Comparative nitric oxide production by LPS-stimulated monocyte-derived macrophages from Ovis canadensis and Ovis aries

Bighorn sheep are more susceptible to respiratory infection by Mannheimia haemolytica than are domestic sheep. In response to bacterial challenge, macrophages produce a number of molecules that play key roles in the inflammatory response, including highly reactive nitrogen intermediates such as nitric oxide (NO). Supernatants from monocyte-derived macrophages cultured with M. haemolytica LPS were assayed for nitric oxide activity via measurement of the NO metabolite, nitrite. In response to LPS stimulation, bighorn sheep macrophages secreted significantly higher levels of NO compared to levels for non-stimulated macrophages. In contrast, levels of NO produced by domestic sheep macrophages in response to M. haemolytica LPS did not differ from levels detected in non-stimulated cell cultures. Nitrite levels detected in supernatants of LPS-stimulated bighorn macrophage cultures treated with an inducible nitric oxide synthase (INOS) inhibitor, N(G)-monomethyl-L-arginine, were similar to that observed in non-stimulated cultures indicating a role for the iNOS pathway.