Developmental increase in β-MHC enhances sarcomere length–dependent activation in the myocardium

Figures

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  Figure 1.. SL dependency of maximal tension and ED in young and adult guinea pig cardiac muscle preparations. Maximal tension was measured by exposing muscle preparations to saturating Ca2+ concentrations (pCa 4.3) in a constantly stirred chamber. ED was estimated as the slope of the linear relationship between F1–Fss and the corresponding ML changes (see Fig. S1) and thus provides an approximate measure of the number of strongly bound XBs. (A and B) Bar graphs showing the SL-dependent effect on (A) maximal tension and (B) ED in young and adult muscle preparations. Two-way ANOVA revealed that the main effect of SL was significant for both maximal tension (P < 0.001) and ED (P = 0.002). The main effect of MHC was not significant for both maximal tension (P = 0.91) and ED (P = 0.24). Post hoc multiple comparisons (Holm–Sidak method) were used to determine significant differences between groups. Asterisks and hash marks indicate a significant difference when compared with data within each group (*, P < 0.05 and ***, P < 0.001 for 1.9 vs. 2.3 µm). The number of preparations measured for all groups was 10. Data are expressed as means ± SEM.

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  Figure 2.. SL dependency of the pCa–tension relation in young and adult guinea pig cardiac muscle preparations. Normalized steady state tensions at various pCa were plotted against pCa to construct the pCa–tension relationships. The Hill model was fitted to pCa–tension relationships to derive myofilament Ca2+ sensitivity (pCa50) and myofilament cooperativity (nH). (A and B) Comparisons of pCa–tension relationships at short and long SL in young (A) and adult (B) muscle preparations. The traces connecting the experimental data points are the Hill model fits. Error bars are obscured by the symbols in some cases. (C and D) Bar graphs showing the SL-dependent effect on pCa50 (C) and nH (D) in young and adult muscle preparations. Two-way ANOVA revealed a significant SL–MHC interaction effect (P = 0.02) on pCa50. For nH, the main effects of both SL (P < 0.001) and MHC (P = 0.0013) were significant. Post hoc multiple comparisons (Holm–Sidak method) were used to determine significant differences between groups. Asterisks and hash marks indicate significant difference when compared with data within each group (*, P < 0.05; **, P < 0.01; and ***, P < 0.001 for 1.9 vs. 2.3 µm; #, P < 0.05 for young vs.…

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  Figure 3.. SL dependency of tension cost and c in young and adult guinea pig cardiac muscle preparations. Tension cost was derived as the slope of the linear relationship between steady state tension and ATPase measurements at various pCa (de Tombe and Stienen, 1995; Stienen et al., 1995). The value of c was estimated by fitting the NLRD model to the family of muscle-elicited force responses to various amplitude step-like ML perturbations at pCa 4.3 (Ford et al., 2010). (A and B) Bar graphs showing the SL-dependent effect on tension cost (A) and c (B) in young and adult muscle preparations. Two-way ANOVA revealed that the main effect of SL was significant for both tension cost (P < 0.001) and c (P < 0.001). The main effect of MHC was significant for both tension cost (P < 0.001) and c (P < 0.001). Post hoc multiple comparisons (Holm–Sidak method) were used to determine significant differences between groups. Asterisks and hash marks indicate a significant difference when compared with data within each group (***, P < 0.001 for 1.9 vs. 2.3 µm; ###, P < 0.001 for young vs. adult). The number of preparations measured for all groups was 10. Data are expressed as means ± SEM.

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  Figure 4.. SL dependency of γ and ER in young and adult guinea pig cardiac preparations. The values of γ and ER were estimated by fitting the NLRD model to the family of muscle-elicited force responses to various amplitude step-like ML perturbations (Ford et al., 2010). γ represents the negative impact of strained XBs on the recruitment of other force-bearing XBs. ER is estimated from the slope of the linear relationship between Fnss–Fss and ML perturbation (see Fig. S1) and thus represents the magnitude of force rise in response to an increase in ML. (A and B) Bar graphs showing the SL-dependent effect on γ (A) and ER (B) in young and adult muscle preparations. Two-way ANOVA revealed that the main effects of SL (P < 0.001) and MHC (P < 0.001) on γ were significant. For ER, two-way ANOVA revealed a significant SL–MHC interaction effect (P = 0.003). Post hoc multiple comparisons (Holm–Sidak method) were used to determine significant differences between groups. Asterisks and hash marks indicate significant difference when compared with data within each group (***, P < 0.001 for 1.9 vs. 2.3 µm; ###, P < 0.001 for young vs. adult). The number of preparations measured for all groups was 10. Data…

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  Figure 5.. SL dependency of ktr in young and adult guinea pig cardiac muscle preparations. ktr is estimated by fitting a monoexponential function to the rising phase of the force response to a large release/restretch perturbation. (A and B) Comparisons of force responses at short and long SL from young (A) and adult (B) guinea pig muscle preparations in response to a large release/restretch perturbation. Forces were normalized by the isometric steady state before ML perturbation. (C) Bar graph showing the SL-dependent effect on ktr in young and adult muscle pig preparations. Two-way ANOVA revealed that the main effect of SL (P = 0.033) and MHC (P < 0.001) on ktr were significant. Post hoc multiple comparisons (Holm–Sidak method) were used to determine significant differences between groups. Asterisks and hash marks indicate significant difference when compared with data within each group (###, P < 0.001 for young vs. adult). The number of preparations measured for all groups was 10. Data are expressed as means ± SEM.