Uptake of calcium by mitochondria: transport and possible function.

Vertebrate mitochondria contain a complex system for transport of Ca2+ and related ions, consisting of two saturable modes of Ca2+ influx and two separate, saturable mechanisms of Ca2+ efflux. The characteristics of the mechanisms of Ca2+ uptake, the uniporter and the RaM, are discussed here and suggestions are made about how the mechanisms may work together and separately to mediate the two physiological roles with which they are most commonly associated-control of the rate of cellular ATP production and induction of the permeability transition and apoptosis. It is argued that more subtlety of control of intramitochondrial free Ca2+ concentration ([Ca2+]m) must be used by the uniporter and the RaM to fulfill their physiological roles than has been commonly recognized. This is because an increase in [Ca2+]m is associated with both increased production of ATP which supports the continued life of the cell and with induction of the permeability transition and possibly apoptosis, which leads to cell death. The saturable mechanisms of mitochondrial Ca2+ efflux and the Ca2+-induced mitochondrial permeability transition, which can transport Ca2+ as well as other ions and molecules and is often considered as a Ca2+ transport mechanism, are being reviewed separately.