Ca²⁺ flux into the mitochondrial matrix through the MCU holocomplex (MCU_cx) has recently been measured quantitatively and with milliseconds resolution for the first time under physiological conditions in both heart and skeletal muscle. Additionally, the dynamic levels of Ca²⁺ in the mitochondrial matrix ([Ca²⁺]_m) of cardiomyocytes were measured as it was controlled by the balance between influx of Ca²⁺ into the mitochondrial matrix through MCU_cx and efflux through the mitochondrial Na⁺ / Ca²⁺ exchanger (NCLX). Under these conditions [Ca²⁺]_m was shown to regulate ATP production by the mitochondria at only a few critical sites. Additional functions attributed to [Ca²⁺]_m continue to be reported in the literature. Here we review the new findings attributed to MCU_cx function and provide a framework for understanding and investigating mitochondrial Ca²⁺ influx features, many of which remain controversial. The properties and functions of the MCU_cx subunits that constitute the holocomplex are challenging to tease apart. Such distinct subunits include EMRE, MCUR1, MICUx (i.e. MICU1, MICU2, MICU3), and the pore-forming subunits (MCU_pore). Currently, the specific set of functions of each subunit remains non-quantitative and controversial. The more contentious issues are discussed in the context of the newly measured native MCU_cx Ca²⁺ flux from heart and skeletal muscle. These MCU_cx Ca²⁺ flux measurements have been shown to be a highly-regulated, tissue-specific with femto-Siemens Ca²⁺ conductances and with distinct extramitochondrial Ca²⁺ ([Ca²⁺]_i) dependencies. These data from cardiac and skeletal muscle mitochondria have been examined quantitatively for their threshold [Ca²⁺]_i levels and for hypothesized gatekeeping function and are discussed in the context of model cell (e.g. HeLa, MEF, HEK293, COS7 cells) measurements. Our new findings on MCU_cx dependent matrix [Ca²⁺]_m signaling provide a quantitative basis for on-going and new investigations of the roles of MCU_cx in cardiac function ranging from metabolic fuel selection, capillary blood-flow control and the pathological activation of the mitochondrial permeability transition pore (mPTP). Additionally, this review presents the use of advanced new methods that can be readily adapted by any investigator to enable them to carry out quantitative Ca²⁺ measurements in mitochondria while controlling the inner mitochondrial membrane potential, ΔΨ_m.

最近，在心脏和骨骼肌的生理条件下，首次以毫秒分辨率定量测量了通过 MCU holocomplex (MCU _cx )进入线粒体基质的Ca ^2+通量。此外，测量了心肌细胞线粒体基质 ([Ca ²⁺ ] _{m ) 中 Ca} ²⁺的动态水平，因为它是由通过 MCU _cx流入线粒体基质的 Ca ²⁺和通过线粒体的流出之间的平衡控制的。 Na ⁺ / Ca ²⁺交换剂（NCLX）。在这些条件下 [Ca ²⁺ ] _m显示仅在几个关键位点调节线粒体的 ATP 产生。文献中继续报道了归因于 [Ca ²⁺ ] _{m的其他功能。}在这里，我们回顾了归因于 MCU _cx功能的新发现，并为理解和研究线粒体 Ca ²⁺流入特征提供了一个框架，其中许多特征仍然存在争议。构成全息复合体的 MCU _cx亚基的特性和功能难以区分。此类不同的亚基包括 EMRE、MCUR1、MICUx（即 MICU1、MICU2、MICU3）和造孔亚基（MCU_孔）。目前，每个亚基的具体功能仍然是非定量的和有争议的。更有争议的问题将在新测量的来自心脏和骨骼肌的原生 MCU _cx Ca ^{2+通量的背景下进行讨论。}这些 MCU _cx Ca ²⁺通量测量已被证明是高度调节的、组织特异性的，具有 femto-Siemens Ca ²⁺电导和明显的线粒体外 Ca ²⁺ ([Ca ²⁺ ] _i ) 依赖性。这些来自心脏和骨骼肌线粒体的数据已被定量检测其阈值 [Ca ²⁺ ] _i水平和假设的守门功能，并在模型细胞（例如 HeLa、MEF、HEK293、COS7 细胞）测量的背景下进行讨论。我们关于 MCU _cx依赖矩阵 [Ca ²⁺ ] _{m信号传导的新发现为 MCU cx}在代谢燃料选择、毛细血管血流控制和病理激活等心脏功能中的作用的持续和新研究提供了定量基础线粒体通透性转换孔（mPTP）。此外，本综述介绍了先进的新方法的使用，任何研究人员都可以很容易地采用这些方法，使他们能够进行定量 Ca ²⁺在控制线粒体内膜电位 ΔΨ _m的同时测量线粒体。