Fig 1.
Mean ± standard deviation of stress versus average sarcomere length of myofibrils.
Myofibrils were stretched in a low-calcium solution (passive stretch, open circles) and high-calcium solution from an initial sarcomere length of 2.4μm (active stretch, black circles) and 3.4μm (active stretch from 3.4μm/sarcomere, grey squares). Based on the cross-bridge model, forces beyond actin-myosin filament overlap (sarcomere length > 4μm; grey vertical bar) in actively stretched myofibrils are predicted to coincide with forces in passively stretched myofibrils. However, forces in actively compared to passively stretched myofibrils are three to four times higher when the stretch starts at an initial sarcomere length of 2.4μm, and around twice as high when the stretch starts at an initial sarcomere length of 3.4μm. Careful experimental and theoretical testing and analysis of all individual sarcomere lengths revealed that these results cannot be explained by the development sarcomere length non-uniformities. A detailed description of the experiments is given elsewhere [2].
Fig 2.
Force versus average sarcomere length of a myofibril first stretched in a low-calcium solution and then shortened again.
The red line corresponds to region (1) where no unfolding of IG domains took place; the green line to region (4) where no further folding-unfolding of IG domains took place.
Fig 3.
Simulation of stretching a single sarcomere beyond actin-myosin overlap.
a, Sketch of an active stretch of a sarcomere without actin-titin interaction and with actin-titin interaction. b, The sarcomere is activated at a length of 2.4μm and stretched actively until the first protein structures start to break. The solid grey circles show the force-elongation relationship based on active force production by cross-bridges and passive force production based on increased titin stiffness and increased IG domain unfolding forces associated with high calcium concentrations, the open grey squares represent the purely passive forces, while the open black squares represent force production based on increased titin stiffness and increased IG domain unfolding forces but inhibited cross-bridge interaction. Finally, the solid black circles represent an active stretch condition involving actin-titin interactions. The small insert shows normalized active force and thereby the dynamic response of cross-bridges to the induced stretch at higher time resolution.
Fig 4.
Force-elongation predictions of sarcomeres stretched beyond actin-myosin filament overlap.
When a sarcomere is activated at 3.4μm and then stretched actively beyond actin-myosin overlap, its force will exceed the purely passive forces, but will not reach the high forces of sarcomeres stretched actively from optimal length.
Fig 5.
Illustration of an active stretch of a sarcomere based on the three filament model.
a, The sarcomere is activated at a length of 2.4μm and b, stretched to 3μm. The resulting force is compared to the corresponding isometric force at 3μm where the sarcomere is activated at a length of 3μm and the elongation of titin’s free spring length is less compared to b independent of whether titin is attached to actin d or not c.
Fig 6.
Simulation of an experiment where the sarcomere is activated at a length of 2.4μm and stretched to 3μm (see also Fig. 5).
The resulting force is normalized to the corresponding isometric force at 3μm. Forces for the regular cross-bridge model (top panel) and the three filament model (second panel) are calculated for a stretching velocity of 100nm/s/Sarcomere length. The corresponding sarcomere length is shown in the third panel. In contrast to the regular cross-bridge model the three filament model predicts force enhancement in a single (half) sarcomere.
Fig 7.
Illustration of active shortening of a sarcomere based on the three filament model.
a, The sarcomere is passively stretched to a length of 2.6μm, activated and b, shortened to 2.4μm. The resulting force is compared to the isometric force at 2.4μm, where the sarcomere is activated at a length of 2.4μm and the elongation of its free spring length is higher, whether titin is attached to actin d or not c.
Fig 8.
Simulation of an experiment where the sarcomere is passively stretched to a length of 2.6μm, activated and shortened to 2.4μm (see also Fig. 7).
The resulting force is normalized to the isometric force at 2.4μm. Forces for the regular cross-bridge model (top panel) and the three filament model (second panel) are calculated for a shortening velocity of 100nm/s/Sarcomere length. The corresponding sarcomere length is shown in the third panel. In contrast to the regular cross-bridge model the three filament model predicts small but significant force depression in a single (half) sarcomere.