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 (PGC1a) following exercise, and drive the generation of reactive oxygen species (ROS). Due to the latter, mitochon-drial Ca2+ overload is recognized as a pathophysiological event but the former events represent important physiological func-tions 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+ redis-tribution 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.