The force generation process in active muscle is strain-sensitive and endothermic: a temperature-perturbation study

In experiments on active muscle, we examined the tension decline and its temperature sensitivity at the onset of ramp shortening and at a range of velocities. A segment (∼1.5 mm long) of a skinned muscle fibre isolated from rabbit psoas muscle was held isometric (sarcomere length ∼2.5µm) at 8-9 °C, maximally Ca-activated and a ramp shortening applied. The tension decline with a ramp shortening showed an early decrease of slope (the P1 transition) followed by a slower decrease in slope (the P2 transition) to the steady (isotonic) force. The tension level at the initial P1 transition and the time t1 to that transition decreased as the velocity is increased; the length change L1 to this transition increased with shortening velocity to a steady value of ∼8 nm / half-sarcomere. A small rapid temperature jump (3-4 °C, <0.2 ms, T-jump) applied coincident with the onset of ramp shortening showed force enhancement by T-jump and changed the tension decline markedly. Analyses showed that the rate of T-jump induced force rise increased linearly with increase of shortening velocity. The results provide crucial evidence that the strain-sensitive crossbridge force generation, or a step closely coupled to it, is endothermic.