CONSTITUTIVE NITRIC-OXIDE PRODUCTION BY RAT ALVEOLAR MACROPHAGES

Results from previous studies suggest that alveolar macrophages must b e exposed to inflammatory stimuli to produce nitric oxide ( NO). In th is study, me report that naive unstimulated rat alveolar macrophages d o produce .NO and attempt to characterize this process. Western blot a nalysis demonstrates that the enzyme responsible is an endothelial nit ric oxide synthase (eNOS). No brain or inducible NOS can be detected. The rate of .NO production is similar to 0.07 nmol.10(6) cells(-1).h(- 1) an amount that is less than that produced by the eNOS found in alve olar type II or endothelial cells. Alveolar macrophage .NO formation i s increased in the presence of extracellular L-arginine, incubation me dium containing magnesium and no calcium, a calcium ionophore (A-23187 ), or methacholine. .NO production is inhibited by N-G-nitro-L-arginin e methyl ester (L-NAME) but not by N-G-nitro-L-arginine, L-N-5-(1-imin omethyl)ornithine hydrochloride, or aminoguanidine. Incubation with AT P, ADP, or histamine also inhibits .NO formation. Some of these proper ties are similar to and some are different from properties of eNOS in other cell types. Cellular .NO levels do not appear to be related to A TP or lactate content. Alveolar macrophage production of NO can be inc reased approximately threefold in the presence of lung surfactant or i ts major component, dipalmitoyl phosphatidylcholine (DPPC). The DPPC-i nduced increase in NO formation is time and concentration dependent, c an be completely inhibited by L-NAME, and does not appear to be relate d to the degradation of DPPC by alveolar macrophages. These results de monstrate that unstimulated alveolar macrophages produce .NO via an eN OS and that lung surfactant increases .NO formation. This latter effec t may be important in maintaining an anti-inflammatory state in vivo.