Calcium Is a Second Messenger in Many Signal Transductions

In many cells that respond to extracellular signals, Ca2 + serves as a second messenger that triggers intracellu-lar responses, such as exocytosis in neurons and endocrine cells, contraction in muscle, and cytoskeletal rearrangement during amoeboid movement. Normally, cytosolic [Ca2+] is kept very low (<10~7 m) by the action of Ca2+ pumps in the ER, mitochondria, and plasma membrane. Hormonal, neural, or other stimuli cause either an influx of Ca2+ into the cell through specific

Hormone (H) binds to a specific receptor.

Extracellular space

Phospholipase C (PLC)

Plasma membrane

Extracellular space

Phospholipase C (PLC)

Plasma membrane

FIGURE 12-19 Hormone-activated phospholipase C and IP3.Two intracellular second messengers are produced in the hormone-sensitive phosphatidylinositol system: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. Both contribute to the activation of protein kinase C. By raising cytosolic [Ca2+], IP3 also activates other Ca2+-dependent enzymes; thus Ca2+ also acts as a second messenger.

TABLE 12-5 Some Signals That Act through Phospholipase C and IP3

Acetylcholine [muscarinic MJ

Gastrin-releasing peptide

Platelet-derived growth factor (PDGF)

^-Adrenergic agonists


Serotonin [5-HT-1c]*


Gonadotropin-releasing hormone (GRH)

Thyrotropin-releasing hormone (TRH)

Angiotensin II

Histamine [HJ*


ATP [P2x and P2y]*

Light (Drosophila)

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