Table 3.
Estimates of stiffness per myosin head (S∗), force per myosin head (F∗), and y0 of individual myosin heads (y0∗) calculated from half-sarcomere stiffness (after subtraction of filament compliance) and isometric force generated by the soleus muscle fibers
Mut R719W∗ | Controls for Mut R719W∗ | Mut R723G†(H27) | Controls for Mut R723G† | |
---|---|---|---|---|
Stiffness per head in rigor (pN/nm) | 0.45 | 0.17 | 0.84 | 0.29 |
Isometric stiffness per head at 10°C (pN/nm) | 0.39 | 0.19 | 0.52 | 0.28 |
Isometric force per head at 10°C (pN) | 1.77 | 0.99 | 1.99 | 1.49 |
Isometric force per head at 20°C (pN) | 2.88 | 1.77 | 2.56 | 1.92 |
y0 per head at 10°C (nm) | 4.54 | 5.14 | 3.83 | 5.32 |
Calculations are based on a filament compliance of 0.025 nm/kPa (Fig. 5). Force and stiffness per head during isometric contraction were calculated based on 111 × 103 myosin heads/μm2 in a half-sarcomere and a fraction of strongly bound myosin heads of 0.66 (52).
For the two controls (that were measured under slightly different conditions) the values for stiffness per head in rigor (0.17 ± 0.05 pN/nm, (n = 7) and 0.29 ± 0.09 pN/nm (n = 6)) and isometric contraction (0.19 ± 0.04 pN/nm, (n = 14) and 0.28 ± 0.07 pN/nm (n = 9)) were not significantly different (all values ± SE).
Calculations based on a fraction of 54% of myosin heads with mutation R719W, 46% wild-type heads (46), and on the values for force and stiffness shown in Table 2 (see also 10). For mutation R719W, changes of cross-bridge turnover kinetics were also taken into account that contribute an increase in Fmax and in active fiber stiffness of ∼15%.