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“Sarcomere Chaos” Driven by Calcium Fluctuations and
Unexpected chaos in the heart’s smallest unit — a new mechanism of rhythm control unveiled
Unexpected chaos in the heart’s smallest unit — a new mechanism of rhythm control unveiled
Although the heartbeat appears highly regular, the “sarcomere” — the microscopic contractile unit inside each cardiomyocyte — shows surprising fluctuations. This study reveals for the first time that those chaotic fluctuations arise from changes in intracellular calcium concentration.
Seine A. Shintani’s group at Chubu University subjected cardiomyocytes to mild heating to induce heat‑induced sarcomere self‑oscillations (HSOs) and compared sarcomere behavior under two conditions: (i) constant calcium and (ii) calcium that rises and falls with each beat. When calcium fluctuated, sarcomere oscillation amplitude and timing became highly irregular, forming a chaotic state the authors name S4C (Sarcomere Chaos Caused by Calcium Changes).
Remarkably, even under chaotic fluctuation, the overall oscillation period of the sarcomere ensemble remained nearly constant. Thus cardiomyocytes maintain a well‑balanced dynamic homeostasis, flexibly responding to calcium‑based rhythm signals while preserving the order of periodicity.
Using high‑speed imaging, the team measured nanometer‑scale changes in sarcomere length (SL) and computed variability in amplitude and timing, as well as Lyapunov exponents (a chaos metric). With constant calcium, neighboring sarcomeres traced smooth ellipses in Lissajous plots, indicating tight coupling; when calcium varied, the coupling broke down and trajectories became complex and disordered.
The S4C phenomenon may help the heart adapt quickly to abrupt changes or stress while safeguarding its rhythm, offering insights that could support early detection of arrhythmias and inspire novel therapies.
Properties of Sarcomere Oscillation Dynamics in Cardiomyocytes
(A) Time series of adjacent sarcomere oscillations with constant calcium.
(B) Corresponding Lissajous figure (well‑ordered elliptical trajectory).
(C) Phase relationship between those sarcomeres.
(D) Sarcomere oscillations during HSOs with fluctuating calcium.
(E) Lissajous figure for (D) (complex chaotic trajectory).
(F) Phase relationship for (D) (frequent phase switching).
(G) Time course of amplitude variability before/after HSO induction; red arrow indicates heating.
(H) Difference in similar trajectories during relaxation (linear growth on log scale = chaos).
(I) Phase relationship before HSOs, during normal excitation–contraction coupling.
Article information & citation
Seine A. Shintani. Observation of sarcomere chaos induced by changes in calcium concentration in cardiomyocytes. Biophysics and Physicobiology, 21(1), e210006, 2024.
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