Force enhancement after stretch of isolated myofibrils is increased by sarcomere length non-uniformities

Figure 1. (a): An image of a single myofibril suspended between two glass needles and visualized at high magnification. (b) Plot profile matching the greyscale pattern of the myofibril in (a). The dark A-bands of the sarcomeres produce peaks that are used in the calculation of sarcomere lengths. (c) A myofibril with an average sarcomere length of 2.73 μm at rest. (d) The tip of the pressure-controlled micro-perfusion system filled with high ionic strength solution is visible above the myofibril. The tip is positioned such that a single sarcomere will be affected by the solution. (e) Contractile proteins in a single sarcomere were inactivated by applying high ionic strength solution. Note the reduced appearance of the A-band and more visible Z-lines. (f) Myofibril with one inactive sarcomere in a contracted state.

Figure 2. Contractions of a control myofibril (a) and a myofibril treated with high ionic strength solution (b). The red traces show an isometric contraction, while the black traces show contractions with an active stretch. Residual force enhancement was calculated as the difference between the force following the stretch and the isometric force. In this myofibril, the residual force enhancement was 6.99% and 22.76% for control and treated myofibril, respectively. Forces were measured at the same sarcomere length, which in this case, was 2.73 μm.

Figure 3. Force values of control (a) and treated (b) myofibrils during isometric contractions (green) and after active stretching (yellow). (c) Isometric forces were significantly lower in myofibrils with an inactive sarcomere compared to control myofibrils (P < 0.01). Residual force enhancement was observed in both control myofibrils (d) and myofibrils with one inactive sarcomere (e). (f) The percent residual force enhancement at a comparable sarcomere length was significantly larger in myofibrils with an inactive sarcomere compared to control myofibrils (P < 0.01).

Figure 4. Sarcomere length (SL) dispersion of the myofibrils. The inactivation of a single sarcomere within the myofibril caused a significant increase in sarcomere length dispersion both before (a) and during (b) contraction (P < 0.01).