Modeling Ca²⁺-Bound Troponin in Excitation Contraction Coupling

To explain disparate decay rates of cytosolic Ca²⁺ and structural changes in the thin filaments during a twitch, we model the time course of Ca²⁺-bound troponin (Tn) resulting from the free Ca²⁺ transient of fast skeletal muscle. In fibers stretched beyond overlap, the decay of Ca²⁺ as measured by a change in fluo-3 fluorescence is significantly slower than the intensity decay of the meridional 1/38.5 nm^-1 reflection of Tn; this is not simply explained by considering only the Ca²⁺ binding properties of Tn alone (Matsuo et al., 2010). We apply a comprehensive model that includes the known Ca²⁺ binding properties of Tn in the context of the thin filament with and without cycling crossbridges. Calculations based on the model predict that the transient of Ca²⁺-bound Tn correlates with either the fluo-3 time course in muscle with overlapping thin and thick filaments or the intensity of the meridional 1/38.5 nm^-1 reflection in overstretched muscle. Hence, cycling crossbridges delay the dissociation of Ca²⁺ from Tn. Correlation with the fluo-3 fluorescence change is not causal given that the transient of Ca²⁺-bound Tn depends on sarcomere length, whereas the fluo-3 fluorescence change does not. Transient positions of tropomyosin calculated from the time course of Ca²⁺-bound Tn are in reasonable agreement with the transient of measured perturbations of the Tn repeat in overlap and non-overlap muscle preparations.