FIGURE 1.

The binding of an agonist stimulates IP₃ production. The agonist activates the membrane-bound enzyme phospholipase C, which hydrolyzes phosphatidylinositol bisphosphate into IP₃ and diacylglycerol. IP₃ activates IP₃R, releasing calcium from the ER. This triggers further release of calcium from the ER through the IP₃R and RyR. There is a positive feedback mechanism whereby calcium can activate phospholipase C. Calcium ions are bound to buffers in the cytosol and ER. Calcium is removed from the ER through the SERCA pump and membrane pump.