Uncoupling in the Wall of the Cartilaginous Bronchus of the Pig Produced by Smooth Muscle Contraction

Abstract

Studies in small non-cartilaginous airways suggest that the cross sectional area of the airway wall (comprising the smooth muscle and mucosa) is unaltered during the contraction–relaxation cycle produced by bronchial provocation. In the present study we examined whether the wall area of large cartilaginous bronchi remained constant during bronchoconstriction produced by acetylcholine (ACh, 10⁻⁴–10⁻²M), and if not the effect of this on the narrowing of the bronchial lumen. Narrowing of the lumen and adventitia of bronchial segments, 3.6 mm id from pigs, was simultaneously recorded using endoscopy and video imaging of the airway lumen. Concurrently, the shortening of the smooth muscle was estimated morphometrically from the same bronchi fixed during muscle contraction. In individual bronchi, ACh produced from 5 to 60% shortening of the smooth muscle, which caused up to 64% narrowing of the bronchial lumen, but only approximately 10% narrowing of the adventitia. Lumen narrowing was greater than predicted. Morphological assessment showed a doubling of the area between the smooth muscle and the cartilage, in response to maximum smooth muscle contraction by ACh. In contrast the area of the smooth muscle and mucosa did not change. Similar changes in bronchial morphology were produced in whole lung slices, incubated in ACh or histamine. We conclude that the inner airway wall uncouples from the outer wall during muscle contraction and that this increases the narrowing of the airway lumen relative to the adventitia.