Dynamically determined contractile states of airway smooth muscle

The contractile state of maximally activated bovine airway smooth muscle is altered during imposed tidal stretches that simulate the action of breathing. When the amplitude of imposed tidal stretch is very small (0.25% of muscle optimal length), the steady-state value of the muscle force, F, approximates the isometric force, the muscle stiffness, E, is large, and the muscle hysteresivity, eta, is small. When the amplitude is increased beyond 1%, however, F and E promptly decrease and eta promptly increases. The provocative stretch amplitude required to cause active force or muscle stiffness to fall by half, or hysteresivity to double, is slightly greater than 2%. By contrast, the stretch amplitude expected during quiet breathing at rest is 4%. Therefore, the isometric force generating capacity of airway smooth muscle may not be applicable to the force generated in normal physiologic circumstances, even during maximal bronchial provocation.