Adventitia-derived nitric oxide in rat aortas exposed to endotoxin: cell origin and functional consequences

The role of adventitial cells in bacterial lipopolysaccharide (LPS)-induced vascular nitric oxide (NO) overproduction has been largely ignored. In rat aortas exposed to LPS in vitro or in vivo, it was found that adventitia co ntained the major part of NO synthase (NOS)-2 protein (Western blot and imm unohistochemistry) and generated the largest amount of NO (electron paramag netic resonance spin trapping). NOS-2 immunoreactive cells were mainly resi dent macrophages at an early stage (5 h, in vitro or in vivo) and fibroblas ts at a later stage (20 h, in vitro). Adventitial NOS-2 activity largely ac counted for 1) the relaxing effect of L-arginine in rings exposed to LPS in vivo, 2) generation of an "NO store" revealed by N-acetylcysteine-induced relaxation, and 3) formation of protein-bound dinitrosyl iron complexes in the medial layer of aortic rings exposed to LPS in vitro. In conclusion, th e adventitia is a powerful source of NO triggered by LPS in the rat aorta. This novel source of NO has an important impact on smooth muscle function a nd might be implicated in various inflammatory diseases.