Smooth muscle mechanics: implications for airway hyperresponsiveness.

Human bronchial smooth muscle preparations from 10 freshly dissected pneumonectomy samples were evaluated for their mechanical characteristics and compared with results obtained for similar samples obtained from porcine trachea. Length-tension relationships of in vitro smooth muscle were evaluated for passive stretching as well as active isometric force generation and isotonic shortening using electrical field stimulation. At the length (Lmax) producing maximal force (Pmax) resting tension was very high (60.0 +/- 8.8% Pmax) compared with porcine trachealis (5.2 +/- 2.3% Pmax). Maximum shortening was 25.0 +/- 9.0% at a length of 75% Lmax with suboptimal shortening occurring at Lmax (12.0 +/- 3.4%) for the human bronchus, whereas optimal shortening of porcine trachealis (71.4 +/- 3.6%) occurred at Lmax. Morphometric evaluation revealed threefold less muscle per cross-sectional area of tissue for human (8.7 +/- 1.5%) versus porcine (24.8 +/- 1.9%) preparations. We conclude that the high passive tension and the decreased maximum shortening are produced by a relatively large load which must be overcome for the muscle to shorten, presumably provided by the greater connective tissue elastic component present in the airway. We suggest that a decrease in airway wall elastance would increase smooth muscle shortening, thereby leading to excessive responses to contractile agonists as seen in vivo in asthma.