Figure 4. CxHcs and Ca²⁺ signalling.

(A) CxHcs open in response to cytoplasmic Ca²⁺ changes and thereby form a conduit for the release of messengers such as ATP and others; CxHcs only open in so-called ‘trigger cells’. (B) ATP diffuses into the extracellular space and activates G-protein-coupled serpentine receptors on neighbouring cells. This results in the activation of phospholipase C, the formation of InsP₃ and the release of Ca²⁺ from the endoplamic reticulum. This pathway underlies paracrine cell–cell communication of Ca²⁺ signals. (C) Ca²⁺ signals can also be communicated by the diffusion of InsP₃ or Ca²⁺ via gap junctions connecting cells. (D) The extracellular ATP concentration gradually decreases and the communication of Ca²⁺ signals stops unless another trigger cell is encountered that regenerates the ATP signal. (E) Ca²⁺-triggered ATP release via CxHcs may also be involved in Ca²⁺ oscillations in the cell via an autocrine signalling path (see the text). (F) Cytoplasmic Ca²⁺ changes can trigger CxHc opening, and conversely, open CxHcs may magnify Ca²⁺ changes by Ca²⁺ entry from the extracellular space.