Calcium signaling in cardiac mitochondria

► Mitochondria occupy a third of the cell volume of cardiac myocytes. ► Mitochondria have an elaborate system of calcium uptake and extrusion mechanisms. ► Mitochondrial Ca signaling is crucial for excitation-metabolism-contraction coupling. ► Mitochondria decode cytosolic Ca signals to respond to energy demands and buffer Ca. ► Beat-to-beat transmission or slow integration of cytosolic Ca signals are debated.