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

Abstract

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 [ 3H]arachidonic acid. The release of [ 3H]leukotriene B 4 and [ 3H]prostanoids (separated by high performance liquid chromatography) was determined. A 23187 was used as stimulus, as rising intracellular Ca 2+ activates directly the phospholipase A 2 and lipoxygenase pathway. A 23187 (10 μM) enhanced [ 3H]leukotriene B 4 release from freshly prepared alveolar macrophages about 65-fold, but only 5- to 6-fold from cultured alveolar macrophages. Evoked [ 3H]leukotriene B 4 release and spontaneous [ 3H]prostanoid release were inhibited when l-arginine (300 μM) was added to the Krebs incubation medium of alveolar macrophages, in which marked NO synthase had been induced by culture with lipopolysaccharides (10 μg/ml). Inhibitory effects of l-arginine were prevented by N G-monomethyl- l-arginine ( l-NMMA, 100 μM). 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 [ 3H]leukotriene B 4 and allowed lipopolysaccharides to cause an increase in spontaneous [ 3H]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.