ABSTRACT
The actual amount of loosely bound or free-ionic calcium in the cytosol is much lower than the total amount of calcium, because most of the calcium in the cells is either sequestered by subcellular organelles or bound to cell structural components or molecules in the cytosol. Measuring cellular calcium and fractionating it among the different cell components is a difficult task, because cellular components can be damaged during fractionation, changing the concentration of calcium they had originally in the intact cell. In summary, although it is difficult to assess the distribution of calcium among the different cell components, it seems that in excitable as well as in nonexcitable cells, calcium associated with the mitochondria, the endoplasmic and sarcoplasmic reticulum, and plasma membrane accounts for most of the cell calcium. Data in show that total calcium concentration in the cytosol of human red blood cells, estimated as the difference between total calcium and calcium bound to the membrane, is low.
