The integration of mitochondrial calcium transport and storage

The extraordinary capacity of isolated mitochondria to accumulate Ca2+ has been established for more than 40 years. The distinct kinetics of the independent uptake and efflux pathways accounts for the dual functionality of the transport process to either modulate matrix free Ca2+ concentrations or to act as temporary stores of large amounts of Ca2+ in the presence of phosphate. One puzzle has been the nature of the matrix calcium phosphate complex, since matrix free Ca2+ seems to be buffered in the region of 1-5 muM in the presence of phosphate while millimolar Ca2+ remains soluble in in vitro media. The key seems to be the elevated matrix pH and the third-power relationship of the PO43- concentration with pH. Taking this into account we may now finally have a model that explains the major features of physiological mitochondrial Ca2+ transport.