Fig. 4. G-protein receptor signaling system. On activation by a ligand (such as acetylcholine), GTP is exchanged for GDP and the α subunit of GTP activates phosphophase C, which, in turn, hydrolyses phosphatidylinositol bisphosphate (PIP2) to form IP3. The IP3-activated calcium channel in the membrane releases stored calcium.92 The state of the store is relayed to a plasma membrane calcium channel by an unknown mechanism, and calcium can enter through this “capacitative” route.99 In the lens, this pathway is quite calcium specific.100 Calcium oscillations are possible as calcium is actively pumped into the emergency department from the cytoplasm by an active, thapsigargin-sensitive calcium pump.