Partitioning of airway responses to inhaled methacholine in the rat

1323, 1987.-We measured the changes in upper and lower airway resistance after inhalation of aerosols of methacholine (MCh) in doubling concentrations (16, 32, 64, and 128 mg/ml) in 11 anesthetized nonintubated spontaneously breathing rats. Upper airway resistance (Ru) increased from a control value of 0.48 t 0.04 cmHzO*ml- ’ l s (mean t SE) to 0.85 t 0.15 after 128 mg/ml MCh, whereas lower airway resistance (Rlo) in-creased from 0.11 t 0.03 to 0.21,t 0.04. However, there was no correlation between the magnitudes of the changes in Ru and Rlo. In a further seven anesthetized spontaneously breathing rats aerosols of MCh were delivered into the lower airways via a tracheostomy and resulted in increases in Rlo from a control value of 0.20-t 0.03 to 0.66 t 0.12 after 128 mg/ml MCh. Ru also increased to approximately double its control value. We conclude that 1) inhaled MCh causes narrowing of both R