ATP Release Via Connexin Hemichannels Controls Intercellular Propagation of Ca Waves in Corneal Endothelial Cells - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 162

Fig. 10.1 ( a ) Mechanism of the Ca 2+ wave propagation between cells. A specific stimulus induces
aCa 2+ transient in the stimulated cell (cell 1) via Ca 2+ influx and/or Ca 2+ release from intracellular
stores (ER). Diffusion of Ca 2+ or IP 3 through the gap junctions (GJ) or secretion of an agonist into
the extracellular space that diffuses to neighboring cells can induce a Ca 2+ increase in the neighbor-
ing cells, resulting in gap junctional intercellular communication (GJIC) or paracrine intercellular
communication (PIC), respectively. ( b ) Structure of connexin, connexon hemichannels and gap
junction channels. Top : One connexin molecule, showing the four typical transmembrane domains
(M1-M4), together with the one cytoplasmic (CL) and the two extracellular (EL1 and EL2) loops.
The N- and C-terminal domains face the cytoplasm and are very divergent between connexins. Six
connexins form a connexon, which can be homomeric or heteromeric, depending on the connexin
isoform composition of the connexon. Connexons from adjacent cells can dock to form gap junc-
tion channels. Each cell can contribute the same or a different type of connexon, giving rise to either
homotypic or heterotypic intercellular channels. Connexons can also be present in non-junctional
membranes as connexon hemichannels