Abstract
1. The basic properties of the ATP-dependent actin-myosin interaction responsible for muscle contraction were studied using an in vitro force-movement assay system, in which a glass microneedle coated with rabbit skeletal muscle myosin was made to slide on the actin filament arrays (actin cables) in the internodal cell of an alga Nitellopsis obtusa with ionophoretic application of ATP. 2. In response to an ATP current pulse (intensity, 5-85 nA; duration, 0.5-10 s), the myosin-coated needle moved for a distance and eventually stopped, indicating reformation of rigor actin-myosin linkages to prevent elastic recoil of the bent needle. A subsequent ATP current pulse again produced the needle movement starting from the baseline force attained by the preceding needle movement. 3. With a constant amount of ATP application, the amount of work done by the ATP-induced actin-myosin sliding first increased with increasing baseline force from zero to 0.4-0.6P0, and then decreased with further increasing baseline force, thus giving a bell-shaped work versus baseline force relation. 4. With increasing amount of ATP application, the amount of work done by the actin-myosin sliding increased more steeply as the baseline force was increased from zero to 0.4-0.6P0. 5. These results are discussed in connection with the basic properties of the actin-myosin sliding in muscle contraction.