Endogenous nitric oxide inhibits leukotriene B-4 release from rat alveolar macrophages

Effects of endogenous nitric oxide (NO) on the release of mediators of the lipoxygenase and cyclo-oxygenase pathway from rat alveolar macrophages were studied. Alveolar macrophages, freshly isolated or after 18-h culture, were incubated in (amino acid-free) Krebs medium and labelled with [H-3]arachidonic acid. The release of [H-3]leukotriene B-4 and [H-3]prostanoids (separated by high performance liquid chromatography) was determined. A 23187 was used as stimulus, as rising intracellular Ca2+ activates directly the phospholipase A(2) and lipoxygenase pathway. A 23187 (10 mu M) enhanced [H-3]leukotriene B-4 release from freshly prepared alveolar macrophages about 65-fold, but only 5- to 6-fold from cultured alveolar macrophages. Evoked [H-3]leukotriene B-4 release and spontaneous [H-3]prostanoid release were inhibited when L-arginine (300 mu M) was added to the Krebs incubation medium of alveolar macrophages, in which marked NO synthase had been induced by culture with lipopolysaccharides (10 mu g/ml). Inhibitory effects of L-arginine were prevented by N-G-monomethyl-L-arginine (L-NMMA, 100 mu M). Inhibition of NO synthase during the culture period by L-NMMA (culture medium, in contrast to Krebs medium, already contains the substrate ofNO synthase, L-arginine), resulted in attenuation of the 'culture-dependent' decline of the evoked release of [H-3]leukotriene B-4 and allowed lipopolysaccharides to cause an increase in spontaneous [H-3]prostanoid release (i.e., to induce cyclo-oxygenase activity). In conclusion, in rat alveolar macrophages, endogenous NO appears to inhibit the release of mediators of the cyclo-oxygenase and lipoxygenase pathway through multiple sites of action.