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Background The down-regulation of constitutive nitric oxide synthase (cNOS) and up-regulation of inducible nitric oxide synthase (iNOS) are associated with the allergen-provocated airway hyperresponsiveness (AHR).This study aimed to determine whether their alteration also plays an important role in the AHR induced by lipopolysaccharide (LPS).Methods Hadley male guinea pigs,weighing between 250 g and 350 g,were injected with LPS at a dose of 1 mg/kg every 24 hours for three days.A non-selective NOS inhibitor,Nω-nitro-L-arginine methyl ester (L-NAME),or a selective inducible NOS inhibitor,aminoguanidine (AG),were used thirty minutes before each injection of LPS.Airway reactions,nitric oxide (NO) production and inflammatory changes were detected 24 hours after the last dose of LPS.Results AG significantly decreased the NO production in the bronchoalveolar lavage fluid (BALF) and sharply reduced the intensity of bronchoconstriction to histamine challenge.L-NAME also significantly decreased the NO production in the BALF,but had no effect on airway reactions or,perhaps,a tendency to enhance the intensity of AHR.Conclusiona The data suggest that inducible NOS contributes to the AHR induced by.repetitive intrapedtoneal LPS,and constitutive NOS was also involved.