Constrained neighboring-sarcomere phase topology shapes mean HSO amplitude in living cardiomyocytes

Abstract

How neighboring sarcomeres redistribute timing while a cardiomyocyte continues to beat, and whether that coordination during warming-induced hyperthermal sarcomeric oscillations (HSOs) is random or structured, remain unresolved. We reanalyzed sarcomere-length recordings from five consecutive sarcomeres in each of seven living neonatal rat cardiomyocytes and represented each valid time point by the four neighboring-pair phase relations that define a 16-state local phase network. During HSOs, the fraction of time with trackable local phase relations increased from 0.298 before warming to 0.956 (paired Wilcoxon P = 0.0156), enabling direct analysis of local reconfiguration. Successive local states were almost always connected by Hamming-1 edges, meaning that only one neighboring-pair relation changed at a time (34/35, 97.1%, before warming; 216/230, 93.9%, during HSOs). HSOs also increased occupancy of anti-phase-rich states with three or more anti-phase neighboring pairs (0.254 to 0.509, P = 0.0156). These results indicate that HSOs do not reflect unstructured local disorder but a constrained neighboring-sarcomere phase topology. As a complementary cycle-level analysis within the same HSO window, we then asked how the observed fast amplitude of the valid-sarcomere mean trace relates to local amplitude and synchrony measured from the same valid sarcomeres. For each cycle, Yvalid, the peak-to-peak HSO amplitude of the valid-sarcomere mean trace, was closely approximated by the product of mean local HSO amplitude (A) and weighted synchrony (Rw; pooled r = 0.992, normalized mean squared error = 0.015, {beta}1 = 0.948, {beta}0 {approx} 0). A simple state-derived synchrony factor computed from the local phase patterns showed a modest positive association with Rw (cell-adjusted {beta} = 0.197, P = 0.0165), providing a bridge between the binary local-state description and the continuous synchrony summary. In blocked cross-validation, the A x Rw summary was markedly more parsimonious than an additive current-state alternative (pooled normalized mean squared error 0.0138 vs 0.1006), whereas simple history terms changed error only marginally. Thus, the main result is a constrained local phase topology during HSOs, and A x Rw serves as a descriptive cycle-level summary of the mean fast signal in the same observed segment, with local amplitude and synchrony as its two components.