Calcium ions play an essential role in regulating cellular responses to many signals, and many GPCRs and other types of receptors exert their effects on cells by influencing the cytosolic concentration of Ca²⁺. As we saw in Chapter 11, the level of Ca²⁺ in the cytosol is tightly maintained at a submicromolar level (0.1 µM = 100 nM) by the continuous action of ATP-powered Ca²⁺ pumps and Na⁺/Ca²⁺ antiporters, which transport Ca²⁺ ions against their concentration gradient across the plasma membrane to the cell exterior or into the lumen of the endoplasmic reticulum.

A small rise in cytosolic Ca²⁺ induces a variety of cellular responses, including hormone secretion by endocrine cells, secretion of digestive enzymes by pancreatic acinar cells, and contraction of muscle (Table 15-4). For example, acetylcholine stimulation of GPCRs in secretory cells of the pancreas and of the parotid (salivary) glands induces a rise in cytosolic Ca²⁺ that triggers the fusion of secretory vesicles with the plasma membrane and release of their protein contents into the extracellular space. Thrombin, an enzyme in the blood-clotting cascade, binds to a GPCR on blood platelets. This binding activates the receptor and triggers a rise in cytosolic Ca²⁺, which in turn causes a conformational change in the platelets that leads to their aggregation, an important step in blood clotting to prevent leakage of blood out of damaged blood vessels.

We learned in Chapter 12 that increases in the concentration of free Ca²⁺ in the mitochondrial matrix accelerate pyruvate oxidation and ATP production. Thus, in muscle, increases in the concentration of Ca²⁺ are used both to induce contractions and to coordinately increase mitochondrial ATP synthesis to provide the energy to fuel those contractions.

In this section, we first discuss experimental tools scientists use to measure the concentration of free Ca²⁺ ions—that is, Ca²⁺ ions that are not tightly bound to proteins—in organelles such as the endoplasmic reticulum and the mitochondrion. Our main focus is on an important signal transduction mechanism that results in an elevation of cytosolic Ca²⁺ ion concentrations: the GPCR-stimulated activation of a phospholipase C (PLC). PLCs are a family of enzymes that hydrolyze a phosphoester bond in certain phospholipids, yielding two second messengers that function in elevating both the cytosolic and mitochondrial-matrix Ca²⁺ levels and in activating a family of cytosolic kinases known as protein kinases C (PKCs); PKCs, in turn, affect many important cellular processes such as growth and differentiation as well as altering the activity of many proteins.

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Some PLCs are activated by GPCRs, as we describe here; others, covered in the following chapter, are activated by other types of receptors. Phospholipases C also produce second messengers that are important for remodeling the actin cytoskeleton (see Chapter 17) and for the binding of proteins important for endocytosis and vesicle fusion (see Chapter 14). Later in this section, we will see how second messengers such as Ca²⁺ are used to help cells integrate their responses to more than one extracellular signal. In the final part of the section, we see how one PLC pathway leads to the synthesis of a gas, nitric oxide (NO), which diffuses out of the cell into adjacent cells, where it can induce activation of a kinase that, in turn, alters several cellular activities.