Mechanism Of Hormone Action: Overview, Functions

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.

Hormone 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:

Cell Membrane Receptors

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

• Hormones do not enter the cell

• Example: Insulin, epinephrine

Cytoplasmic Receptors

• For steroid hormones

• Hormones enter the cell and bind receptors in cytoplasm.

Nuclear Receptors

• For thyroid hormones (thyroxine)

• Hormone–receptor complex acts on receptors in the nucleus.

Classification of Hormones (Based on Chemical Nature)

The hormones are classified based on their chemical nature:

Peptide, Polypeptide & Protein Hormones

• Examples: Insulin, growth hormone

Steroids Hormones

• Examples: Cortisol, estrogen

Iodothyronines

• Examples: thyroxine (T4), triiodothyronine (T3)

Amino Acid Derivatives

• Examples: Epinephrine, norepinephrine

Mechanisms of Hormone Action

The mechanism of hormone action 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.

Steps:

• The hormone binds to a specific receptor on the cell membrane.

• Activation of adenyl cyclase

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

• cAMP causes a series of enzymatic reactions

• 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.

• Hormones diffuse across the plasma membrane.

• Hormones bind to the receptors present in the cytoplasm or nucleus.

• The hormone-receptor complex starts the DNA transcription.

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

Hormones as Regulators (Feedback Mechanisms)

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

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

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 (Neurohormones)

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 in Hormone Action

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 Hormone Action

Steroid and thyroid hormone mechanisms are described as follows:

• Hormone diffuses into the cell.

• Binds to receptors in the cytoplasm or the nucleus.

• Hormone-receptor complex initiates gene transcription.

• Newly synthesized proteins cause 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.

Hormonal Synergism and Antagonism

The effects of hormones are described below:

Synergistic Hormones

• Some hormones have effects that enhance each other.
• Example includes estrogen and progesterone.

Antagonistic Hormones

• Some hormones have effects on the body that are opposite or antagonistic to one another.
• Examples include insulin and glucagon.

Hormone Receptors & Action NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Hormone receptors and their location

• Mechanism of hormone action

Practice Questions for NEET

Q1. Which ONE of the following statements is TRUE concerning the human ovary?

1. Estrogen is secreted by Graafian follicles and progesterone by the corpus luteum.

2. Estrogen is secreted by the corpus luteum and progesterone by Graafian follicles.

3. Both estrogen and progesterone are secreted by the corpus luteum.

4. Both estrogen and progesterone are secreted by Graafian follicles.

Correct answer: 1) Estrogen is secreted by Graafian follicles and progesterone by the corpus luteum.

Explanation:

Graafian follicle – a fluid-filled structure in the mammalian ovary within which an ovum develops before ovulation. A corpus luteum refers to the composition of the cells that produce the hormone progesterone. The corpus luteum forms into the ovary after the discharge of an ovum. However, it degenerates after some days as its roles depend on fertilization.

Hence, the correct answer is option 1) Estrogen is secreted by Graafian follicles and progesterone by corpus luteum.

Q2. _____ Regulates sex characters, mammary gland development, development of growing ovarian follicles etc

1. Estrogen

2. Relaxin

3. Prolactin

4. Progesterone

Correct answer: 1) Estrogen

Explanation:

Estrogens produce wide-ranging actions such as stimulation of growth and activities of female secondary sex organs, development of growing ovarian follicles, the appearance of female secondary sex characters (e.g., the high pitch of voice, etc.), and mammary gland development. Estrogens also regulate female sexual behaviour

Hence, the correct answer is option 1) Estrogen.

Q3. Name a peptide hormone which acts mainly on hepatocytes, and adipocytes and enhances cellular glucose uptake and utilization.

1. Insulin

2. Glucagon

3. Secretin

4. Gastrin

Correct answer: 1) Insulin

Explanation:

Insulin - Insulin is a peptide hormone, which plays a major role in the regulation of glucose homeostasis. Insulin acts mainly on hepatocytes and adipocytes (cells of adipose tissue) and enhances cellular glucose uptake and utilization

Hence, the correct answer is option 1) Insulin.

Also Read-

• MCQs on Mechanism of Hormone Action

• Pituitary Gland

• Adrenal Gland

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