Figure 5.

Schematic drawing of the proposed mechanism of LDA, showing tropomyosin (gray lines), actin (ovals), and cardiac troponin units (tricolored rectangles). In our version of the three-state model, the impact of calcium on the activation of the neighboring units (dashed black line) at short SL works to activate the neighboring cTn unit, and thus, calcium needs to bind to more Tn units (arrows), whereas at longer SL, the activation of one cTn unit, due to the proposed stiffening of the tropomyosin, is able to activate more cTn units, thus requiring less calcium to activate the same number of cTn units (arrows). In other words, stretching the SL theoretically shifts the requirement for calcium binding from every 1–2 troponins to approximately every 3–4 troponins, allowing for more binding sites to be activated with the same levels of calcium activation.