Eicosanoid release in the endotoxin-primed isolated perfused rat lung and its pharmacological modification

Abstract.

Objective: Recent observations have demonstrated a central role of the "inducible" isoform of the cyclooxygenase (COX), COX-2, in the rat lung. Therefore, the reported capacity of selective COX-2 inhibitors to potentiate the formation of leukotriene (LT) B₄ may raise concern about pro-inflammatory side effects of such drugs in the respiratory system. The present study was aimed at determining the effects of the COX-2 inhibitor NS-398 on the release of COX and 5-lipoxygenase (LOX) metabolites of arachidonic acid in isolated perfused lungs obtained from endotoxin-treated rats before and after stimulation with the leukocyte secretagogue N-formyl-methionyl-leucyl-phenylalanine (FMLP).¶Methods: Two hours after rats had received endotoxin i.v., the lung was dissected and perfused via the pulmonary artery with physiological salt solution. After an equilibration period of 20 min the outflow was collected (5-min fractions). In the respective treatment groups, indomethacin, NS-398, or the 5-LOX inhibitor MK886 were present throughout the experiment, while FMLP was added to the perfusate during a single 5-min period. The concentration of eicosanoids in the outflow was determined by radioimmunoassay.¶Results: Endotoxin treatment of rats resulted in in-creased expression of COX-2 mRNA in lung tissue, and an elevated basal release of the prostaglandin (PG)I₂ metabolite 6-keto PGF_1α, without a detectable increase of leukotriene (LT) formation. In-vitro exposure to FMLP stimulated LT and prostanoid release, which was significantly enhanced in endotoxin-primed lungs, and was suppressed by the 5-LOX inhibitor MK-886 (3 μM) and the COX-inhibitor indomethacin (5 μM), respectively. Either compound showed selective inhibition of the respective pathway of arachidonic acid metabolism. In endotoxin-primed lungs, the COX-2 inhibitor NS-398 (0.3-1.0 μM) depressed basal as well as FMLP-stimulated release of 6-keto PGF_1α, but did not cause a significant increase of LTB₄ or cysteinyl-LT release.¶Conclusions: These results suggest that FMLP, presumably acting on inflammatory cells trapped in the pulmonary circulation of endotoxin treated rats, induced prostanoid formation mainly via the COX-2 pathway, and that its inhibition by NS-398 had no detectable potentiating effect on LTB₄ or cysteinyl-LT biosynthesis.