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

Objective: Recent observations have demonstrated a central role of the "ind ucible" isoform of the cycloxygenase (COX), COX-2, in the rat lung. Therefo re, the reported capacity of selective COX-2 inhibitors to potentiate the f ormation of leukotriene (LT) B-4 may raise concern about pro-inflammatory s ide 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 relea se of COX and 5-lipoxygenase (LOX) metabolites of arachidonic acid in isola ted perfused lungs obtained from endotoxin-treated rats before and after st imulation with the leukocyte secretagogue N-formyl-methionyl-leucylphenylal anine (FMLP). Methods: Two hours after rats had received endotoxin i.v., the lung was dis sected and perfused via the pulmonary artery with physiological salt soluti on. After an equilibration period of 20 min the outflow was collected (5-mi n fractions). In the respective treatment groups, indomethacin, NS-398, or the 5-LOX inhibitor MK886 were present throughout the experiment, while FML P was added to the perfusate during a single 5-min period. The concentratio n of eicosanoids in the outflow was determined by radioimmunoassay. Results: Endotoxin treatment of rats resulted in increased expression of CO X-2 mRNA in lung tissue, and an elevated basal release of the prostaglandin (PG)I-2 metabolite 6-keto PGF(1 alpha), without a detectable increase of l eukotriene (LT) formation. In-vitro exposure to FMLP stimulated LT and pros tanoid release, which was significantly enhanced in endotoxin-primed lungs, and was suppressed by the 5-LOX inhibitor MK-886 (3 mu M) and the COX-inhi bitor indomethacin (5 mu M), respectively. Either compound showed selective inhibition of the respective pathway of arachidonic acid metabolism, in en dotoxin-primed lungs, the COX-2 inhibitor NS-398 (0.3-1.0 mu M) depressed b asal as well as FMLP-stimulated release of 6-keto PGF(1 alpha), but did not cause a significant increase of LTB4 or cysteinyl-LT release. Conclusions: These results suggest that FMLP, presumably acting on inflamma tory cells trapped in the pulmonary circulation of endotoxin treated rats, induced prostanoid formation mainly via the COX-2 pathway, and that its inh ibition by NS-398 had no detectable potentiating effect on LTB4 or cysteiny l-LT biosynthesis.