Twenty years of calcium imaging: Cell physiology to dye for

The use of fluorescent dyes over the past two decades has led to a revolution in our understanding of calcium signaling. Given the ubiquitous role of Ca²⁺ in signal transduction at the most fundamental levels of molecular, cellular, and organismal biology, it has been challenging to understand how the specificity and versatility of Ca²⁺ signaling is accomplished. In excitable cells, the coordination of changing Ca²⁺ concentrations at global (cellular) and well-defined subcellular spaces through the course of membrane depolarization can now be conceptualized in the context of disease processes such as cardiac arrhythmogenesis. The spatial and temporal dimensions of Ca²⁺ signaling are similarly important in non-excitable cells, such as endothelial and epithelial cells, to regulate multiple signaling pathways that participate in organ homeostasis as well as cellular organization and essential secretory processes.