Mechanisms for the mechanical response of airway smooth muscle to length oscillation

Shen, X., M. F. Wu, R. S. Tepper, and S. J. Gunst. Mechanisms for the mechanical response ofairway smooth muscle to length oscillation. J. Appl.Physiol. 83(3): 731-738, 1997.Airway smoothmuscle tone in vitro is profoundly affected by oscillations in musclelength, suggesting that the effects of lung volume changes on airwaytone result from direct effects of stretch on the airway smooth muscle.We analyzed the effect of length oscillation on active force andlength-force hysteresis in canine tracheal smooth muscle at differentoscillation rates and amplitudes during contraction with acetylcholine.During the shortening phase of the length oscillation cycle, the activeforce generated by the smooth muscle decreased markedly below theisometric force but returned to isometric force as the muscle waslengthened. Results indicate that at rates comparable to those duringtidal breathing, active shortening and yielding of contractile elementscontributes to the modulation of force during length oscillation;however, the depression of force during shortening cannot be accountedfor by cross-bridge properties, shortening-induced cross-bridgedeactivation, or active relaxation. We conclude that the depression ofcontractility may be a function of the plasticity of the cellularorganization of contractile filaments, which enables contractileelement length to be reset in relation to smooth muscle cell length asa result of smooth muscle stretch.