Mitochondrial Ca 2+ dynamics in cells and suspensions

Mitochondria play a significant role in shaping cytosolic Ca 2+ signals. Thus, transfer of Ca 2+ across the mitochondrial membrane is much studied, not only in intact cells but also in artificial systems such as mitochondrial suspensions or permeabilised cells. Observed rates of Ca 2+ changes vary by at least one order of magnitude. In this work, we investigate the relationship between the Ca 2+ dynamics observed in various experimental conditions using a computational model calibrated on experimental data. Results confirm that mitochondrial Ca 2+ exchange fluxes through the mitochondrial Ca 2+ uniporter ( MCU ) and the Na + /Ca 2+ exchanger obey the same basic kinetics in cells and in suspensions, and emphasise the important role played by the high Ca 2+ levels reached in mitochondria‐associated endoplasmic reticulum membranes in intact cells. Tissue specificity can be ascribed to the different modes of regulation of the MCU by Ca 2+ , probably related to the specific levels of expression of the Ca 2+ sensing regulator subunit of this channel. The model emphasises the importance of mitochondrial density and buffering in controlling the rate of Ca 2+ exchanges with mitochondria, as verified experimentally. Finally, we show that heterogeneity between individual mitochondria can explain the large range of amplitudes and rates of rise in mitochondrial Ca 2+ concentration that have been observed experimentally.