Hormones That Use Second Messengers

Hormones that are catecholamines (epinephrine and norepinephrine), polypeptides, and glycoproteins cannot pass through the lipid barrier of the target cell's plasma membrane. Although some of these hormones may enter the cell by pinocytosis, most of their effects result from their binding to receptor proteins on the outer surface of the target cell membrane. Since they exert their effects without entering the target cells, the actions of these hormones must be mediated by other molecules within the target cells. If you think of hormones as "messengers" from the endocrine glands, the intracellular mediators of the hormone's action can be called second messengers. (The concept of second messengers was introduced in connection with synaptic transmission in chapter 7.) Second messengers are thus a component of signal-transduction mechanisms, since extracellular signals (hormones) are transduced into intracellular signals (second messengers).

Dimerization

TR receptor

— (for triiodothyronine)

TR receptor

— (for triiodothyronine)

9-cis-Retinoic acid

Dimerization

9-cis-Retinoic acid

Triiodothyronine

Hormone- Genetic response transcription element I

Triiodothyronine

Hormone- Genetic response transcription element I

VmRNA

■ Figure 11.7 The receptor for triiodothyronine (T3). The nuclear receptor protein for T3 forms a dimer with the receptor protein for 9-cis-retinoic acid, a derivative of vitamin A. This occurs when each binds to its ligand and to the hormone-response element of DNA. Thus, 9-cis-retinoic acid is required for the action of T3. The heterodimer formed on the DNA stimulates genetic transcription.

When these hormones bind to membrane receptor proteins, they must activate specific proteins in the plasma membrane in order to produce the second messengers required to exert their effects. On the basis of the membrane enzyme activated, we can distinguish second-messenger systems that involve the activation of (1) adenylate cyclase, (2) phospholipase C, and (3) tyrosine kinase.

Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

Get My Free Ebook


Post a comment