillary bed is critically dependent on intravascular precursor fatty acid supply, with EPA representing the preferred substrate compared with AA. PMN-related transcellular eicosanoid synthesis is suggested to underlie the predominant generation of cysteinyl-LTs. The supply of n-3 versus n-6 precursor fatty acid may thus have a major impact on inflammatory mediator generation.