Evaluation of a novel model of skeletal muscle fatigue

几个世纪以来，骨骼肌疲劳一直令生理学家们着迷，然而，在重复激活过程中和之后影响收缩特性的细胞过程并不是很清楚。大多数目前的疲劳理论认为，代谢产物的积聚及其抑制或损害收缩反应的假定能力是疲劳的可能机制。然而，这与已知的疲劳的主要机制不一致：抑制兴奋-收缩偶联(E-CC)。现在是时候进一步了解E-CC是如何受损的，以真正理解肌肉在新陈代谢压力下作为组织的行为。据推测：疲劳是E-CC维持肌肉中ATP的细胞调节。从本质上讲，这一理论代表了一种现实的灾难避免模式。肌球蛋白ATPase和钙离子ATPase对ATP的使用也必须受到调节。建议进行一系列实验来验证这一理论，并提供这一调节如何导致疲劳的细节。这项工作将使用最近开发的结构准确的肌节三维计算机模型来模拟E-CC、收缩反应和肌原纤维水平的能量学。此外，还提出了使用不同组织级别的实验。肌节模型将允许对自由和结合的[Ca~(2+)]以及[ATP]的分布进行纳米级的评估，使我们远远超出原本可以可视化的范围。单纤维和全肌肉实验将补充这种计算机模拟方法。所有的模拟和拟议的实验都将在哺乳动物的生理温度下或接近生理温度进行，因为已经证明，在室温下导致疲劳的机制与哺乳动物的生理温度无关。建议开展几个项目：1)肌肉能量学的计算机模拟，以了解代谢物的亚细胞分布，包括：ATP、ADP、AMP、肌酸和肌酸磷酸；2)整个肌肉研究[ATP]的变化和对疲劳的影响；3)单纤维测量钙敏感性和改变的钙释放，以探索具体的机制。计算机模拟将使用我们目前结构上准确的半肌节三维模型，并根据已知的ATP使用和再生的速度和位置监测[ATP]和其他代谢物的地区差异。这将揭示兰诺定受体周围的[ATP]，以及在重复收缩过程中它是如何变化的。整个肌肉研究将证明E-CC的快速(收缩到收缩)调节，表现为EMG和肌力的快速变化，间歇刺激没有变化，但通过快速调整长度改变了能量需求。预计ATP需求的短暂减少将导致激活增强，从而产生更大的威力。当长度从接近最佳的位置减少时，主动力和ATP的使用量就会减少。对单纤维进行的类似工作将评估当前的理论，即钙敏感性的变化会导致疲劳，以及评估肌肉收缩能量成本变化后钙释放快速调整的药物干预。拟议的实验将为E-CC的调节如何导致疲劳提供证据，促进我们在一个为一些新思维做好准备的研究领域的知识。这项拟议的工作代表了肌肉疲劳研究的一个新方向，并将极大地揭示骨骼肌这一常见而有趣的特性的拟议模型。

Skeletal muscle fatigue has fascinated physiologists for centuries, yet the cellular processes impacting contractile properties during and following repeated activations are not well understood. Most current theories of fatigue consider build-up of metabolic products and their assumed ability to inhibit or impair the contractile response as the likely mechanism of fatigue. Yet, this is not consistent with the known primary mechanism of fatigue; inhibition of excitation-contraction coupling (E-CC). It is time to advance an understanding of how E-CC is impaired to truly comprehend how muscle behaves as a tissue when exposed to a metabolic stress. It is postulated that: fatigue is cellular regulation of E-CC to preserve ATP in the muscle. Essentially, this theory represents a realistic model of catastrophe avoidance. ATP use by both myosin ATPase and Ca2+ ATPase must also be regulated. A series of experiments are proposed to test this theory and to provide details for how this regulation contributes to fatigue. This work will use a recently developed structurally accurate 3-dimensional computer model of a sarcomere to simulate E-CC, contractile response and energetics at the myofibrillar level. In addition, experiments using various levels of organization are proposed. The sarcomere model will permit nanoscale evaluation of free and bound [Ca2+] as well as distribution of [ATP], taking us well beyond the scale that can otherwise be visualized. Single fibre and whole muscle experiments will supplement this computer simulation approach. All simulations and proposed experiments will be conducted at or close to physiological temperature for mammals because it has been demonstrated that mechanisms contributing to fatigue at room temperature are not relevant at mammalian physiological temperature. It is proposed to undertake several projects: 1) Computer modeling of muscle energetics, to gain an understanding of subcellular distribution of metabolites including: ATP, ADP, AMP, creatine, and creatine phosphate; 2) Whole muscle study of changes in [ATP] and the impact on fatigue; 3) Single fibre measurement of Ca2+ sensitivity and altered Ca2+ release to probe specific mechanisms. The computer simulation will use our current structurally accurate 3-dimensional model of the half sarcomere and monitor regional differences in [ATP] and other metabolites, based on known rates and locations of ATP use and regeneration. This will reveal the [ATP] around the ryanodine receptor, and how it changes during repeated contractions. The whole muscle studies will be undertaken to demonstrate rapid (contraction to contraction) regulation of E-CC, as manifest in rapid changes in EMG and muscle force with no change in the intermittent stimulation, but altered energy requirements by rapid adjustment of length. It is anticipated that a brief decrease in ATP requirement will result in enhanced activation and therefore greater force. When length is decreased from near optimal, active force and therefore ATP use decreases. Similar work with single fibres will evaluate the current theory that changes in Ca2+ sensitivity contribute to fatigue, and drug interventions to evaluate rapid adjustments in Ca2+ release following changes in energy cost of muscle contractions. The proposed experiments will provide evidence for how the regulation of E-CC can result in fatigue, advancing our knowledge in a field of study that is primed for some novel thinking. The proposed work represents a new direction for the study of muscle fatigue and should be remarkably revealing in support of the proposed model of this common yet intriguing property of skeletal muscle.