Airway smooth muscle, tidal stretches, and dynamically determined contractile states.

In the classic theory of airway lumen narrowing in asthma, active force in airway smooth muscle is presumed to be in static mechanical equilibrium with the external load against which the muscle has shortened. This theory is useful because it identifies the static equilibrium length toward which activated airway smooth muscle would tend if given enough time. The corresponding state toward which myosin-actin interactions would tend is called the latch state. But are the concepts of a static mechanical equilibrium and the latch state applicable in the setting of tidal loading, as occurs during breathing? To address this question, we have studied isolated, maximally contracted bovine tracheal smooth muscle subjected to tidal stretches imposed at 0.33 Hz. We measured the active force (F) and stiffness (E), which reflect numbers of actin-myosin interactions, and hysteresivity ( η ), which reflects the rate of turnover of those interactions. When the amplitude of imposed tidal stretch ( e ) was very small, 0.25...