Bronchial reactivity: interaction between vagal stimulation and inhaled histamine.

The interaction between the effects of vagal stimulation and inhaled histamine on the bronchi was studied in anesthetized dogs. Reactivity was assessed by measuring changes in bronchial caliber visualized with tantalum bronchograms. In seven vagotomized dogs the bronchoconstrictor response to a combination of electrical stimulation of the vagus nerves and inhaled histamine solution produced a mean reduction in airway diameter (Daw) of 2.21 mm which was significantly greater than the additive results of the two stimuli applied separately (mean decrease in Daw 0.29 +/- 0.91 mm). In three dogs the effect of vagal stimulation was to produce a shift in the dose-response curve to inhaled histamine. These results indicate that the effect of the base-line bronchomotor tone must be considered in the evaluation of the effect of vagal blockade on airway reactivity. An increase in the resting degree of bronchomotor tone may contribute to the hyperreactivity observed in patients with asthma.