FIGURE 6.

Effect of slowed cross-bridge detachment on the rate of Ca²⁺ dissociation from rabbit myofibrils. Panel A represents the biphasic kinetic trace for TnC_IANBD^T53C myofibrils in Buffer A + 200 μm Ca²⁺ + 2 mm ADP when mixed with an equal amount of Buffer A + 10 mm EGTA and 2 mm ATP at 15 °C (Myofibrils + ADP versus EGTA + ATP trace, biphasic). The Ca²⁺-free myofibrils + ADP versus ATP trace from Fig. 3, panel B (6.9 ± 0.6/s), was shown for a graphic comparison. Panel B shows the effect that ADP and cross-linking had on the rate of Ca²⁺ dissociation at 15 °C. The Ca²⁺-free myofibrils + ADP versus ATP trace (panel A) was subtracted from the biphasic myofibrils + ADP versus EGTA + ATP trace (panel A) to recover the Ca²⁺ dissociation signal (subtraction, ∼ 25/s). TnC_IANBD^T53C myofibrils in Buffer A + 200 μm Ca²⁺ + 2 mm ADP were rapidly mixed with an equal volume of the Buffer A + 10 mm EGTA at 15 °C (Myofibril + ADP versus EGTA trace, 24 ± 1/s). Cross-linked TnC_IANBD^T53C myofibrils in Buffer A + 200 μm Ca²⁺ were rapidly mixed with an equal volume of the Buffer A + 10 mm EGTA + 2 mm ATP (Cross-linked Myofibrils versus EGTA + ATP trace, 22 ± 1/s).