Hysteresis of contracted airway smooth muscle.

How does smooth muscle tone affect the properties of airways during breathing maneuvers? We have studied the dynamic relationships of airway pressure and cross-sectional area in excised dog lobes and isolated tracheal segments during simulated breathing maneuvers. When contracted by carbachol, airways developed complex hysteretic behavior, becoming less compliant in small tidal breaths and rapid deflations, and grossly hysteretic in large tidal maneuvers. For a given sequence of maneuvers the patterns of hysteresis varied with timing. Analogous behavior was seen during length-cycling of dog trachealis muscle strips. The mechanisms suggested by the hysteretic patterns of muscle length-tensional plots are 1) slipping of contractile filaments at threshold forces during lengthening and 2) active shortening of contractile filaments that is slow relative to many respiratory events. The observed behavior indicates that measurements of pulmonary function that depend on airway caliber or stiffness should become increasingly affected by the sequence, amplitude, and timing of breathing maneuvers as smooth muscle tone increases. The stiffness for small amplitude cycling may be favorable for local control of ventilation by parenchymal smooth muscle.