Tiny changes in cytoplasmic [Ca²⁺] cause large changes in mitochondrial Ca²⁺: what are the triggers and functional implications?

Ca²⁺ is an integral component of the functional and developmental regulation of the mitochondria. In skeletal muscle, Ca²⁺ is reported to modulate the rate of ATP resynthesis, regulate the expression of peroxisome proliferator-activated receptor-gamma coactivator 1 (PGC1α) following exercise, and drive the generation of reactive oxygen species (ROS). Due to the latter, mitochondrial Ca²⁺ overload is recognized as a pathophysiological event but the former events represent important physiological functions in need of tight regulation. Recently, we described the relationship between [Ca²⁺]_mito and resting [Ca²⁺]_cyto and other mitochondrial Ca²⁺-handling properties of skeletal muscle. An important next step is to understand the triggers for Ca²⁺ redistribution between intracellular compartments, which determine the mitochondrial Ca²⁺ load. These triggers in both physiological and pathophysiological scenarios can be traced to the coupled activity of the ryanodine receptor 1 (RyR1) and store-operated Ca²⁺ entry (SOCE) in the resting muscle. In this piece, we will discuss some issues regarding Ca²⁺ measurements relevant to mitochondrial Ca²⁺-handling, the steady-state relationship between cytoplasmic and mitochondrial Ca²⁺, and the potential implications for Ca²⁺ handling by muscle mitochondria and cellular function.