Fig. 1. Diagram showing the topological structure of connexin and roles of gap junction channels and hemichannels. (A) A connexin (Cx) protein has one intracellular and two extracellular loops and intracellular N- and C-terminals, and six homomeric or heteromeric Cx proteins compose a functional gap junction channel. (B) Homotypic or heterotypic gap junction channels coupled between cells pass molecules less than 1 kD, such as Ca²⁺, IP₃, cAMP, cGMP, glutamate, glucose, ATP, and ADP, but unopposed hemichannels, upon their activation, release those molecules, particularly, ATP, glutamate, and prostaglandins.