Responses of isolated canine airways to electric stimulation and acetylcholine

Contractility of tracheal smooth muscle strips and spiral strips of intrapulmonary airways 5 mm and 1.5 mm in diameter was studied in organ baths. Acetylcholine (ACh) sensitivity of canine airways closely resembled that of other species, and concentration-response curves for this drug were similar for all three preparations. Electric field stimulation (ES) elicited sustained contractions and, in contrast to guinea pig and human airways, never produced relaxations below the base line. Atropine antagonized the responses to both ES and exogenous ACh whereas tetrodotoxin abolished the response to supramaximal ES without altering ACh concentration-response curves. Thus, ES contracts airway smooth muscle via release of neurotransmitter from autonomic nerve endings whereas exogenous ACh acts directly on smooth muscle cells. The ratio of maximal force produced by ES to that produced by ACh decreased significantly from the trachea to the 1.5-mm airways. The latter result suggested that either the innervation of airway smooth muscle or the type of smooth muscle intercellular contacts, or both, vary depending on location within the tracheobronchial tree.