Muscle Contraction Group, Department of Physiology and Pharmacology, School of Medical Sciences, University of Bristol, Bristol BS8 1TD, UK.
J Physiol. 2010 Feb 1;588(Pt 3):479-93. doi: 10.1113/jphysiol.2009.179200. Epub 2009 Nov 30.
We examined the tension responses to ramp shortening and rapid temperature jump (<0.2 ms, 3-4 degrees C T-jump) in maximally Ca(2+)-activated rabbit psoas muscle fibres at 8-9 degrees C (the fibre length (L(0)) was approximately 1.5 mm and sarcomere length 2.5 microm). 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 V(max) (V range approximately 0.01 to approximately 1.5 L(0) s(1)). In the isometric state, a T-jump induced a biphasic tension rise consisting of a fast (approximately 50 s(1), phase 2b) and a slow (approximately 10 s(1), phase 3) component, but if treated as monophasic the rate was approximately 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 V(max) it was approximately 200 s(1), approximately 10x faster than in the isometric state. Relative to the tension reached after the T-jump, the amplitude increased with shortening velocity, and near V(max) it was 4x 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 L(0) 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.
我们研究了在 8-9°C（纤维长度（L(0)）约为 1.5 毫米，肌节长度 2.5 微米）下，最大 Ca(2+)-激活的兔腰大肌纤维对斜坡缩短和快速温度跃变（<0.2ms，3-4°C T-跃变）的张力反应。目的是研究肌肉中横桥力产生的应变敏感性。在接近 V(max)的广泛速度范围内（V 范围约为 0.01 至约 1.5 L(0)s(1)），在稳定缩短过程中检查 T-跃变引起的张力上升。在等长状态下，T-跃变引起双相张力上升，包括快速（约 50s(1)，相位 2b）和缓慢（约 10s(1)，相位 3）成分，但如果视为单相，则速率约为 20s(1)。在稳定缩短期间，T-跃变引起的张力上升是单相的；张力上升速率与缩短速度呈线性关系，在接近 V(max)时，速率约为 200s(1)，比等长状态快约 10 倍。与 T-跃变后的张力相比，振幅随缩短速度增加而增加，在接近 V(max)时，比等长状态大 4 倍。因此，在稳定缩短期间，如在稳定状态实验中发现的那样，肌肉力的温度敏感性随着速度的增加而显著增加。斜坡缩短期间张力下降的速率也随着速度的增加而显著增加。T-跃变张力上升的绝对幅度在较低速度（<0.5L(0)s(1)）下大于等长状态，但在较高速度下下降到低于等长状态。T-跃变张力上升的绝对幅度的这种两相速度依赖性表明，至少有两个过程相互作用，随着缩短速度的增加，这两个过程对张力输出有相反的影响，可能是增强了横桥力的产生和通过负应变更快（stroke 后）的横桥分离。总的来说，我们的结果表明，T-跃变力产生是应变敏感的，当暴露于负应变时会变得快得多。因此，肌肉中的横桥力产生步骤既对温度敏感（吸热）又对应变敏感。
