Lilliu Elena, Choi Rocky, Hilber Karlheinz, Launikonis Bradley, Koenig Xaver
Center for Physiology and Pharmacology, Medical University of Vienna, Wien, Austria.
School of Biomedical Sciences, Faculty of Medicine, The University of Queensland, Brisbane, Queensland, Australia.
Acta Physiol (Oxf). 2025 Jun;241(6):e70059. doi: 10.1111/apha.70059.
This study investigates the activation and regulation of phasic store-operated calcium entry (pSOCE) in fast- and slow-twitch skeletal muscle fibers. Specifically, we aimed to enhance the sensitivity of pSOCE detection in slow-twitch fibers by optimizing ionic conditions and to compare the physiological relevance of pSOCE between fiber types.
We employed mechanically skinned fast-twitch extensor digitorum longus (EDL) muscle fibers loaded with spectrally distinct Ca-sensitive dyes to simultaneously measure action potential-induced sarcoplasmic reticulum Ca release and t-tubular system Ca dynamics with millisecond resolution. Experimental conditions were optimized by reducing cytosolic Mg and EGTA buffering to enhance Ca release in slow-twitch soleus fibers. Confocal microscopy was used to track t-tubular system Ca depletion and reuptake during electric field stimulation.
Skinned soleus fibers exhibited ~8-fold lower Ca release per action potential compared to EDL fibers, yet pSOCE amplitudes were comparable. Reducing Mg and EGTA levels increased Ca release and left pSOCE kinetics in EDL fibers unaltered, but enabled pSOCE measurements in soleus fibers. While pSOCE in EDL fibers followed a linear dependence on the ambient Ca concentration in the t-tubular system, such a relationship was violated in soleus fibers.
These findings reveal a novel, fiber-type-specific difference in pSOCE regulation. When compared to EDL fibers, soleus fibers exhibited a higher sensitivity to SOCE activation despite releasing less Ca from the sarcoplasmic reticulum upon an action potential. These differences may allow soleus fibers to sustain Ca homeostasis more effectively, be more resilient against disruptions in Ca handling, and entail protection against disease states.
本研究调查快速和慢速收缩骨骼肌纤维中阶段性储存-操作性钙内流（pSOCE）的激活与调节。具体而言，我们旨在通过优化离子条件提高慢速收缩纤维中pSOCE检测的灵敏度，并比较不同纤维类型之间pSOCE的生理相关性。
我们使用加载了光谱不同的钙敏染料的机械去表皮快速收缩趾长伸肌（EDL）肌纤维，以毫秒分辨率同时测量动作电位诱导的肌浆网钙释放和横管系统钙动力学。通过降低胞质镁和乙二醇双乙酸盐（EGTA）缓冲来优化实验条件，以增强慢速收缩比目鱼肌纤维中的钙释放。共聚焦显微镜用于跟踪电场刺激期间横管系统钙的消耗和再摄取。
与EDL纤维相比，去表皮的比目鱼肌纤维每个动作电位的钙释放量低约8倍，但pSOCE幅度相当。降低镁和EGTA水平增加了钙释放，且EDL纤维中的pSOCE动力学未改变，但使得比目鱼肌纤维中的pSOCE测量成为可能。虽然EDL纤维中的pSOCE对横管系统中环境钙浓度呈线性依赖关系，但比目鱼肌纤维中这种关系并不成立。
这些发现揭示了pSOCE调节中一种新的、纤维类型特异性差异。与EDL纤维相比，比目鱼肌纤维尽管在动作电位时从肌浆网释放的钙较少，但对SOCE激活表现出更高的敏感性。这些差异可能使比目鱼肌纤维更有效地维持钙稳态，对钙处理的破坏更具弹性，并提供疾病状态防护。
