Effect of duration of stimulation on mechanical properties of trachealis muscle

We have studied the effect of duration of stimulation on the dynamic behavior of isolated dog trachealis muscle tonically contracted by alternating current or carbachol. With the muscle contracted quick stretch and quick release maneuvers were performed. The force-length curves during the quick maneuvers were similar for both methods of stimulation and regardless of the duration of stimulation. This behavior is consistent with an undamped passive series elastic component. The plateaux of force preceding these quick maneuvers were also similar. Isometric stress relaxations after quick stretch were similar. However, after quick release, isometric stress recovery and isotonic shortening velocity were more rapid with alternating current stimulation than with carbachol, and both isometric stress recovery and isotonic shortening velocity decreased with increasing duration of stimulation by either means of stimulation. We conclude that the difference in velocity of shortening is not due to the different method of stimulation but rather to the duration of stimulation. Active respiratory smooth muscle may show different dynamic properties depending on the duration of stimulation but not on the differences in means of stimulation.