Ca2+-accumulating capacity of mitochondria, sarcolemma and sarcoplasmic reticulum of rat heart

Using the isotope exchange technique including 45Ca, the Ca2+-binding and Ca2+-accumulating capacity of mitochondria, sarcolemma and sarcoplasmic reticulum of rat heart was studied. The ATP-independent binding of Ca2+ to isolated membrane fractions is by 1--2 orders of magnitude less than the ATP-dependent Ca2+-accumulating capacity of the fractions. The Ca2+-accumulating capacity of mitochondria is increased 6--8 fold after addition of physiological concentrations of succinate and Pi to the incubation medium. Under these conditions the ratio of Ca2+-accumulating capacity of mitochondria, sarcolemma and sarcoplasmic reticulum of the heart is 100:3,12:2,9. The initial rate of Ca2+-uptake by the sarcoplasmic reticulum is much higher in comparison with sarcolemma and mitochondria. A high Ca2+-accumulating capacity of heart mitochondria probably determines a long-term regulation of the concentration of "troponin-accessible" Ca2+ in the sarcoplasm, whereas the high initial rate of Ca2+ accumulation by the sarcoplasmic reticulum provides for a rapid decrease of Ca2+ concentration during rhythmic contractions of the heart.