Signal transduction is highly regulated

There are several ways in which cells can regulate the activity of a transducer. The concentration of NO, which breaks down quickly, can be regulated only by how much of it is made. The level of Ca2+, on the other hand, is determined by both membrane pumps and ion channels. For protein kinase cascades, G proteins, and cAMP, there are enzymes that convert the activated transducer back to its inactive precursor:

► Protein phosphatases remove the phosphate groups from phosphorylated proteins.

► GTPases convert the GTP on an active G protein back to GDP, inactivating the protein.

► cAMP phosphodiesterase converts cAMP into its precursor, AMP, which has no second messenger activity.

ACh ACh

ACh receptor

ACh ACh

ACh receptor

Action Ach Endothelial Cells

I Acetylcholine binds to receptors on endothelial cells of blood vessels, opening Ca2+ channel.

NO synthase, the enzyme that makes nitric oxide gas (NO) from arginine.

I. . .where it stimulates cGMP synthesis.

I Acetylcholine binds to receptors on endothelial cells of blood vessels, opening Ca2+ channel.

Smooth muscle cell

NO synthase, the enzyme that makes nitric oxide gas (NO) from arginine.

I. . .where it stimulates cGMP synthesis.

Smooth muscle cell

15.13 Nitric Oxide as a Second Messenger Nitric oxide (NO) is an unstable gas, which nevertheless serves as a second messenger between a primary signal, acetylcholine, and its effect, the relaxation of smooth muscles. The endothelial cells that line blood vessels, seen in (a), are crucial intermediaries in this signal transduction pathway (b).

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Essentials of Human Physiology

Essentials of Human Physiology

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