MECHANISMS FOR THE MECHANICAL RESPONSE OF AIRWAY SMOOTH-MUSCLE TO LENGTH OSCILLATION

Airway smooth muscle tone in vitro is profoundly affected by oscillati ons in muscle length, suggesting that the effects of lung volume chang es on airway tone result from direct effects of stretch on the airway smooth muscle. We analyzed the effect of length oscillation on active force and length-force hysteresis in canine tracheal smooth muscle at different oscillation rates and amplitudes during contraction with ace tylcholine. During the shortening phase of the length oscillation cycl e, the active force generated by the smooth muscle decreased markedly below the isometric force but returned to isometric force as the muscl e was lengthened. Results indicate that at rates comparable to those d uring tidal breathing, active shortening and yielding of contractile e lements contributes to the modulation of force during length oscillati on; however, the depression of force during shortening cannot be accou nted for by cross-bridge properties, shortening-induced cross-bridge d eactivation, or active relaxation. We conclude that the depression of contractility may be a function of the plasticity of the cellular orga nization of contractile filaments, which enables contractile element l ength to be reset in relation to smooth muscle cell length as a result of smooth muscle stretch.