TRACHEAL SMOOTH-MUSCLE MECHANICS INVIVO

We applied the technique of sonomicrometry to directly measure length changes of the trachealis muscle in vivo. Pairs of small 1-mm piezoelectric transducers were placed in parallel with the muscle fibers in the posterior tracheal wall in seven anesthetized dogs. Length changes were recorded during mechanical ventilation and during complete pressure-volume curves of the lung. The trachealis muscle showed spontaneous fluctuations in base-line length that disappeared after vagotomy. Before vagotomy passive pressure-length curves showed marked hysteresis and lenght changed by 18.5 ± 13.2% (SD) resting length at functional residual capacity (L(FRC)) from FRC to total lung capacity (TLC) and by 28.2 ± 16.2% L(FRC) from FRC to residual volume (RV). After vagotomy hysteresis decreased considerably and length now changed by 10.4 ± 3.7% L(FRC) from FRC to TLC and by 32.5 ± 14.6% L(FRC) from FRC to RV. Bilateral supramaximal vagal stimulation produced a mean maximal active shortening of 28.8 ± 14.2% resting length at any lung volume (L(R)) and shortening decreased at lengths above FRC. The mean maximal velocity of shortening was 4.2 ± 3.9% L(R)·s-1. We conclude that sonomicrometry may be used to record smooth muscle length in vivo. Vagal tone strongly influences passive length change. In vivo active shortening and velocity of shortening are less than in vitro, implying that there are significant loads impeding shortening in vivo.