Figure Legends
Figure 1: Schematic introducing dynamic filament coupling between thick- and thin-filaments. Thin-filament regulation involves Ca2+ binding to troponin and subsequent movement of tropomyosin to expose actin sites along the thin filament, to which myosin can bind and form force-generating cross-bridges. Thick-filament regulation involves myosin OFF-ON transition kinetics, which is a mechanosensitive equilibrium that shifts myosin heads from OFF to ON as muscle force increases. Myosin heads in the OFF state cannot bind actin, while those in the ON state can bind actin to form force-generating cross-bridges. This dynamic regulatory coupling implies that any modification to thin-filament function will in turn change the status of thick-filament regulation, and vice versa (figure adapted from (Campbell et al., 2018) ).
Figure 2: Effects of mavacamten on the isometric force-pCa relationship at 1.9 and 2.3 µm sarcomere length. (A-B) Steady-state force values (normalized to cross-sectional area of each myocardial strip) are plotted against pCa (pCa = -log10[Ca2+]). Lines represent 4-parameter Hill fits to Eq. 1. Dashed lines show fits at 1.9 µm sarcomere length, replotted in panel B. Data were gathered from 6 hearts, with a total of 17 control strips and 18 mavacamten strips at 1.9 μm sarcomere length, and 18 control strips and 17 mavacamten strips at 2.3 μm sarcomere length. Data shown as mean±SEM, error bars within symbol if not visible.
Figure 3: Effects of mavacamten on maximal Ca2+-activated force and passive force at 1.9 and 2.3 µm sarcomere length. (A) Maximal and (B) passive force values from fits to Eq. 1 are shown for each myocardial strip from each experimental group. Significant main effects and the associated interaction from linear mixed models analysis are listed above each panel for respective data therein. Jitter plots (colored symbols) show measurements for each myocardial strip, with n listed in the legend of Fig. 2. Black symbols show mean±SEM for each group plotted to the left of individual measurements.
Figure 4: Effects of mavacamten on calcium activation of contraction at 1.9 and 2.3 µm sarcomere length. (A) pCa50 values and (B) nH values from fits to Eq. 1 are shown for each myocardial strip from each experimental group. Significant main effects and the associated interaction from linear mixed models analysis are listed above each panel for respective data therein. Jitter plots (colored symbols) show measurements for each myocardial strip, with n listed in the legend of Fig. 2. Black symbols show mean±SEM for each group plotted to the left of individual measurements.
Figure 5: Effects of mavacamten on viscoelastic myocardial stiffness at pCa 4.5 for at 1.9 and 2.3 µm sarcomere length. Elastic (A-B) and viscous (C-D) moduli are plotted against frequency for maximal Ca2+-activated conditions. Data shown as mean±SEM, with n listed in the legend of Fig. 2.
Figure 6: Effects of mavacamten on frequency-dependent shifts in the minimum and maximum viscous modulus at 1.9 and 2.3 µm sarcomere length. The (A) frequency producing the minimum viscous modulus and (B) frequency producing the minimum viscous modulus from polynomial fits to these associated regions of interest. Frequency shifts in the minimum and maximum viscous modulus describe relative changes in cross-bridge recruitment and detachment rates, respectively. Significant main effects and the associated interaction from linear mixed models analysis are listed above each panel for respective data therein. Jitter plots (colored symbols) show measurements for each myocardial strip, with n listed in the legend of Fig. 2. Black symbols show mean±SEM for each group plotted to the left of individual measurements.