Effects of bronchoconstriction on breathing during normoxia and hypoxia in anesthetized cats

The effect of an increase in bronchomotor tone on control of breathing during both normoxia and hypoxia, and the role of vagal afferents in regulating these responses were studied in 15 anesthetized cats. Minute ventilation (V̇e) was measured with a pneumotachograph connected in series with tracheal cannula. Total diaphragmatic EMG activity per minute ( X ̄p × f, peak EMG moving average × respiratory fequency) was measured to assess the central inspiratory drive. Bronchoconstriction was generated by inhalation of methacholine aerosol (10-30 breaths, 0.5% solution) which increased total lung resistance to approximately 400% of the control value. Transient hypoxia was induced by allowing the cats to rebreathe a hypoxic gas mixture (4.5% O₂ balanced N₂) for approximately 1 min. During normoxia, bronchoconstriction increased V̇e from a baseline of 100 to 129 ± 7% (mean ± SEM; P < 0.05) and increased ( X ̄p × f) from 100 to 174 ± 16% (P < 0.01). During hypoxia, the response of (-Xp × f) to bronchoconstriction 404 ± 40%) was still greater than without bronchoconstriction (304 ± 35%; P < 0.01), but the response of V̇e were not significantly different between these conditions (P < 0.05). After sectioning both vagus nerves the bronchoconstriction-induced increse in central inspiratory drive was either reduced (during normoxia) or abolished (during hypoxia). These results suggest that stimulation of vagal bronchopulmonary afferents are involved in regulating the ventilatory responses to bronchoconstriction. Other non-vagal factors, such as intrinsic properties and reflex responses of the respiratory muscles, may also contribute, in part, to the observed responses.