Goldilocks calcium concentrations and the regulation of oxidative phosphorylation: Too much, too little, or 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+) modulates oxidative phosphorylation in mouse liver mitochondria and intact hepatocyte cell lines. To do so, we monitored the effects of more modest versus supraphysiological 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.