Journal of Applied Physiology, Vol 68, Issue 1 209-219, Copyright © 1990 by American Physiological Society
Tracheal smooth muscle mechanics in vivo
M. Okazawa, P. Pare and J. Road
Department of Medicine, University Hospital, Vancouver, British Columbia, Canada.
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 length changed by 18.5
+/- 13.2% (SD) resting length at functional residual capacity (LFRC) from
FRC to total lung capacity (TLC) and by 28.2 +/- 16.2% LFRC from FRC to
residual volume (RV). After vagotomy hysteresis decreased considerably and
length now changed by 10.4 +/- 3.7% LFRC from FRC to TLC and by 32.5 +/-
14.6% LFRC 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 (LR) and shortening decreased at lengths above FRC. The
mean maximal velocity of shortening was 4.2 +/- 3.9% LR.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.
