Department of Public Health, Aarhus University, Aarhus C, Denmark.
Department of Biomedicine, Aarhus University, Aarhus C, Denmark.
Acta Physiol (Oxf). 2018 Mar;222(3). doi: 10.1111/apha.12977. Epub 2017 Oct 19.
In dynamically contracting muscles, increased curvature of the force-velocity relationship contributes to the loss of power during fatigue. It has been proposed that fatigue-induced reduction in [Ca ] causes this increased curvature. However, earlier studies on single fibres have been conducted at low temperatures. Here, we investigated the hypothesis that curvature is increased by reductions in tetanic [Ca ] in isolated skeletal muscle at near-physiological temperatures.
Rat soleus muscles were stimulated at 60 Hz in standard Krebs-Ringer buffer, and contraction force and velocity were measured. Tetanic [Ca ] was in some experiments either lowered by addition of 10 μmol/L dantrolene or use of submaximal stimulation (30 Hz) or increased by addition of 2 mmol/L caffeine. Force-velocity curves were constructed by fitting shortening velocity at different loading forces to the Hill equation. Curvature was determined as the ratio a/F with increased curvature reflecting decreased a/F .
Compared to control levels, lowering tetanic [Ca ] with dantrolene or reduced stimulation frequency decreased the curvature slightly as judged from increase in a/F of 13 ± 1% (P = < .001) and 20 ± 2% (P = < .001) respectively. In contrast, increasing tetanic [Ca ] with caffeine increased the curvature (a/F decreased by 17 ± 1%; P = < .001).
Contrary to our hypothesis, interventions that reduced tetanic [Ca ] caused a decrease in curvature, while increasing tetanic [Ca ] increased the curvature. These results reject a simple causal relation between [Ca ] and curvature of the force-velocity relation during fatigue.
在动态收缩的肌肉中，力-速度关系的曲率增加导致疲劳时功率损失。有人提出，疲劳导致[Ca]的减少会导致这种曲率增加。然而，早期的单纤维研究是在低温下进行的。在这里，我们研究了在接近生理温度的情况下，在分离的骨骼肌中，[Ca]的减少是否会导致张力增加的假设。
在标准的 Krebs-Ringer 缓冲液中，以 60 Hz 的频率刺激大鼠比目鱼肌，并测量收缩力和速度。在一些实验中，通过添加 10 μmol/L 丹曲林或使用亚最大刺激（30 Hz）来降低[Ca]，或通过添加 2 mmol/L 咖啡因来增加[Ca]。通过将不同加载力下的缩短速度拟合到 Hill 方程来构建力-速度曲线。曲率通过 a/F 的比值来确定，曲率增加反映了 a/F 的降低。
与对照水平相比，丹曲林降低[Ca]或降低刺激频率分别使 a/F 增加了 13%±1%（P<0.001）和 20%±2%（P<0.001），从而使曲率略有增加。相比之下，用咖啡因增加[Ca]增加了曲率（a/F 降低了 17%±1%；P<0.001）。
与我们的假设相反，降低[Ca]的干预措施导致曲率减小，而增加[Ca]增加了曲率。这些结果否定了[Ca]与疲劳时力-速度关系的曲率之间的简单因果关系。
