Figure 2.

IP₃ receptors are stimulated by IP₃ and Ca²⁺. (A) Many receptors, including G-protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs), can stimulate phospholipase C (PLC), leading to production of IP₃, which then binds to IP₃Rs in the endoplasmic reticulum (ER). (B) IP₃ binding to IP₃R primes them to bind Ca²⁺, which then stimulates the channel to open, allowing Ca²⁺ to flow out of the ER. (C) This dual regulation of IP₃Rs by IP₃ and Ca²⁺ allows them to mediate regenerative signals propagated by Ca²⁺-induced Ca²⁺ release (CICR). PM, Plasma membrane.