Hypocapnia-induced constriction of the canine peripheral airways exhibits tachyphylaxis

Hypocapnia-induced constriction of peripheral airways may be important in regulating the distribution of ventilation in pathological conditions. We studied the response of the peripheral lung to hypocapnia in anesthetized, paralyzed, mechanically ventilated dogs using the wedged bronchoscope technique to measure resistance of the collateral system (Rcs). A 5-min hypocapnic challenge produced a 161 ± 19% (mean ± SE) increase in Rcs. The magnitude of this response was not diminished with repeated challenge or by atropine sulfate (1 mg base/kg iv), chlorpheniramine maleate (5 mg base/kg iv), or indomethacin (5 mg/kg iv). The response was reduced by 75% by isoproterenol (5 μg/kg iv) (P<0.01) and reduced by 80% by nifedipine (20 μg/kg iv) (P<0.05). During 30-min exposure to hypocapnia the maximum constrictor response occurred at 4-5 min, after which the response attenuated to ~50% of the maximum response (mean = 53%, range 34-69%). Further 30-min challenges with hypocapnia resulted in significantly decreased peak responses, the third response being 50% of the first (P<0.001). The inability of indomethacin or propranolol to affect the tachyphylaxis or attenuation of the response suggests that neither cyclooxygenase products nor β-adrenergic activity was involved. Hence, hypocapnia caused a prompt and marked constrictor response in the peripheral lung not associated with cholinergic mechanisms or those involving histamine H₁-receptors or prostaglandins. With prolonged exposure to hypocapnia there was gradual attenuation of the constrictor response with continued exposure and tachyphylaxis to repeated exposure both of which would tend to diminish any compensatory effect of hypocapnic airway constriction on the distribution of ventilation.