Abstract
We developed an electromagnetic apparatus to perform a quick change in load in the motility system, using magnetizable beads on which myosin thick filaments from molluscan smooth muscle or green algae, Chara, myosin were attached. The quick change in load to beads (diameter 4.5 μm) was applied in the range of 0–85 pN. The movement of beads was recorded by a video-system and analyzed with special software. When the quick increase in load was applied during the movement of beads under no load, the beads showed a transient movement to the reverse direction before the steady slower movement to the normal direction. When the application of load was stopped, the beads showed a transient fast phase of movement. The change in load-sustaining ability was measured by a double load step. The backward velocity at the second constant test load was smaller when the first preceding step was increased, suggesting that the ability to sustain load was higher with a higher preceding step. These phenomena were observed both in molluscan thick filaments and in Chara myosin, and the time course of the movement of a bead was quite similar to those observed previously in frog single muscle fibers. This suggested that the velocity transients are the intrinsic properties induced by the interaction between actin and myosin, irrespective of the hexagonal lattice structure of filaments, the regular sarcomere structure, and myosin type, namely, that the molecule of myosin itself has the ability to adjust to mechanical circumstances.
