Figure 8.

(A) k_tr measurements in 6-day-old Neb^ΔExon55 and wild-type (wt) fibres. Left: Example of a k_tr protocol in a Neb^ΔExon55 fibre preparation; right: k_tr is significantly lower in Neb^ΔExon55 compared with wild-type fibres. (B) Tension cost measurements in Neb^ΔExon55 and wild-type fibres. Left: Example of maximally activated Neb^ΔExon55 fibres with developed force at the bottom and [ATP] at the top. The slope of the [ATP] versus time trace was divided by fibre volume (in mm³) to determine ATP consumption rate. ATP consumption rate was normalized to tension to determine the tension cost. Right: Tension cost is significantly higher in Neb^ΔExon55 compared to wild-type fibres. (C) An example of a protocol to determine crossbridge cycling kinetics in a myofibril. Myofibrils are rapidly activated (pCa 4.5), followed by rapid relaxation (pCa 8.0) after steady state tension is reached. From the force tracings, the rate constants of activation (k_act) and relaxation are determined. Relaxation consists of an initial slow phase (slow k_rel) followed by a fast phase (fast k_rel). k_tr is determined during a separate protocol that is similar to that applied on fibres. (D) Both k_act and k_tr are significantly lower in Neb^ΔExon55 compared with wild-type myofibrils, whereas slow and fast k_rel are significantly higher in Neb^ΔExon55 compared with wild-type myofibrils.