Role of the Transverse Tubular System in Mammalian Skeletal Muscle Excitability

描述（由申请人提供）：该提案的中心假设是横管系统（TTS）在哺乳动物骨骼肌的整体特性中发挥着如此重要的作用，为了理解肌肉通道病的病理生理学，有必要仔细表征该膜室的电特性。由离子通道激活介导的 TTS 膜电位变化不仅影响肌纤维的电特性，而且还负责触发兴奋-收缩耦合 (ECC) 机制。我们将使用电生理学方法、最先进的光学技术（允许测量 TTS 电压变化）以及该室中去极化径向扩散的数学模型，以探究离子电导在这些过程中发挥的详细作用。首先，我们将表征电压钳条件下正常小鼠肌肉纤维的无源电特性和每个主要导电通路（目标 1）。然后，我们将研究受刺激以引发重复放电的纤维中 TTS 传播的特性和局限性，并测试个体电导（特别是钠和氯）的变化对这些特性的影响。目标是阐明 TTS 管腔中 K 积累在与周期性麻痹和肌强直等通道病相关的现象学中可能发挥的潜在作用（目标 2）。由于通道病功能研究中的一个难题是肌强直和麻痹之间的微妙界限，因此在目标 3 中，我们将研究 ECC 电压调节的复杂性是否会导致 Ca2 释放过程的废除或保留，具体取决于 TTS 中的电活动模式。最后，利用之前目标中获得的知识，我们将研究是否可以通过 TTS 电传播的变化来理解在肌强直和高钾性周期性麻痹动物模型中观察到的发病机制（目标 4）。通过这些研究获得的知识不仅与理解通道病的病理生理学相关，而且由于它们将提供有关 TTS 电传播的生理机制的基本信息，因此对于理解许多肌肉疾病具有重要意义。

DESCRIPTION (provided by applicant): The central hypothesis of this proposal is that the transverse tubular system (TTS) plays such a preponderant role in the overall properties of mammalian skeletal muscle that, in order to understand the pathophysiology of muscle channelopathies it will be necessary to carefully characterize the electrical properties of this membrane compartment. Changes in membrane potential of the TTS, which are mediated by the activation of ion channels, not only affect the electrical properties of the muscle fiber, but also are responsible for triggering the mechanisms of excitation-contraction coupling (ECC). We will use electrophysiological methods, state-of-the-art optical techniques (which permit to measure TTS voltage changes), and mathematical modeling of the radial spread of the depolarization in this compartment, in order to probe the detailed role that ionic conductances play in these processes. First, we will characterize the passive electrical properties and each of the major conductive pathways in normal mouse muscle fibers under voltage clamp conditions (Aim 1). We will then study the properties and limitations of the TTS propagation in fibers stimulated to elicit repetitive firing and test the effects that alterations in individual conductances (sodium and chloride in particular) have on these properties. The goal is to elucidate the potential role that K accumulation in the lumen of the TTS lumen may play in the phenomenology associated with channelopathies such as periodic paralysis and myotonia (Aim 2). Since a conundrum in the functional investigation of channelopathies is the tenuous demarcation between myotonia and paralysis, in Aim 3 we will investigate whether intricacies of the voltage regulation of the ECC can result in abolition or preservation of the Ca2+ release process depending on the pattern of electrical activity in the TTS. Finally, with the knowledge acquired in previous aims, we will investigate whether the pathogenesis observed in animal models of myotonia and hyperkaelemic periodic paralysis can be understood from alterations in the electrical propagation at the TTS (Aim 4). The knowledge gained with these investigations will not only be relevant towards the understanding of the pathophysiology of channelopathies, but since they will provide basic information about the physiological mechanisms of TTS electrical propagation, they will be of significance for understanding a number of muscle diseases.