Mechanics and Structure of Cross-Bridges During Contractions Initiated by Photolysis of Caged Ca²⁺

Abstract

Cross-bridge structure and mechanics were studied during development of skinned frog muscle fiber contractions initiated by photolysis of DM-nitrophen (a caged Ca²⁺). Stiffness rises earlier than tension following photo-release of Ca²⁺. A similar lead of stiffness in electrically stimulated fibers and the early rise of the I₁₁/I₁₀ ratio of equatorial X-ray reflections are thought to signal attachment of cross-bridges into states with lower force than in steady-state contraction. We investigated the structure of the early attachments by electron microscopy of fibers activated by photolysis of DM-nitrophen and then ultra-rapidly frozen and freeze substituted with tannic acid and OsO₄. Sections from relaxed fibers show helical tracks of myosin heads on the thick filaments surface. Optical diffraction patterns show strong meridional intensities and layer lines up to the 6th order of 1/43 nm, indicating preservation and resolution of periodic structures smaller than 10 nm. Following photo-release of Ca²⁺, the 1/43 nm myosin layer line becomes less intense, and higher orders disappear. A ∼1/36 nm layer line appears early (12–15 ms) and becomes stronger at later times. The 1/14.3 nm meridional spot weakens initially and recovers at a later time, while it broadens laterally. The 1/43 nm meridional spot is present during contraction, but the 2nd order meridional spot (1/21.5 nm) is weak or absent. These results are consistent with time resolved X-ray diffraction data on the periodic structures within the fiber. In sections along the 1,1 plane of activated fibers, the individual cross-bridges have a wide range of shapes and angles, perpendicular to the fiber axis or pointing toward or away from the Z-line. Fibers frozen at 13 ms, 33 ms, and 220 ms after photolysis all show surprisingly similar cross-bridges. Thus, a highly variable distribution of cross-bridge shapes and angles is established early in contraction.