Induction of nitric oxide production by bovine alveolar macrophages in response to Pasteurella haemolytica A1.

We assessed the kinetics of inducible nitric oxide synthase (iNOS) mRNA expression and production of nitric oxide (NO) in bovine alveolar macrophages (AMs) stimulated with purified lipopolysaccharide (LPS) from Pasteurella haemolytica strain 12296. The effect of LPS on iNOS gene expression was dose-dependent and was expressed maximally at 24 h after stimulation with 10 micrograms/ml of LPS. Production of NO measured as secreted nitrite in supernatants took place in a time and dose-dependent manner with peak production at 24 h after LPS stimulation. Recombinant bovine gamma interferon (rb gamma IFN) augmented the LPS-induced iNOS gene expression and production of NO. The ability of LPS to induce iNOS gene expression and NO production either alone or in combination with rb gamma IFN was significantly abrogated by polymyxin B. In addition, the iNOS inhibitor NG-monomethyl-Larginine (L-NMMA) significantly inhibited LPS and rb gamma IFN + LPS induced NO production. Our results also demonstrated that NO produced from an exogenous NO donor sodium nitroprusside (SNP), and NO generated from LPS-stimulated AMs (endogenous) caused cytotoxic injury to bovine pulmonary artery endothelial cells in a dose-dependent manner. The cytotoxic injury caused by NO generated from LPS stimulated AMs was inhibited by polymyxin B or L-NMMA. There was a markedly increased concentration of nitrite in the lung lavage fluids of calves following P. haemolytica infection. These findings support a role for NO in the pathogenesis of lung injury in bovine pneumonic pasteurellosis.