This is from Wikipedia:The mitochondrial calcium uniporter (MCU)...

This is from Wikipedia:

The mitochondrial calcium uniporter (MCU) is a transmembrane protein that allows the passage of calcium ions from a cell's cytosol into mitochondria. Its activity is regulated by MICU1 and MICU2, which together with the MCU make up the mitochondrial calcium uniporter complex.^[1]

The MCU is one of the primary sources of mitochondria uptake of calcium, and flow is dependent on membrane potential of the inner mitochondrial membrane and the concentration of calcium in the cytosol relative to the concentration in the mitochondria. Balancing calcium concentration is necessary to increase the cell's energy supply and regulate cell death. Calcium is balanced through the MCU in conjunction with the sodium-calcium exchanger.

The MCU has a very low affinity for calcium, so the cytosolic calcium concentration needs to be approximately 5-10 uM for significant transport of calcium into the mitochondria. Mitochondria are closely associated with the endoplasmic reticulum (ER), at contact sites, which contains stores of cellular calcium ions for calcium signaling. The presence of 1,4,5-triphosphate (IP₃) triggers the release of calcium from these intracellular stores, which creates microdomains of high calcium concentration between the ER and the mitochondria, creating the conditions for the MCU to take up calcium.^[2]