title: Modeling Ca2+-Bound Troponin in Excitation Contraction Coupling.
abstract: 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-1reflection 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-1reflection 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.
authors.affiliation: Department of Biology, University of West Georgia Carrollton, GA, USA.
authors.forename: Henry G
authors.initials: HG
authors.lastname: Zot
authors.affiliation: Department of Physics, University of West Georgia Carrollton, GA, USA.
authors.forename: Javier E
authors.initials: JE
authors.lastname: Hasbun
keywords: EC-coupling; calcium; contraction; excitation; kinetics; model; muscle; troponin