Department of Biochemistry and Tissue Biology, Institute of Biology, University of Campinas (Unicamp), Campinas, Brazil.
Department of Medicine, University of California San Diego, La Jolla, CA, USA.
J Physiol. 2018 Mar 15;596(6):1079-1089. doi: 10.1113/JP275470. Epub 2018 Feb 11.
Skeletal muscle contractile activity is associated with an enhanced reactive oxygen species (ROS) generation. At very low PO2, ROS generation by mitochondria can be elevated in intact cells. An elevated intracellular oxidant activity may affect muscle force development and recovery from fatigue. We treated intact single muscle fibres with a mitochondrial antioxidant and stimulated the fibres to contract at a low extracellular PO2 that is similar to the intracellular PO2 that is observed during moderate to intense exercise in vivo. The mitochondrial antioxidant prevented a sustained decrease in the myofibrillar Ca sensitivity and improved muscle submaximal force development after fatigue at low extracellular PO2.
Skeletal muscle can develop a prolonged low frequency-stimulation force depression (PLFFD) following fatigue-inducing contractions. Increased levels of reactive oxygen species (ROS) have been implicated in the development of PLFFD. During exercise the skeletal muscle intracellular PO2 decreases to relatively low levels, and can be further decreased when there is an impairment in O diffusion or availability, such as in certain chronic diseases and during exercise at high altitude. Since ROS generation by mitochondria is elevated at very low PO2 in cells, we tested the hypothesis that treatment of muscle fibres with a mitochondrial-targeted antioxidant at a very low, near hypoxic, PO2 can attenuate PLFFD. We treated intact single fibres from mice with the mitochondrial-specific antioxidant SS31, and measured force development and intracellular [Ca ] 30 min after fatigue at an extracellular PO2 of ∼5 Torr. After 30 min following the end of the fatiguing contractions, fibres treated with SS31 showed significantly less impairment in force development compared to untreated fibres at submaximal frequencies of stimulation. The cytosolic peak [Ca ] transients (peak [Ca ] ) were equally decreased in both groups compared to pre-fatigue values. The combined force and peak [Ca ] data demonstrated that myofibrillar Ca sensitivity was diminished in the untreated fibres 30 min after fatigue compared to pre-fatigue values, but Ca sensitivity was unaltered in the SS31 treated fibres. These results demonstrate that at a very low PO2, treatment of skeletal muscle fibres with a mitochondrial antioxidant prevents a decrease in the myofibrillar Ca sensitivity, which alleviates the fatigue induced PLFFD.
骨骼肌收缩活动与活性氧（ROS）生成增加有关。在非常低的 PO2 下，完整细胞中线粒体产生的 ROS 可以升高。细胞内氧化剂活性的升高可能会影响肌肉力量的发展和疲劳的恢复。我们用一种线粒体抗氧化剂处理完整的单个肌纤维，并在非常低的细胞外 PO2 下刺激纤维收缩，该 PO2 类似于体内中等至高强度运动期间观察到的细胞内 PO2。线粒体抗氧化剂防止肌原纤维钙敏感性的持续降低，并改善了低细胞外 PO2 疲劳后肌肉次最大力的发展。
骨骼肌在疲劳诱导收缩后会产生长时间的低频刺激力抑制（PLFFD）。活性氧（ROS）水平升高与 PLFFD 的发展有关。在运动过程中，骨骼肌细胞内 PO2 降低到相对较低的水平，当 O 扩散或可用性受损时，例如在某些慢性疾病和高海拔运动中，PO2 会进一步降低。由于细胞中线粒体产生的 ROS 在非常低的 PO2 下升高，我们测试了这样一个假设，即在非常低的、接近缺氧的 PO2 下用一种线粒体靶向抗氧化剂处理肌纤维可以减轻 PLFFD。我们用线粒体特异性抗氧化剂 SS31 处理来自小鼠的完整单个纤维，并在细胞外 PO2 约为 5 托的情况下测量疲劳 30 分钟后的力发展和细胞内[Ca2+]。在疲劳收缩结束后的 30 分钟内，与未处理纤维相比，用 SS31 处理的纤维在亚最大刺激频率下显示出明显较少的力发展受损。与疲劳前值相比，两组的胞浆峰[Ca2+]（峰[Ca2+]）均显著降低。综合力和峰[Ca2+]数据表明，与疲劳前值相比，未处理纤维在疲劳后 30 分钟时肌原纤维钙敏感性降低，但 SS31 处理的纤维钙敏感性未改变。这些结果表明，在非常低的 PO2 下，用一种线粒体抗氧化剂处理骨骼肌纤维可防止肌原纤维钙敏感性降低，从而缓解疲劳诱导的 PLFFD。
