Regulation of free Ca2+ by liver mitochondria and endoplasmic reticulum

Electrode measurements were made of the free Ca2+ concentration maintained by suspensions of isolated rat liver mitochondria and microsomes, as well as by hepatocytes whose plasma membranes had been made permeable by treatment with digitonin. When the KCl, ATP, Mg2+, and phosphate concentrations were made similar to that of cytosol, the steady state free Ca2+ concentration in the presence of respiring mitochondria alone was about 0.5 microM. The additional presence of rat liver microsomes resulted in a steady state level of close to 0.2 microM, which was maintaied for greater than 1 h at 25 degrees C. This concentration of Ca2+ was also maintained by suspensions of hepatocytes permeabilized by digitonin and thus may approximate the actual cytosolic free Ca2+ concentration in vivo. The "set point" for free Ca2+ homeostasis in these systems is determined by mitochondrial Ca2+ influx-efflux cycling, which is dependent on the level of intramitochondrial Ca2+ and can be adjusted by sequestration of Ca2+ in microsomes.