The dynamics of mitochondrial Ca2+ fluxes

We have investigated the kinetics of mitochondrial Ca2+ influx and efflux and their dependence on cytosolic [Ca2+] and [Na+ using low-Ca2+-affinity aequorin. The rate of Ca2+ release from mitochondria increased linearly with mitochondrial [Ca2+] ([Ca2+](M)). Na+-dependent Ca2+ release was predominant al low [Ca2+](M) but saturated at [Ca2+](M) around 400 mu M, while Na+-independent Ca2+ release was very slow at [Ca2+](M) below 200 mu M, and then increased at higher [Ca2+](M), perhaps through the opening of a new pathway. Half-maximal activation of Na+-dependent Ca2+ release occurred at 5-10 mM [Na+], within the physiological range of cytosolic [Na+]. Ca2+ entry rates were comparable in size to Ca2+ exit rates at cytosolic [Ca2+] ([Ca2+](c)) below 7 mu M, but the rate of uptake was dramatically accelerated at higher [Ca2+](c). As a consequence, the presence of [Na+ considerably reduced the rate of [Ca2+](M) increase at [Ca2+](c) below 7 mu M, but its effect was hardly appreciable at 10 mu M [Ca2+](c). Exit rates were more dependent on the temperature than uptake rates, thus making the [Ca2+](M) transients to be much more prolonged at lower temperature. Our kinetic data suggest that mitochondria have little high affinity Ca2+ buffering, and comparison of our results with data on total mitochondrial Ca2+ fluxes indicate that the mitochondrial Ca2+ bound/Ca2+ free ratio is around 10- to 100-fold for most of the observed [Ca2+](M) range and suggest that massive phosphate precipitation can only occur when [Ca2+](M) reaches the millimolar range. (C) 2010 Elsevier B.V. All rights reserved.