Time-resolved two-dimensional X-ray diffraction study of the effect of shortening on activation of contracting skeletal muscle

In twitch contractions of frog skeletal muscle, the isometric tension peaks when intracellular calcium has fallen to near-resting levels. To understand the mechanism of this delayed tension maintenance in the context of calcium regulation, the time course of the tropomyosin movement on actin was monitored by recording the intensity of the 2nd actin layer lines in a time-resolved two-dimensional X-ray diffraction study. The intensity rose ahead of tension, reflecting the tropomyosin movement from its "off" to "on" positions, but it fell with a time course similar to that of tension. Muscle shortening applied at the tension peak was followed by a poor recovery of tension, and accelerated the fall of the reflection intensity. The results suggest that the force-generating myosin heads retain the tropomyosin in its "on" position after the fall of intracellular calcium, and their shortening-induced detachment makes the tropomyosin return to its "off" position, thereby preventing myosin reattachment to actin.