Contraction measurement
Bronchial rings were connected to isometric force transducers Fort25 (WPI, UK). The signal was amplified by a Powerlab 8/36 and Octal Bridge Amp system (AD instruments, UK), recorded and analyzed via the LabChart 7 interface software (AD instruments, UK). Tissues were mounted on hooks, and attached with thread to a stationary rod and the other tied with thread to an isometric force displacement transducer. Airways were allowed to equilibrate by flushing with fresh KH buffer solution. Passive tension was determined by gentle stretching of tissue (0.5–1.0 g) during equilibration. The isometric change in tension was measured by the transducer and the tissue vitality assessed by EFS25Hz. These procedures allowed the bronchial rings to be correctly positioned between the hooks. When the passive contractile tone reached the plateau, rings were washed three times with fresh KH buffer solution and allowed to equilibrate for further 45 min.