Exposure of sheep to Pasteurella haemolytica alters ex vivo pulmonary vascular smooth muscle alpha adrenergic responses: role of vascular endothelium.

Sheep were exposed to a live- vaccine derived strain of Pasteurella haemolytica (10(5) colony forming units/sheep, i.m.) and euthanized 3 days later. Pulmonary artery and vein ex vivo responses to cumulative doses of methoxamine were determined. In some experiments endothelium was removed. In the pulmonary artery, removal of the endothelium significantly impairs the potency of methoxamine as a contractile agent in control sheep. Also, in sheep exposed to P. haemolytica removal of the endothelium ex vivo impairs the efficacy of methoxamine as a contractile agent. In the pulmonary vein, removal of the endothelium significantly increases the maximum contractile response obtained in control sheep. Conversly, in the pulmonary vein from P. haemolytica exposed sheep, removal of the endothelium reduces the efficacy of methoxamine. These experiments suggest that vaccination with live P. haemolytica impairs pulmonary vascular alpha adrenergic responses. Such impairments may contribute to development of pulmonary edema and congestion during stressful periods such as post-vaccination.