How do hormones work ? What is the mechanism of hormone action ?

I have been asked this question a lot of times. That is, how do hormones work? What is the mechanism of hormone action?

Several theories of hormone action have been pro­posed by scientist and doctors. Many polypeptide and amine hormones act at the cell membrane, whereas the steroid hor­mones have to reach the cell nucleus before influencing the cellular events.

Mechanism of peptide hormone action

A peptide hormone (first messenger) interacts at the receptor site on the cell membrane. These interactions lead to an increase or decrease in the activity of adenyl cyclase-A component of the cell membrane which results in an increase or decrease in the concentration of cyclic AMP (second mes­senger) within the cell.

The cyclic AMP acts on protein kinases which have two sub-units-regula­tory (R) and catalytic (C). The regulatory unit inhibits the catalytic unit. Cyclic AMP combines with the regulatory unit and releases the catalytic unit which phosphorylates proteins.

In addition to cyclic AMP, many other second messengers have been postulated, one of which is calcium. Both hormone and cyclic AMP increase calcium entry into the cell. This increase in calcium in turn inhibits adenyl cyclase, thus forming a negative feed-back mechanism.

Mechanism of steroid hormone action

The steroid hormone on reaching the target cells diffuses into the cytoplasm where it gets attached to a specific binding protein-receptor. This steroid- receptor complex then enters the nucleus and interacts with the chromatin to bring about in­creased rate of synthesis of specific mRNA.

The resulting mRNA molecules enter the cytoplasm where they are translated into molecules of corre­sponding specific proteins. These proteins account for the ultimate biologic effects of the hormone.

The hypothalamus, along with the pituitary gland, thus acts as an intimate link between the nervous system and the endocrine system.

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