Direct and reflex bronchoconstriction induced by histamine aerosol inhalation in dogs

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Abstract
Histamine aerosols were administered to 9 anesthetized, paralyzed, mechanically ventilated mongrel dogs with the cervical vagi first intact, then sectioned and then peripherally stimulated at 2 intensities. Pulmonary resistance (RL) was measured and dose-response curves were constructed in the 4 conditions. All dogs had dose-related increases in RL with increasing histamine aerosol concentrations. After the vagi were cut, the degree to which constant levels of vagal stimulation altered the dose-response relationships was assessed by examining the slopes and positions of the RL vs. histamine curves. Of the 9 animals, 4 showed evidence of increasing vagal efferent activity as the concentration of histamine in the aerosol increased, but 3 did not. Of the dogs, 2 had equivocal responses; increasing activity was suggested but not clearly demonstrated. Apparently, in addition to direct effects of histamine, the extent of and basis for vagal interaction with this stimulus varies among dogs. Apparently, the basis of vagal interaction is related to constant efferent activity (tone) or increasing efferent activity (reflexes).