Transient oscillatory force-length behavior of activated airway smooth muscle.

Airway smooth muscle (ASM) is cyclically stretched during breathing, even in the active state, yet the factors determining its dynamic force-length behavior remain incompletely understood. We developed a model of the activated ASM strip and compared its behavior to that observed in strips of rat trachealis muscle stimulated with methacholine. The model consists of a nonlinear viscoelastic element (Kelvin body) in series with a force generator obeying the Hill force-velocity relationship. Isometric force in the model is proportional to the number of bound crossbridges, the attachment of which follows first-order kinetics. Crossbridges detach at a rate proportional to the rate of change of muscle length. The model accurately accounts for the experimentally observed transient and steady-state oscillatory force-length behavior of both passive and activated ASM. However, the model does not predict the sustained decrement in isometric force seen when activated strips of ASM are subjected briefly to large stretches. We speculate that this force decrement reflects some mechanism unrelated to the cycling of crossbridges, and which may be involved in the reversal of bronchoconstriction induced by a deep inflation of the lungs in vivo.