Changing face of contractile activation in striated muscle at physiological temperature

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  Figure 1.. Schematic illustration of three state model (McKillop and Geeves, 1993) of thin filament activation in relation to Ishii et al. (2023). The equilibrium distribution between the three thin filament states (B, C, and O) is modified by binding of Ca2+ to troponin (troponin C) and strong-binding myosin crossbridges (rigor-like) to actin. Numbers in parentheses give proportional occupation of different states based on results from skeletal muscle (Geeves et al., 2019). According to most models of muscle contraction, both high-tension development and high actin–myosin sliding velocity, require the capability of crossbridges to enter strong-binding states. Therefore, the results in Ishii et al. (2023), together with data from some of the papers they cite suggest that increasing temperature in the physiological range shifts the equilibrium from the B and C states to the O state. Additionally, it seems likely that the intervention also shifts the equilibrium of crossbridge binding to the strongly bound state.