Summary of the mechanisms governing mitochondrial Ca²⁺ dynamics. Mitochondrial Ca²⁺ uptake occurs through MCU_mode1 with a high Ca²⁺ affinity, but low Ru360 sensitivity, whereas the bulk of mitochondrial Ca²⁺ loading occurs through the low Ca²⁺ affinity, high Ru360-sensitive MCU_mode2. Ca²⁺ extrusion occurs through the mNCE. Rapid Ca²⁺ entry via MCU_mode1 saturates fixed mitochondrial Ca²⁺ buffers (B_m) to produce a large change in [Ca²⁺]_mito, whereas larger amounts of Ca²⁺ entry, along with P_i uptake, result in reversible precipitation of calcium-phosphate (CaP_pt) with little effect on steady-state [Ca²⁺]_mito. P_i is transported into mitochondria through the phosphate carrier (PiC). Mitochondrial matrix contents are released when the mitochondrial PTP opens. PTP opening correlates with total mitochondrial Ca²⁺ load but not [Ca²⁺]_mito, perhaps through the formation of calcium-phosphate complexes.