Goldilocks calcium and the mitochondrial respiratory chain: too much, too little, just right

Calcium (Ca ²⁺ ) is a key regulator in diverse intracellular signaling pathways, and has long been implicated in metabolic control and mitochondrial function. Mitochondria can actively take up large amounts of Ca ²⁺ , thereby acting as important intracellular Ca ²⁺ buffers and affecting cytosolic Ca ²⁺ transients. Excessive mitochondrial matrix Ca ²⁺ is known to be deleterious due to opening of the mitochondrial permeability transition pore (mPTP) and consequent membrane potential dissipation, leading to mitochondrial swelling, rupture, and cell death. Moderate Ca ²⁺ within the organelle, on the other hand, can directly or indirectly activate mitochondrial matrix enzymes, possibly impacting on ATP production. Here, we aimed to determine in a quantitative manner if extra or intramitochondrial Ca ²⁺ modulate oxidative phosphorylation in mouse liver mitochondria and intact hepatocyte cell lines. To do so, we monitored the effects of more modest versus supra-physiological increases in cytosolic and mitochondrial Ca ²⁺ on oxygen consumption rates. Isolated mitochondria present increased respiratory control ratios (a measure of oxidative phosphorylation efficiency) when incubated with low (2.4 ± 0.6 μM) and medium (22.0 ± 2.4 μM) Ca ²⁺ concentrations in the presence of complex I-linked substrates pyruvate plus malate and α-ketoglutarate, respectively, but not complex II-linked succinate. In intact cells, both low and high cytosolic Ca ²⁺ led to decreased respiratory rates, while ideal rates were present under physiological conditions. High Ca ²⁺ decreased mitochondrial respiration in a substrate-dependent manner, mediated by mPTP. Overall, our results uncover a Goldilocks effect of Ca ²⁺ on liver mitochondria, with specific “just right” concentrations that activate oxidative phosphorylation.