Mitochondrial Ca2+ uptake: Tortoise or hare?

Mitochondria are equipped with an efficient machinery for Ca2+ uptake and extrusion and are capable of storing large amounts of Ca2+. Furthermore, key steps of mitochondrial metabolism (ATP production) are Ca2+-dependent. In the field of cardiac physiology and pathophysiology, two main questions have dominated the thinking about mitochondrial function in the heart: 1) how does mitochondrial Ca2+ buffering shape cytosolic Ca2+ levels and affect excitation–contraction coupling, particularly the Ca2+ transient, on a beat-to-beat basis, and 2) how does mitochondrial Ca2+ homeostasis influence cardiac energy metabolism. To answer these questions, a thorough understanding of the kinetics of mitochondrial Ca2+ transport and buffer capacity is required. Here, we summarize the role of mitochondrial Ca2+ signaling in the heart, discuss the evidence either supporting or arguing against the idea that Ca2+ can be taken up rapidly by mitochondria during excitation–contraction coupling and highlight some interesting new areas for further investigation.