Calcium retention capacity (CRC) is a measure of the capability of mitochondria to retain calcium, primarily in the form of calcium phosphates, in the mitochondrial matrix. By storing calcium in the form of osmotically inactive precipitates the mitochondria contribute to the buffering of cytosolic free calcium levels and thereby to the regulation of calcium-dependent cellular processes. Alterations of CRC are believed to be important in stress phenomena associated with energy limitation and have also been linked to neurodegenerative diseases (Starkov 2013 FEBS J). Experimentally, CRC has been indirectly assessed by determination of respiratory rates of isolated mitochondria which were exposed to continuously increasing levels of calcium by use of the Titration-Injection microPump TIP2k. The upper limit of CRC was observed as a sudden decrease of respiration presumed to reflect opening of the permeability transition pore (Hansson 2010 J Biol Chem).