Figure 2.

Basic mechanisms driving a diffusive Ca²⁺ wave across a cell (either initiating or communicating the wave; see sect. III). Inositol 1,4,5-trisphosphate (IP₃) diffuses across the cell and binds to an IP₃ receptor (IP₃R) to stimulate Ca²⁺ release from the endoplasmic reticulum (ER). The positive feedback (+ve) of Ca²⁺ on the IP₃R initiates Ca²⁺-induced Ca²⁺ release (CICR) through the IP₃R. The elevated [Ca²⁺]_i has a negative feedback (−ve) on the IP₃R to terminate CICR and limit the [Ca²⁺]_i increase. Some Ca²⁺ may diffuse to adjacent ryanodine receptors (RyR) to initiate CICR via these receptors. However, IP₃ diffusion occurs more quickly than Ca²⁺ diffusion (due to cytosol protein buffers) and propagates the Ca²⁺ wave more efficiently. IP₃ may have been produced in response to cell stimulation or may have entered the cell via a gap junction.