Calcium ion distribution in cytoplasm visualised by aequorin: diffusion in cytosol restricted by energized sequestering.

The distribution of Ca2+ in the cytoplasm following a local rise in Ca2+ concentration is visualized by means of aequorin luminescence and a television system with an image intensifier. Diffusion of Ca2+ through the cytosol is so constrained that a rise in cytoplasmic Ca2+ concentration produced by local Ca2+ entry through cell membrane or by local Ca2+ injection is confined to the immediate vicinity of these sites. The diffusion constraints are lifted by treatment with cyanide or ruthenium red. Thus, energized calcium sequestering, probably by mitochondria, is the dominant factor in the constraints. In cell regions where the sequestering machinery is sufficiently dense, different Ca2+ message functions inside a cell may be effectively segregated, permitting private-line intracellular communication.