Structured neighboring-sarcomere switching is coupled to variable-reach internal length redistribution during hyperthermal sarcomeric oscillations in living cardiomyocytes.

Hyperthermal sarcomeric oscillations (HSOs) expose rapid sarcomere-level motion in living cardiomyocytes and provide a mesoscopic window between actomyosin activity and robust cellular contraction. I reanalyzed high-speed sarcomere-length recordings from five consecutive sarcomeres in each of seven neonatal rat cardiomyocytes. During HSOs, local phase relations became trackable through most of the oscillatory segment (valid fraction, 0.298 before warming and 0.956 during HSOs; paired Wilcoxon P=0.0156). Neighboring-sarcomere reconfiguration was dominated by one-link switches, in which one adjacent-pair relation changed while the other three were maintained (216/230 HSO phase transitions), and anti-phase-rich occupancy increased from 0.254 to 0.509 (P=0.0156). I then measured event-local relative internal length redistribution. For each reach-qualified one-link event, compensation reach, S, was defined as the expected sarcomere-index distance between relative shortening and relative lengthening. The same directed IAAI-to-IAII switch was accompanied by short-reach redistribution in one event (S=1.29) and cross-chain redistribution in another (S=2.88). Across 248 reach-qualified events, S increased with the pre-event number of I-type links, with a cell-fixed slope of 0.148 span units per added I-link supported by cell-blocked permutation and cell-cluster bootstrap analyses. Thus, HSO reveals a mesoscale organizing process in which a local switch in neighboring-sarcomere synchrony is linked to spatially distributed relative internal length redistribution whose reach is shaped by the pre-event phase context.