Figure 2.

Ca²⁺ sensitivity of force in cardiac myofibrils and its modulation via TnI phosphorylation. (A) The graph shows the concentrations of Ca²⁺ required for half-maximal force for donor and DCM patient hearts with truncating mutations in the TTN gene. Cardiac myofibrils were treated with PKA and λ phosphatase to fully phosphorylate and dephosphorylate TnI, respectively. Statistical analysis was performed using one-way ANOVA with Fisher’s least significant difference test. ^#P < 0.05, ^##P < 0.01, and ^###P < 0.001 vs. donor NM. *P < 0.05, **P < 0.01, and ***P < 0.001 vs. no treatment or other treatment. Numbers on bars indicate number of myofibril samples. (B) Correlation between myofilament Ca²⁺ sensitivity and TnI phosphorylation. The EC₅₀ for Ca²⁺ required for half-maximal force responses in untreated, PKA-, and λ phosphatase-treated cardiac donor myofibrils (closed dots) are plotted against TnI phosphorylation level. The solid line is a least-squares linear regression line with 95% confidence interval (dotted line). Pearson correlation coefficient r = 0.91, n = 8 donor hearts (four untreated, two PKA-treated and two λ phosphatase-treated). The open circles are for DCM myofibrils (untreated, PKA-, and λ phosphatase-treated). (C) The EC₅₀ values for the combined group of DCMs with TTNtv vs. healthy donors. Sarcomere length was 2.2 µm. Statistical analysis was performed using linear mixed model. Bars show estimated marginal means ± SE. Grey circles represent mean values of individual heart samples. *P < 0.05 and ***P < 0.001. Measurements were performed at 17°C.