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Neighboring-Sarcomere Coordination Is Updated
The same local switch can be accompanied by short- or broad-reach length redistribution
The same local switch can be accompanied by short- or broad-reach length redistribution
The heartbeat appears highly regular, yet it has remained unclear how several sarcomeres connected in series coordinate their motion inside a living cardiomyocyte. This study reanalyzed high-speed recordings obtained during hyperthermal sarcomeric oscillations (HSOs), a rapid oscillatory state induced by locally warming living neonatal rat cardiomyocytes to approximately 41 °C. Five consecutive sarcomeres were tracked individually in each cell at 500 frames per second.
The five-sarcomere segment contains four neighboring-pair relations. Of 230 phase-state transitions detected during HSOs, 216 (93.9%) were one-link switches, meaning that only one neighboring relation changed while the other three were preserved. Thus, local sarcomere coordination was updated mainly one link at a time rather than through simultaneous reorganization of the entire segment.
The distance between relative shortening and relative lengthening was quantified as compensation reach, S. Across 248 events for which S could be calculated, even the same IAAI-to-IAII switch could be accompanied by short-reach redistribution (S=1.29) or redistribution spanning a wider part of the observed five-sarcomere chain (S=2.88). Compensation reach was also broader when more neighboring pairs were in phase before the switch.
These findings identify an intermediate, multi-sarcomere coordination process between stochastic actomyosin activity and robust whole-cell contraction. The conclusions are restricted to the observed five-sarcomere window.
The upper half shows two events with the same IAAI-to-IAII one-link switch across five consecutive sarcomeres. The red boxes mark the single neighboring relation that changes. In the left example, relative shortening (blue) and relative lengthening (orange) remain close together, giving S=1.29. In the right example, they are separated across a wider part of the observed five-sarcomere chain, giving S=2.88. The lower-left panel shows the distribution of compensation reach, and the lower-right panel shows that more pre-event in-phase links are associated with broader reach.
Article information & citation
Seine A. Shintani. Structured neighboring-sarcomere switching is coupled to variable-reach internal length redistribution during hyperthermal sarcomeric oscillations in living cardiomyocytes. Biophysics and Physicobiology 23: e230024 (2026).
Advance Publication: 15 July 2026
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