Distribution of airway contractile responses in major resistance airways of the dog.

The topographical distribution of airway contractile responses within major diameter bronchi was examined isometrically in vitro in 15 mongrel dogs. Responses were compared in the same animals among airway Generations 2-5. Concentration-response curves were generated in the same strips with methacholine (MCh) and potassium chloride (KCl) in vitro. Force of contraction was assessed as grams force (g) divided by tissue wet weight (g/g) and was normalized further for the proportion of smooth muscle in each airway by means of computerized morphometry (g/g smooth muscle mass; g/gM). Both techniques revealed substantial heterogeneity of response that was not dependent upon the mechanism of contraction. Maximal isometric contraction to MCh increased from 1911 +/- 245 (Generation 2) to 6693 +/- 850 g/gM (Generation 5) (P less than 0.005). Similarly, maximal contraction to KCl, an agonist causing contraction by a non-receptor-mediated mechanism, was nearly three times greater in Generation 5 than in Generation 2 bronchi (P less than 0.005). It is concluded that the force of isometric contraction of bronchial smooth muscle increases through the first five generations of bronchi, even after normalization for smooth muscle mass. These differences are not agonist-dependent and do not depend upon the topographical distribution of receptors in the airways.