Table 2.
Mechanics using Mg·ATP
| Force and shortening velocity | ||
|---|---|---|
| Diameter (μm) | Force (nN μm−2) | Vo (L s−1)a |
| 78 ± 2.4 (22) | 98 ± 7.8 (22) | 0.88 ± 0.04 (19) |
| Force recovery: ramp–stretch cycle and slack tests | |||||
|---|---|---|---|---|---|
| Shortening protocol | kr1 (s−1) | krs (s−1) | krf (s−1) | A′f | krf/krs |
| Ramp–restretch | 5.9 ± 0.28 (26) | 3.6 ± 0.34 (13) | 10.6 ± 0.62 (13) | 0.54 ± 0.03 (13) | 3.1 ± 0.1 (13) |
| Prestretch slack tests | 5.0 ± 0.34 (9) | 2.3 ± 0.18 (7) | 8.2 ± 0.52 (7) | 0.66 ± 0.02 (7) | 3.6 ± 0.2 (7) |
Means ± s.e.m. Values in parentheses refer to number of fibres in mechanics experiments. The number of repeats per fibre is discussed in Methods. Temperature = 5°C.
Unloaded shortening velocity (Vo) obtained from restretch–slack tests. L s−1= lengths per second at sarcomere length (SL) = 2.4 μm. Mechanical properties are from the first Mg·ATP activation for each of n fibres. Magnitudes of exponential components were calculated by extrapolating back to the time of the restretch in the case of a ramp–restretch protocol, the step release applied after a restretch, or zero force in a slack test. Symbols: kr= rate constant of an exponential component fitted to tension recovery, where r1 = single exponential, rs = slow component of double exponential, rf = fast component, A′f= amplitude of the fast component of a double-exponential fit relative to total force recovery (amplitudes extrapolated to the beginning of recovery as discussed in Methods). The rate constants for ramp–stretch cycles are averages of SL and fibre length (PM) control protocols as the two groups of data were within 5–12% of each other as discussed in the text. All slack tests were in PM control. The rate constants for slack tests are average values for all release sizes, which did not vary significantly for prestretch slack tests as discussed in Methods.