Calcium may be triggered to enter a cell by the binding of a chemical (primary) messenger to a plasma membrane protein receptor. A protein pore associated with the receptor may then open allowing calcium entry. Alternatively, the following pathway is initiated:
- the ligand-receptor complex, via activation of a membrane-bound G protein, activates the enzyme phosphoinositidase (phospholipase) C
- the enzyme cleaves a membrane lipid, phosphoinositide-4,5-biphosphate (PIP2) into diacylglyerol (DG) and inositol-1,4,5-triphosphate (IP3)
- IP3 is able to diffuse to intracellular calcium stores e.g. microsomes where it can trigger calcium ion release
- diacylglycerol is capable of activating a membrane-bound enzyme, protein kinase C, which phosphorylates, and so activates, further enzymes within the cell
- IP3 is broken down by a number of phosphatase enzymes within the cell and PIP2 is regenerated
Receptors which are capable of activating phospholipase C include:
- muscarinic receptors
- alpha-1-adrenoreceptors
- histamine H1 receptors
- serotonin receptors
- angiotensin receptors