Physiology of airway narrowing

1. Responsiveness of the bronchial tree is measured at the mouth in vivo and represents narrowing responses of multiple airways in the lung. Responsiveness of individual bronchi has not been adequately investigated and the dynamic changes associated with airway narrowing are largely undefined. 2. Responsiveness to cholinergic drugs was determined in bronchial segments in vitro, from lumen flow and narrowing, and from the force and shortening of the smooth muscle producing these responses. 3. Maximum flow reduction was produced by submaximum smooth muscle force and shortening. In 2 mm inner diameter bronchi, flow was abolished by acetylcholine but the lumen was not physically closed. Flow sensitivity was increased more in small diameter bronchi than in large, but this was not associated with differences in force by smooth muscle. Smooth muscle contraction increased the thickness of the total airway wall via a tensioning and expansion of mural elastic fibres. Flow responses were more sensitive to drugs placed adventitially than luminally. 4. Results define a major role for airway geometry and location, the epithelial barrier and the intercellular matrix, which couples the smooth muscle to the cartilaginous airway adventitia, in determining responsiveness of intact airways.