Regulation of KATP and Cl channel activity by AMPK, ATP and adenosine receptors: impact on membrane excitability during muscle activity and fatigue

值得注意的是在骨骼肌活动期间细胞膜中离子通道活性的复杂变化。第一个变化发生在第1阶段，开始于肌肉活动的开始，其特征在于ClC-1 Cl-通道活性降低70%。第2阶段发生的特征是ClC-1和KATP通道活性大幅增加，最高达刺激前水平的3倍。在中度运动期间，平均间质[K+]达到10毫米5分钟内。我们的研究K+和Cl-对膜兴奋性的影响提供的证据表明，减少ClC-1通道活性在第1阶段是重要的，不仅要防止任何K+诱导的力抑郁症，但事实上，以增加K+的能力，以增强力在10毫米K+。在第二阶段期间，ClC-1和KATP通道活性的增加被认为与疲劳有关，这被称为当肌肉被重复刺激时力或功的减少。现在普遍认为肌肉疲劳的一个主要因素涉及肌浆网的Ca 2+释放减少，这反过来又部分归因于膜兴奋性降低。兴奋性降低是由ClC-1活性增加触发的，将K+效应从增强变为抑制，而通过KATP通道的K+流出增加降低了动作电位振幅，最终降低了力保留ATP。 问题是从肌肉活动开始是什么控制ClC-1和KATP通道活性的复杂变化（即，阶段1）到疲劳的发展（即，阶段2）。已经表明，在运动开始时ClC-1通道活性的降低与PKC的磷酸化有关。还已知ATP受体的激活降低ClC-1通道活性，但仍有待确定这些ATP受体是否参与运动开始。在膜片钳条件下，A1腺苷受体激活KATP通道，但A1受体是否参与疲劳期间ClC-1和KATP通道的激活仍有待确定。在心肌中，AMPK是一种重要的细胞能量传感器，激活KATP通道;因此，在骨骼肌疲劳期间，AMPK也可能激活ClC-1和KATP通道。 长期目标是研究从肌肉活动开始到疲劳的细胞内信号通路，调节膜兴奋性。本提案的短期目标是研究i）ATP及其受体在第1阶段优化肌肉性能中的作用，以及ii）AMPK和腺苷在触发疲劳中的作用（第2阶段）。提出了两个假设：1）“ATP受体通过降低ClC-1 Cl-通道活性、激活Na+ K+ ATP酶泵和促进K+诱导的力增强来优化肌肉性能”和2）“AMPK和腺苷通过降低泵活性和激活KATP和ClC-1通道来触发疲劳”。

Noteworthy are complex changes in ion channel activity in the cell membrane during skeletal muscle activity. The first change, occurring during Phase 1, starts at the onset of muscle activity and is characterized by a 70% decreases in ClC-1 Cl- channel activity. Phase 2 occurs is characterized by large increases ClC-1 and KATP channel activity up to 3-times above the pre-stimulation level. During moderate exercise, mean interstitial [K+] reaches 10 mM within 5 min. Our studies on K+ and Cl- effects on membrane excitability are providing evidence that the reduction in ClC-1 channel activity during Phase 1 is important not only to prevent any K+-induced force depression, but in fact to augment the capacity of K+ to potentiate force at 10 mM K+. The increase in ClC-1 and KATP channel activity during Phase 2 is believed to be associated with fatigue, known as the decrease in force or work when muscles are repetitively stimulated. It is now well accepted that a major factor in muscle fatigue involves a decrease in Ca2+ release by the sarcoplasmic reticulum, which in turn is in part due to a decreased membrane excitability. The decreased excitability is triggered by increase in ClC-1 activity changing the K+ effect from a potentiation to a depression, while the increase in K+ efflux through KATP channels reduces action potential amplitude to eventually reduce force sparing ATP.      The question is what controls the complex changes in the activity of ClC-1 and KATP channel activity from the onset of muscle activity (i.e., Phase 1) to the development of fatigue (i.e., Phase 2). It has been shown that the decrease in ClC-1 channel activity at the onset of exercise is related to a phosphorylation by PKC. It is also known that activation of ATP receptors reduce ClC-1 channel activity, but it remains to be determined whether these ATP receptors are involved at the onset of exercise. Under patch clamp conditions, the A1 adenosine receptors activate KATP channels, but it remains to be determined if the A1 receptors are involved in the activation of ClC-1 and KATP channels during fatigue. In cardiac muscle, AMPK, an important cell energy sensor, activate KATP channels; it is therefore possible that it also activates the ClC-1 and KATP channels during fatigue in skeletal muscle.      The long term objective is to study the intracellular signaling pathways that regulate membrane excitability from the onset of muscle activity to fatigue. The short term objectives of this proposal are to investigate i) the role of ATP and its receptors in optimizing muscle performance during Phase 1 and ii) the role of AMPK and adenosine in triggering fatigue (Phase 2). Two hypotheses are proposed: 1) “ATP receptors optimize muscle performance by lowering ClC-1 Cl- channel activity, activating Na+ K+ ATPase pump, and promoting K+-induced force potentiation” and 2) “AMPK and adenosine triggers fatigue by reducing the pump activity and activating KATP and ClC-1 channels”.