Filament sliding and energy absorbed by the cross-bridge in active muscle subjected to cycical length changes

1. The effects of single and double cycles of stretch and release on the tension response and relative sliding movement of the actin and myosin filaments in active frog's muscle were investigated. 2. The cross-bridges linking the filaments together are able to accommodate a greater range of filament displacement before becoming detached during a second cycle stretch, providing it commences without delay following the preceding release: sarcomere 'give' then occurs for displacements of around 18 nm, as compared with 12 nm for a first cycle stretch. It is postulated that the difference arises because the myosin heads adopt different 'preferred' positions in the isometric steady-state and at the end of a previous release. 3. Muscle length-tension loops were recorded and used to measure the energy absorbed when a muscle is subjected to cycles of stretch and release. The work absorbed per unit length change increases with increasing displacement of the cross-bridges from their initial (isometric) steady-state position, up to the point at which sarcomere 'give' occurs (S2); thereafter it remains constant. 4. More work is absorbed during the first cycle of a double stretch-release combination than during the second. The greater amount absorbed during the first cycle is associated with a correspondingly greater amount of filament sliding in the period following sarcomere 'give'. Sarcomere length-tension loops were constructed and these showed that not less than 80-85% of the work done on a muscle is absorbed by the sarcomeres themselves. 5. A greater amount of work is done on stretching up to (but not beyond) S2 during second cycle stretch as compared to a first. The difference amounts about 1 mJ.m-2 per half-sarcomere. 6. The results are compatible with the mechanism for force production proposed by Huxley & Simmons (1973), in which each myosin head generates force in a number of stepping movements, from one attached state to another. It is concluded that (a) during an unloaded isotonic contraction the working 'stroke' of the head would result in a 10-13 nm relative sliding movement of the filaments, and (b) the potential energy difference separating the two 'preferred' states is 6-9.6 kT per cross-bridge, or 3-4.8 kT per S-1 sub-units, assuming that each one interacts simultaneously with the actin filament.