EUCAPNIC HYPERVENTILATION-INDUCED BRONCHOCONSTRICTION IN RABBITS

We examined whether eucapnic hyperventilation with dry air produces the bronchoconstriction in anesthetized, non-sensitized rabbits and in ovalbumin sensitized rabbits. Eucapnic hyperventilation challenge with dry air containing 5% CO2 at room temperature was performed with 4 non-sensitized and 7 sensitized rabbits by mechanical ventilation for 15 min (120 breaths/min, 7 ml tidal volume/kg body weight). Total lung resistance (R(L)) and dynamic compliance (Cdyn) were measured before and 0, 5, 15, and 30 min after hyperventilation. In non-sensitized rabbits, R(L) and Cdyn did not change significantly. However, in sensitized rabbits, R(L) increased maximally by 48.9% +/- 9.0% at 5 min, and then decreased to the baseline level at 30 min after challenge. Cdyn decreased maximally by 12.5% +/- 3.5% at 15 min after challenge. These changes were significantly different from the baselines (p < 0.05). Furthermore, to investigate the role of histamine on hyperventilation-induced bronchoconstriction (HIB) in sensitized rabbits, we performed the hyperventilation challenges in 5 sensitized rabbits with the pretreatment of H-1-receptor antagonist (chlorpheniramine, 1 mg/kg, i.v.) and found that the maximum increment of R(L) was suppressed to 24.2% +/- 7.4% of the control, which was significantly lower than the maximal R(L) in nontreated sensitized rabbits (p < 0.05). We concluded that HIB occurrs only in sensitized rabbits and that histamine may play an important role in the development of HIB in sensitized rabbits.