Canine pulmonary response to aerosol histamine: direct versus vagal effects

Histamine, a potent bronchoconstrictor, has been shown to produce bronchoconstriction both directly and by a vagal reflex. To define the relative roles of direct and reflex effects, we studied the pulmonary response of dogs exposed to increasing doses of aerosol histamine before and after vagal blockade or vagotomy. In addition, the relative contributions of aerodynamically large and small airways to the overall response were determined by the measurement of pulmonary resistance on sulfur hexafluoride-oxygen and helium-oxygen mixtures. Histamine aerosol caused a similar dose-dependent increase in resistance of aerodynamically large and small airways and fall in dynamic compliance. The dose-response relationships were not consistently altered by either vagal blockade or vagotomy. The following variables were found not to alter the experimental results: anesthesia, type of aerosol generator, control of breathing during aerosol exposure, spontaneous breathing vs. controlled ventilation after aerosol exposure, cold block of vagi vs. vagotomy. We conclude that 1) histamine aerosol in dogs causes a local dose-dependent constriction of bronchial smooth muscle, and 2) the vagus nerve played a relatively minor role in the pulmonary response to aerosol histamine in these experiments.