Mechanism of Hormone Action: Have you ever watched a game of football? Once the referee releases the ball, the players start passing the ball from one player to another till it reaches the goal. Every team player makes a combined effort to pass the hurdles by the opposite team and take the ball to the goal. This football analogy can be used to understand the mechanism of action of hormones if you consider the ball as a hormone and goal post as a target organ. Wonder how?? Our neural system controls and coordinates every activity of our body either through the neural network or by hormones. In this article, we will learn about the mechanism of hormone action in detail.

Hormones

The hormone is an organic substance released in a trace amount by the glandular tissue. Hormones act as a chemical messenger that carries information from our neural system to the target organ. Hormones are released directly into the blood by endocrine glands, where they travel to the target cells.

When a hormone is released in the bloodstream, it is directed to a particular kind of cell called the target cell. For example, Thyroid-stimulating hormones (TSH) will act only on the glandular tissue of the thyroid gland.

The target cells possess hormone receptors. These receptors are proteins in chemical nature and receive the hormone. These receptors are of two types:

1. Fixed membrane receptor: Attached to the surface of a cell
2. Mobile receptor: Intracytoplasmic, i.e., present inside cytoplasm.

Location of the Receptors

1. Protein Hormones (Insulin) – Cell surface
2. Steroid Hormones (Androgen) – In the cytoplasm
3. Thyronine (Thyroxine) – Cell nucleus

Fig: A cell Can be the Target for One or More than One Hormone at a Time

Types of Hormones

Hormones can be categorized into based on their chemical composition:

1. Peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.)
2. Steroids (e.g., cortisol, testosterone, estradiol and progesterone)
3. Iodothyronines (thyroid hormones)
4. Amino-acid derivatives (e.g., epinephrine).

Hormones approach the target cell through their plasma membrane, and since lipid is a major constituent of the plasma membrane, hormones are broadly classified into two types:

1. Lipophobic or protein Hormones: These hormones are amino acid derivatives and made up of protein. Since they are water-soluble and insoluble in lipids, they are unable to diffuse through the plasma membrane. These hormones require a localized receptor on the cell membrane and do not actually enter inside the cell. As soon as the hormone binds to the receptor, a cascade is triggered inside the cytoplasm, which involves secondary messengers, such as cyclic AMP (cAMP). Example: Thyroid-stimulating hormones (TSH), follicle-stimulating hormones (FSH), luteinizing hormones (LH), and insulin.
2. Lipophilic hormones: These hormones are not made up of protein. They can easily diffuse through the plasma membrane and do not require any receptor on the cell surface. A receptor for lipophilic hormones is located inside the cytoplasm or nucleus. As soon as the hormone enters the cytoplasm, they bind to the receptor and activate the transcription cascade. The majority of Lipophilic hormones act as transcription factors which means that they can initiate transcription of a particular gene to obtain gene products. Example: thyroid hormones – Iodothyronines, Steroids- glucocorticoids, mineralocorticoids, androgens, oestrogens.

Mechanism of Action of Hormone

The mechanism of action of hormones depends upon the chemical nature of the hormone and the type of receptor. lipophobic hormones require fixed membrane receptors, whereas lipophilic hormones require mobile receptors to trigger the action.

Mechanism of Action for Lipophobic or Protein Hormones through Fixed Receptor

Receptors for Lipophobic hormones like Thyroid-stimulating hormones (TSH), follicle-stimulating hormones (FSH), luteinizing hormones (LH), and insulin are present on the cell surface attached to the plasma membrane.

Once the hormone binds to a specific receptor on the target cell, it activates the enzyme Adenylyl Cyclase in the cell membrane and causes the production of cyclic AMP (cAMP). cAMP is a secondary messenger. It diffuses through the cell membrane and activates (Protein Kinase). Protein kinases dephosphorylate ATP, i.e. use ATP to activate various enzymatic reactions to cause biochemical changes. After the target cell responded to the changes, cAMP is deactivated by a group of the enzyme phosphodiesterase. The action is depicted below in the form of a flow chart.

Fig: Mechanism of Action of Lipophobic Hormone

Fig: Diagrammatic Representation of the Mechanism of Action of Lipophobic Hormone

Once the desired physiological response is achieved, cAMP is deactivated by an enzyme called phosphodiesterase.

Mechanism of Action of Lipophilic Hormones through Intracellular Receptors

Receptors for hormones like Iodothyronines, glucocorticoids, mineralocorticoids, androgens, etc., are present inside the cytoplasm. Since these hormones are lipophilic, they are readily diffusible inside the cytoplasm and bind with the mobile receptor present inside the cytoplasm. The hormone-receptor complex moves to the nucleus, where they initiate transcription of the DNA to form specific mRNA. mRNA is translated into protein in the cytoplasm. The protein (enzyme) causes biochemical changes in the cell.

The action is depicted below in the form of a flow chart.

Fig: Mechanism of Action of Lipophilic Hormones

Fig: Mechanism of Action of Lipophilic Hormone

Difference Between Intracellular Receptors and Fixed Membrane Receptors

Intracellular Receptor	Fixed Membrane Receptor
These are present inside the cytoplasm.	These are present on the cell surface.
Their action is mediated by the receptor-hormone complex.	Their action is mediated by secondary messenger.
They act as a transcription factor.	They do not act as a transcription factor.
They bind to hydrophobic ligands (hormone).	They bind to hydrophilic ligands (hormones).
Examples: Steroids, thyroxine	Examples: Protein hormones like insulin

The Function of Hormone Action

There are mainly two functions achieved due to hormone action:

1. They serve as messengers: Neurohormones are produced and released by neuroendocrine cells (also called neurosecretory cells)of hypothalamic nuclei into the blood. The neurohormones are released by the hypothalamus.  The neurohormones stimulate the pituitary gland to release various hormones. Hence they are also called “releasing factors”. These actions of releasing factors stimulate another gland or organ for functioning; hence they are called neuro messengers.
2. They serve as regulating factors: Hormones regulate the internal body temperature in humans by homeostasis. They also regulate the secretion of a certain hormone based on the cell and body requirements by the feedback mechanism. The feedback mechanism focuses on returning the body to its normal state. There are two kinds of feedback mechanisms- positive and negative. The majority of hormones are regulated by negative feedback mechanisms. It also includes the concept of positive feedback control and negative feedback control.
   (a) In negative feedback regulation, a stimulus triggers the release of a chemical like enzymes etc. Once the substance reaches a certain level, it sends a signal to stop further release of the substance so that the concentration of hormones in the blood is maintained within a narrow range. For example- the anterior pituitary signals the thyroid to release thyroid hormones, and when there is an adequate amount of thyroxine in the blood, it sends negative feedback.
   (b) Positive feedback mechanism triggers to increase production of certain substances. For example, milk production by a mother for her baby when the baby suckles on mothers nipples, as the nerve in the nipple sends the message to the pituitary to produce prolactin to enhance milk production.

Summary

Hormones are the chemical messengers which are released for the controlled and coordinated function of the body. Hormones act on a specific target cell that possesses receptors. These receptors can be fixed on the surface of the cytoplasm or can be mobile and be present inside the cytoplasm. The hormones are of two types: Lipophilic and Lipophobic. Lipophobic hormones are insoluble in water, and they can not diffuse into the cytoplasm. They require a fixed receptor for action. Once the hormone binds to the receptor, the chain of biochemical response occurs, which ultimately causes a physiological response.

Lipophilic hormones readily diffuse inside the cytoplasm through the plasma membrane and bind to the mobile or intracellular receptor. Once the hormone enters the cytoplasm, it forms a hormone-receptor complex, moving to the nucleus and triggering gene expression. mRNA produced due to gene expression is then translated to obtain the desired physiological effect. The hormone acts in two main ways: regulatory action and Messenger action. The regulatory function involves roles like maintaining internal body temperature. Messenger action involves releasing neurohormones to direct other glands to release hormones based on body requirements.

Frequently Asked Question (FAQs) on the Mechanism of Action of Hormone

Q.1. What do you mean by hormone action?
Ans: Hormone action refers to the release and activity of hormones on the target cell. Hormone activates the target cell by binding to the receptor and causing a chain of biochemical events that manifests in physiological response.

Q.2. What is the function of a hormone?
Ans: The main function of the hormone is to control and coordinate bodily function. Hormones have a regulatory role, which means they can start and stop certain biochemical reactions.

Q.3. What is the use of hormones in our body?
Ans: Hormone controls the entire functionality of the body. It decides the growth, metabolism, sexuality, and even emotional response of the human body.

Q.4. What are steroidal hormones?
Ans: Steroid hormones are a group of hormones derived from cholesterol that act as chemical messengers in the body. Example: Androgens, estrogens, etc.

Q.5.How many hormones are in our body?
Ans: There are more than 50 types of hormones identified in the body so far.

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