CROSS-BRIDGE DETACHMENT AND ATTACHMENT FOLLOWING A STEP STRETCH IMPOSED ON ACTIVE SINGLE FROG-MUSCLE FIBERS

1. The time course of cross-bridge detachment-attachment following a s tep stretch was determined in single frog muscle fibres (at 4 degrees C and 2 . 1 mu m sarcomere length) by imposing, under sarcomere length control by a striation follower, test step releases of various amplit udes (2-13 nm per half-sarcomere) at successive times (4-55 ms) after a conditioning stretch of similar to 4 nm per half-sarcomere. 2. The c omparison with the control tension transients, elicited by releases no t preceded by the conditioning stretch, shows that, early after the co nditioning stretch, the quick tension recovery following small release s is depressed and the quick tension recovery following large releases is potentiated. Both effects are expected as a consequence of the str ain produced in the cross-bridges by the conditioning stretch. 3. Thes e effects disappear and the tension transient is reprimed, indicating substitution of freshly attached cross-bridges for strained cross-brid ges, with a time constant of similar to 10 ms. 4. A novel multiple-exp onential equation, based on the hypothesis of complete substitution of freshly attached cross-bridges for the cross-bridges that underwent t he stretch, has been used to fit the whole tension transient following step stretches of different sizes (2-6 nm per half-sarcomere), For a stretch of 4nm, the time constant of the exponential process responsib le for cross-bridge detachment (tau(d), 9 . 1. ms) almost coincides wi th the time constant of repriming as measured by the double-step exper iments. The time constant of the exponential process representing the cumulative effects of attachment and force generation (tau(3)) is 13 . 6 ms. 5. For stretches of different sizes the amount of quick tension recovery attributable to the reversal of the working stroke elicited by the stretches is estimated by subtracting, from the original tensio n transient, the contribution to tension recovery due to detachment-at tachment of cross-bridges as estimated by the multiple-exponential ana lysis. Following this calculation, the structural change in the myosin heads responsible for the reversal of the working stroke can be 2 nm at maximum, suggesting that the elastic component in the crossbridges is at least twice as rigid as previously thought.