| The muscle mechanics work described herein is broken into two main sections, which deal with force enhancement with stretch, and elasticity of vertebrate thick filaments.; Experiments examining the relation between active tension and filament overlap have consistently yielded intriguing results: when activated specimens are ramp-stretched to less filament overlap, tension increases transiently and decays to a steady value larger than the prestretch value. This unexpected result—higher tension with less overlap—conflicts with the accepted length-tension relation. To follow up, we carried out similar experiments on single myofibrils. Any ambiguity that might arise from hidden or undetected sarcomeres is averted in the single myofibril because all sarcomeres can be clearly seen and because they are in series and bear the same tension. Thus, the length and tension of each sarcomere can be measured. Repeatability of activation was a problem in myofibril experiments. While it is possible to activate repeatably on the plateau of the length-tension curve, myofibrils do not activate repeatably on the descending limb. After any given activation, repeated stretch release cycles yielded similar force and sarcomere length responses suggesting that response to stretch was repeatable. Data show that a few sarcomeres elongate to longer lengths, which are supported by passive tension. However, all sarcomeres that are on the descending limb are stretched to less overlap yet generate more tension. These data suggests that more tension with stretch to less overlap is a basic property inherent to the sarcomere.; Thick filaments are generally thought to be effectively inextensible. Previous X-ray diffraction results showed a 1.0–1.6% increase in myosin head spacing with activation (Haselgrove, 1975; Bordas et al., 1993; H. E. Huxley et al., 1994; Yagi et al., 1995); however, this increase in spacing has been interpreted as change in the state of the crossbridges, not as elasticity in the thick filament backbone. Here we use novel nanofabricated cantilevers to carry out the first force-elongation measurements on single vertebrate thick filaments. Synthetic thick filaments from rabbit myosin were suspended between two cantilevers and stretched. With stretch, stiffness increased gradually and then became nearly constant after about 100 piconewtons. Under physiological loads (about 440 pN with full activation) filaments elongated by 1.5 ± 0.5%. Comparison with published X-ray diffraction data suggests that changes in the myosin X-ray reflections seen during activation may be due to elongation of the thick filament backbone. |
