Instantaneous view of actomyosin structure in shortening muscle.

In the previous study), utilizing the rapid-freeze technique for electron microscopy, a new type of actomyosin assembly was reported to occur during isometric contraction. In this study, we have developed a new freezing apparatus which permits rapid freezing of the "isotonically-shortening" muscle at high shortening velocity, allowing simultaneous measurement of its tension and length. Using glycerinated rabbit skeletal muscle rapidly frozen with this system, we have compared the freeze-substituted images of the actomyosin assembly in the shortening muscle at various tension levels with those in isometrically-contracting muscle. The optical diffraction patterns obtained from whole A bands and from single actin filaments decorated with crossbridges have revealed that the unique actomyosin assembly formed in isometrically-contracting muscle is maintained even during high-velocity shortening under near-zero load.