Twenty Years of Calcium Imaging: Cell Physiology to Dye For

Putative mechanisms that might accomplish the versatility of Ca²⁺signal transduction. A. Quantitative and kinetic determination of the Ca²⁺ message by amplitude and frequency of global/cytosolic Ca²⁺ elevation. B. By generation of locally isolated Ca²⁺ gradients, Ca²⁺ selectively affects Ca²⁺-sensitive proteins. The schema illustrates the appearance of such areas with high (i.e., subplasmalemmal Ca²⁺ Control Unit; SCCU) and low (mitochondrial buffer unit; MBU) Ca²⁺ concentration between the plasma membrane and the respective organelle. C. By distribution of Ca²⁺-sensitive proteins to areas of distinct Ca²⁺ regulation, specificity of Ca²⁺-initiated signal transduction can be achieved. D. A local Ca²⁺ elevation recruits a Ca²⁺-activated scaffold protein that assembles a certain signaling pathway independently of global Ca²⁺ concentration. E. Ca²⁺ exhibits auto-regulatory properties and modulates subsequent Ca ²⁺ signaling.