EVIDENCE FOR N-ACETYLCYSTEINE-SENSITIVE NITRIC-OXIDE STORAGE AS DINITROSYL-IRON COMPLEXES IN LIPOPOLYSACCHARIDE-TREATED RAT AORTA

1 The aim of this study was to assess whether or not vasoactive nitric oxide (NO) stores exist within vascular tissue after lipopolysacchari de (LPS)-treatment. 2 Rat thoracic aortic rings (for contraction exper iments) or whole thoracic aortae (for electron paramagnetic resonance (e.p.r.) spectroscopy) were incubated for 18 h at 37 degrees C in the absence (control) or in the presence of LPS (10 mu g ml(-1)), with or without L-arginine (L-Arg, 1 mM), the substrate of NO synthase (NOS) o r N-omega-nitro-L-arginine methyl ester (L-NAME, 1 mM), an inhibitor o f NOS. 3 Incubation of rat aortic rings with LPS and L-Arg resulted in a significant decrease of the maximum contractile response to noradre naline (NA, 3 mu M). Addition of L-NAME (3 mM) enhanced contraction to wards control values. After precontraction with NA and L-NAME, additio n of N-acetyl-L-cysteine (NAC, 0.1 to 10 mM) evoked a concentration-de pendent relaxation in rings incubated with LPS and L-Arg, but not in c ontrol rings, rings incubated with LPS in the absence of L-Arg or ring s incubated with LPS in the presence of L-Arg and L-NAME. Removal of t he endothelium did not significantly modify the relaxation induced by NAG. Methylene blue (3 mu M), an inhibitor of the activation of guanyl yl cyclase by NO, completely abolished the relaxing effect of NAC. 4 T he presence of protein-bound dinitrosyl non-haem iron complexes (DNIC) was detected by e.p.r. spectroscopy in aortae incubated with LPS and L-Arg, but not in control aortae. Furthermore in LPS-treated aortae, a ddition of NAC (20 mM) gave rise to the appearance of an e.p.r. signal characteristic of low molecular weight DNIC. 5 These results provide evidence that, within vascular tissue, NO generated from L-Arg by LPS- induced NOS activity can be stored as protein-bound DNIC in non-endoth elial cells. Upon addition of NAG, low molecular weight DNIC are relea sed from these storage sites and induce vascular relaxation probably t hrough guanylyl cyclase activation.