Intracellular levels and distribution of Ca2+ in digitonin-permeabilized cells.

Digitonin and other saponins can be used to selectively permeabilize the plasma membrane of a wide variety of cells without significantly affecting the gross structure and function of Ca2+-sequestering organelles such as mitochondria and endoplasmic reticulum. These characteristics have allowed digitonin to be used in the determination of the intracellular levels and distribution of Ca2+, as well as the measurement of Ca2+ fluxes by organelles "in situ". Studies conducted with several different types of digitonin-permeabilized cells indicate that the endoplasmic reticulum functions as a high affinity and low-capacity intracellular Ca2+ buffer, whereas mitochondria operate as a relatively low affinity but high capacity Ca2+ buffering system. However, recent findings suggest that mitochondria have a comparable affinity for net Ca2+ uptake in the presence of physiological concentrations of polyamines. The use of permeabilized cells has also been important in the identification of the endoplasmic reticulum as a site at which the recently discovered second messenger inositol trisphosphate acts to bring about an increase in the cytosolic free Ca2+ concentration. Thus, the selective permeabilization of cells with digitonin and its analogues has been a powerful yet simple tool in the study of intracellular Ca2+ homeostasis.