Imaging Calcium in the Cytoplasm and in Organelles with Fluorescent Dyes: General Principles

Abstract

The notion that Ca²⁺ might play an important role in cell function became apparent only a little more than 100 years ago, when Ringer observed that Ca²⁺ ions were essential for the normal contraction of the frog heart. In 1957 Hodgkin and Keynes used ⁴⁵Ca²⁺ to trace the movements of the ion in the squid giant axon, and since that time many investigators have employed isotopes to examine Ca²⁺ fluxes in cells, tissues, or whole organs. These studies provided an important framework for future developments in the Ca²⁺ transport field, and continue to give important information to this day. Efforts to develop effective probes for measuring free cytosolic Ca²⁺, however, came much later, and were hampered in part by the lack of appreciation of just how low basal cytoplasmic Ca²⁺ could be. In fact, the tools for reproducing physiological Ca²⁺ concentrations ([Ca²⁺]) such as those found in the cytoplasm were unavailable until the introduction of Ca²⁺ selective chelators such as EGTA in 1964.