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.