Mechanism Of Hormone Action: Overview, Functions

The mechanism of hormone action explains how hormones interact with target cells to regulate various biological processes. Hormones bind to specific receptors on or inside the cells, triggering a series of events that influence cell function. This process can involve changes in gene expression, enzyme activity, or cell signalling pathways. In this article, hormones, receptors and their locations, classification of hormones, mechanisms of hormone action, hormones as regulators, hormones as messengers, signalling pathways, steroid and thyroid hormones, examples of hormone action, regulation of hormonal activity, and hormonal synergism and antagonism are discussed. Mechanism of hormone action is a topic of the chapter Chemical Coordination and Integration chapter of Biology.

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This Story also Contains

1. Definition of Hormones
2. Receptors and their Locations
3. Classification of Hormones
4. Mechanisms of Hormone Action
5. Hormones As Regulators
6. Hormones As Messengers
7. Signaling Pathways
8. Steroid and Thyroid Hormones
9. Examples Of Hormone Action
10. Regulation of Hormonal Activity
11. Hormonal Synergism And Antagonism

Definition of Hormones

A hormone is a small chemical messenger that travels in the blood to help maintain internal balance or homeostasis in the human body. The definition only scratches the surface, as hormones play roles in many complicated functions within varied systems.

Receptors and their Locations

Hormones work through specific receptors, and sensitivity and responsiveness depend on the number of receptors along with their affinity for the hormone. Receptors are located in various sites, such as:

At the Cell Membrane:

Example: Protein or peptide hormones, and also catecholamines, act through receptors on the cell membrane.

Inside the Cytoplasm:

Example: Steroid hormones bind to receptors inside the cytoplasm.

In the Cell Nucleus:

For example, thyroxine acts on receptors in the nucleus.

Classification of Hormones

The hormones are classified based on their chemical nature:

Peptide, Polypeptide, and Protein Hormones: Examples are insulin and growth hormone.

Steroids:

Examples are cortisol and estrogen.

Iodothyronines:

Examples are thyroxine and triiodothyronine.

Amino Acid Derivatives:

Examples are epinephrine and norepinephrine.

Generally, hormones acting through membrane-bound receptors do not directly enter the target cells. On the contrary, they form second messengers that act to control cellular metabolism. In contrast, hormones acting through intracellular receptors most often work by regulating gene expression or chromosome function by the interaction of hormone-receptor complexes with the genome. All these biochemical effects ultimately lead to physiological and developmental responses.

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Mechanisms of Hormone Action

The mechanism of action of the hormone is broadly classified into two types:

Fixed Membrane Receptor Mechanism

This mechanism is characteristic of water-soluble hormones such as amines or proteins, including growth hormone, oxytocin, and antidiuretic hormone. These hormones cannot pass through the lipid membrane and thus bind to receptors located on the cell membrane.

Hormone Binding:

The hormone binds to its specific receptor on the cell membrane.

Activation Of Adenyl Cyclase:

The binding activates the enzyme adenyl cyclase.

Production Of Camp:

The adenyl cyclase converts ATP to cyclic AMP, which acts as the secondary messenger.

Cellular Response:

cAMP freely diffuses in the cell and causes a series of enzymatic reactions finally leading to Biochemical changes

Inactivation Of Camp:

The enzyme phosphodiesterase inactivates the action of cAMP.

Mobile Receptor Mechanism

This mechanism occurs through lipid-soluble hormones like fatty acids and steroids which easily pass through the plasma membrane. The actions of these hormones are through intracellular receptors.

Diffusion In Cell:

The hormones diffuse across the plasma membrane.

Binding To Intracellular Receptor:

The hormones bind to the receptors present in the cytoplasm or nucleus.

Gene Transcription:

The hormone-receptor complex starts the DNA transcription.

Protein Synthesis:

mRNA is translated into proteins, which then produce biochemical changes within the cell.

Hormones As Regulators

The hormones form an important part of the regulation of the body's internal environment. The secretory activity of the hormone can be regulated through the feedback mechanism, which includes :

Positive Feedback Control:

This is a process whereby the end products of an action further enhance the action in a feedback loop. Examples include blood clotting and the menstrual cycle.

Negative Feedback Control:

The final product of an action reduces the stimulus for the same action. Examples include thermoregulation and control of levels of blood sugar.

Hormones As Messengers

The hypothalamic neurosecretory cells release the hormones, also called neurohormones, into the blood. These neurohormones diffuse to the pituitary gland and there trigger the release of several other hormones. For this reason, they are also called "releasing factors."

Signaling Pathways

The signalling pathways are:

Receptor Binding:

The hormone binds to specific receptors on the exterior cell membrane.

Second Messenger Activation:

The binding activated second messengers inside the cell, namely, cAMP, IP3, or DAG.

Cellular Response:

The second messengers trigger a series of actions inside the cell, which ultimately lead to physiological responses.

Steroid and Thyroid Hormones

Steroid and thyroid hormone mechanisms are described as follows:

Cell Diffusion:

The hormone diffuses through the plasma membrane into the cell.

Binding To Intracellular Receptor:

The hormone binds to receptors in the cytoplasm or the nucleus.

Gene Transcription:

The hormone-receptor complex binds to DNA and initiates gene transcription and protein synthesis.

Cellular Response:

The newly synthesized proteins then cause the physiological response.

Examples Of Hormone Action

The examples are mentioned below:

Insulin:

Binds to insulin receptors on the plasma membrane of cells and initiates the signaling transduction which provokes glucose uptake of the cells to decrease blood sugar.

Cortisol:

Diffuses into cells, binds with the intracellular receptor and controls the gene expression that provokes increased production of glucose and anti-inflammatory effects.

Epinephrine:

Binds with the adrenergic receptors of plasma membranes of cells and activates the signal transduction pathway that provokes heartbeats and energy availability.

Regulation of Hormonal Activity

Read about the regulation of hormonal activity:

Feedback Mechanisms:

Feedback mechanisms ensure that the levels of hormones are kept within the homeostatic range. High levels of hormones result in negative feedback that inhibits further release of the hormone.

Receptor Sensitivity:

Receptors may change their sensitivity to the hormone, either intensifying or decreasing the response to a given amount of hormone.

Hormonal Synergism And Antagonism

The effects of hormones are described below:

Synergistic Effects:

Some hormones have effects that enhance each other. The classic example is that of estrogen and progesterone, working together to regulate the menstrual cycle.

Antagonistic Effects:

Some hormones have effects on the body that are opposite or antagonistic to one another. Some classic examples include insulin and glucagon, which respectively lower and raise blood sugar.

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