Effect of length history on contractile behavior of canine tracheal smooth muscle.

The effects of shortening on force redevelopment were studied in canine tracheal smooth muscles that were tonically contracted with electrical stimulation or acetylcholine. Muscle strips were suspended in vitro so that muscle length, force, and rate of change of force could be continuously monitored. The strips were rapidly shortened from different initial lengths to the same final length, at which the effects of shortening distance on force redevelopment were evaluated. Both the rate and magnitude of force redevelopment declined as the shortening distance was increased. Decreasing the activation of the muscle or increasing the prerelease contraction time enhanced the depressive effect of shortening. The time course of the latter corresponded to a decline in the velocity of unloaded shortening. However, force redevelopment was also substantially decreased by releases performed before the onset of force development. The shortening process per se had a depressive effect on both the rate and magnitude of force redevelopment in contracted canine tracheal smooth muscle, and this effect was reduced as the activation of the muscle was increased.