Airway responsiveness was assessed as a change in airway function after challenge with aerosolized methacholine (MCh) via the airways. Anesthetized, tracheostomized mice were mechanically ventilated and lung function was assessed using methods similar to those described by Martin et al. (8 (link)). A four-way connector was attached to the tracheostomy tube, with two ports connected to the inspiratory and expiratory sides of a ventilator (model 683; Harvard Apparatus, South Natick, MA). Ventilation was achieved at 160 breaths/min and a tidal volume of 0.15 ml with a positive end–expiratory pressure of 2–4 cm H2O.
The Plexiglas chamber containing the mouse was continuous with a 1.0-liter glass bottle filled with copper gauze to stabilize the volume signal for thermal drift. Transpulmonary pressure was detected by a pressure transducer with one side connected to the fourth port of the four-way connector and the other side connected to a second port on the plethysmograph. Changes in lung volume were measured by detecting pressure changes in the plethysmographic chamber through a port in the connecting tube with a pressure transducer and then referenced to a second copper gauze–filled 1.0-liter glass bottle. Flow was measured by digital differentiation of the volume signal. Lung resistance (Rl) and dynamic compliance (Cdyn) were continuously computed (Labview, National Instruments, TX) by fitting flow, volume, and pressure to an equation of motion.
Aerosolized agents were administered for 10 s with a tidal volume of 0.5 ml (9 (link)). From 20 s up to 3 min after each aerosol challenge, the data of Rl and Cdyn were continuously collected. Maximum values of Rl and minimum values of Cdyn were taken to express changes in murine airway function.