Temperature jump induced force generation in rabbit muscle fibres gets faster with shortening and shows a biphasic dependence on velocity

Abstract

We examined the tension responses to ramp shortening and rapid temperature jump (<0.2 ms, 3-4°C T-jump) in maximally Ca2+-activated rabbit psoas muscle fibres at 8-9°C (the fibre length (L0) was ∼1.5 mm and sarcomere length 2.5 μm). The aim was to investigate the strain sensitivity of crossbridge force generation in muscle. The T-jump induced tension rise was examined during steady shortening over a wide range of velocities (V) approaching the Vmax (V range ∼0.01 to ∼1.5 L0 s-1). In the isometric state, a T-jump induced a biphasic tension rise consisting of a fast (∼50 s-1, phase 2b) and a slow (∼10 s-1, phase 3) component, but if treated as monophasic the rate was ∼20 s-1. During steady shortening the T-jump tension rise was monophasic; the rate of tension rise increased linearly with shortening velocity, and near Vmax it was ∼200 s-1, ∼10× faster than in the isometric state. Relative to the tension reached after the T-jump, the amplitude increased with shortening velocity, and near Vmax it was ∼4× larger than in the isometric state. Thus, the temperature sensitivity of muscle force is markedly increased with velocity during steady shortening, as found in steady state experiments. The rate of tension decline during ramp shortening also increased markedly with increase of velocity. The absolute amplitude of T-jump tension rise was larger than that in the isometric state at the low velocities (<0.5 L0 s-1) but decreased to below that of the isometric state at the higher velocities. Such a biphasic velocity dependence of the absolute amplitude of T-jump tension rise implies interplay between, at least, two processes that have opposing effects on the tension output as the shortening velocity is increased, probably enhancement of crossbridge force generation and faster (post-stroke) crossbridge detachment by negative strain. Overall, our results show that T-jump force generation is strain sensitive and becomes considerably faster when exposed to negative strain. Thus the crossbridge force generation step in muscle is both temperature sensitive (endothermic) and strain sensitive. © 2010 The Authors. Journal compilation © 2010 The Physiological Society.