Induction of nitric oxide synthase by endotoxin in rat isolated aorta but not in rat aortic smooth muscle cells grown in culture from explant

Incubation of endothelium-denuded rings of rat aorta at 37 degrees C for 18 hours in Krebs solution led to a profound depression of the contractile actions of phenylephrine (1 nM-10 mu M). A major component of this depression of vasoconstriction was due to the relaxant actions of nitric oxide since it was reversed following inhibition of the synthesis of nitric oxide with N-G-nitro-L-arginine methyl ester or its actions with haemoglobin (30 mu M) or methylene blue (10 mu M). The depression was also reversed upon treatment with LY83583 (0.1-1 mu M) which generates superoxide anions, intracellularly and extracellularly, but was unaffected by hypoxanthine (100 mu M)/xanthine oxidase (16 mu/ml) which generates superoxide anion only extracellularly. The ability of polymixin B (30 mu M) to inhibit the development of the depression of vasoconstriction suggests that it results from the expression of an inducible form of nitric oxide synthase, stimulated by bacterial lipopolysaccharide, contaminating the Krebs solution. In contrast to aortic rings, we found that lipopolysaccharide (10-10,000 ng/ml) alone from S. typhosa was unable to stimulate the expression of the inducible form of nitric oxide synthase in rat aortic smooth muscle cells grown in culture from explant, as assessed either by measuring the accumulation of nitrite into the culture medium during a 24 hour incubation period or by measuring the smooth muscle cyclic GMP content. Interferon-gamma (1-100 IU/ml) and interleukin-1 alpha (1-10 IU/ml) alone were, however, able to stimulate the accumulation of nitrite in a concentration-dependent manner. These inductions of nitrite accumulation were abolished following treatment with N-G-monomethyl-L-arginine (0.5 mM), N-G-nitro-L-arginine methyl ester (1 mM) and dexamethasone (1 mu M). Further investigations are required to determine whether the ability of bacterial lipopolysaccharide to induce the inducible form of nitric oxide synthase in rat aortic rings, but not in rat aortic smooth muscle cells in culture, results from the presence of an endotoxin-sensitive, cytokine-secreting cell type in the vessel wall which is absent in culture, or from differences in smooth muscle phenotype in situ and in culture.