Figure 6.

D137L ααTm-reconstituted myofibrils show a greater active tension than control at submaximal Ca²⁺ activation. (A) Myofibrils replaced with control ααTm (upper trace) and D137L ααTm (lower trace) were activated at pCa 5.95 and then subjected to [Ca²⁺] jumps to pCa 4.50 at 15°C. At each level of Ca²⁺ activation, a release-restretch was applied to the preparation under steady-state conditions of tension generation. Bars above the traces correspond to the timing of the solution change, and numbers correspond to the pCa values of relaxing, submaximal, and maximal activating solutions. Tension calibration (vertical bar): 100 mN mm⁻²; time calibration (horizontal bar): 1 s. Control ααTm myofibril: SL 2.31 μm, resting tension 20 mN mm^–2, P_{o 5.95}/P_{o 4.50} = 0.17; D137L ααTm myofibril: SL 2.18 μm, resting tension 90 mN mm^–2, P_{o 5.95}/P_{o 4.50} = 0.36. (B) Mean values for the ratio of submaximal pCa 5.95 to maximal pCa 4.5 activated tension obtained from [Ca²⁺] jump experiments in control and D137L ααTm-replaced myofibrils. Values are given as means ± SEM; numbers in parentheses are the number of myofibrils; ^∗p < 0.02, Student’s t-test.