Vaccination with live Pasteurella haemolytica alters vascular adrenergic reactivity

Rats were injected with sterile saline (controls) or 0.7 x 10(5) c.f.u. of Pasteurella haemolytica (biotype A) obtained from a commercial vaccine. Three days following vaccination animals were killed and aortas removed from the heart base. Isolated aorta was placed in a tissue bath and the biophysical responses to phenylephrine (alpha-1 agonist), clonidine (alpha-2 agonist) and isoproterenol (beta agonist) determined. The results indicate that the alpha-2 adrenergic contractile response to clonidine is significantly enhanced while the response to phenylephrine is not significantly altered. The relaxant response to isoproterenol is eliminated following vaccination. These results suggest that exposure to vaccine-derived strains of P. haemolytica cause an imbalance in the adrenergic responsiveness of vascular smooth muscle.