2+
concentrations tropomyosin is located at the edge of the thin filament, thereby interfering with the formation of strong actin-myosin linkages (blocked state). An increase in Ca2+ activity causes an azimuthal shift of tropomyosin around the filament (by about 30°), thereby increasing the probability of low-force crossbridge interaction, a process which by cooperative effects induces further tropomyosin movement (by an additional 10°) which results in the open state of the filament characterized by forceful crossbridge interaction. (This mechanism may be analogous to that in ligand-gated ion channels, where ligand binding increases the open probability of the pore.) The extent of activation then depends on the free Ca2+ concentration and on the calcium sensitivity of the thin filament that may be affected by protein phosphorylation, crossbridge attachment, the troponin isoform composition of the filament, and novel calcium-sensitizing drugs that act on the contractile or regulatory proteins and thus increase the force of the heart.
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Rüegg, J. Cardiac Contractility: How Calcium Activates the Myofilaments. Naturwissenschaften 85, 575–582 (1998). https://doi.org/10.1007/s001140050554
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DOI: https://doi.org/10.1007/s001140050554