QS Study

A hormone is a chemical substance that is secreted into the internal body fluid by one cell or group of cells and exerts a physiological control effect on other cells of the body. It affects our body’s functions, from growth and sexual development and mood to how well we sleep, how we manage stress, and how our body breaks down food.

Mechanism of action of hormones

(A) Hormone receptor and their activation: The first step of hormone action is to bind to a specific receptor at the target cell. When the hormone combines with its receptor-hormone complex is formed. This usually initiates a cascade of reactions in the cell. These hormones are water soluble and cannot passes through the lipid membrane and they have their target receptor on the cell membrane.

Locations for the different types of hormone receptors are –

  1. In or on the surface of cell membrane for protein, peptide and catecholamine hormones.
  2. In the cell cytoplasm for steroid hormones.
  3. In the cell nucleus for thyroid hormones.

The decrease in the production of the receptor is known as Down-Regulation of a receptor. It results in the decreased responsiveness of the target tissue to the hormone.

Increased production of the receptor is known as up-regulation of the receptor. It results in the increased responsiveness of the target tissue to the hormone.

Examples of down-regulation –

  • Receptor-mediated endocytosis,
  • Desensitization,

In this process, receptors are chemically modified.

When a hormone or a neurotransmitter is present in excess, downregulation usually occurs.


  • Angiotensin II in its action on the Adrenal cortex.
  • It increases rather than decreases the number of its receptors in the adrenal.

Fig: Mechanism of Hormonal Action

(B) Intracellular signaling after hormone receptor activation:

(1) Change in membrane permeability: Hormone binds to a membrane receptor and causes a change in the structure of the receptor, usually opening or closing a channel for one or more ions. Thus it changes membrane permeability.


  • Insulin increases the permeability of glucose and ammo and into the cell.
  • Aldosterone promotes Na+ influx to distal constituted tubular cells and efflux and K+ and H+.
  • ADH promotes the permeability of water to the cells of Distal Convoluted Tubule and collecting ducts of the kidney.

(2) Activation of intracellular enzymes when the hormone combines with membrane receptor.

Hormone binds to membrane receptors and activates an enzyme immediately inside the cell membrane. Example:

  • Insulin binds with membrane receptor that protrudes the exterior of the cell,
  • A structural change in receptor molecule itself,
  • A portion of the receptor that protrudes to the inside to become an activated kinase,
  • Promotes phosphorylation of different substances inside the cell.

(3) Activation of genes by binding with the intracellular receptor.

  • Steroid and thyroid hormones bind with protein receptors,
  • Activate specific portion of DNA strand,
  • Initiate transcription of specific genes,
  • Formation of mRNA,
  • Translocation of a new protein,
  • Performance of new function.

(C) “Second Messenger” Mechanism for mediating intracellular Hormonal function.

Hormones exert intracellular actions by 2nd messenger.

  • Hormones bind with membrane receptor,
  • Activation of the 2nd messenger,
  • An intracellular function takes place.

Different 2nd messenger systems are –

(i) Adenyl cyclase,

(ii) Ca-Calmodulin,

(iii) A breakdown product of phospholipase

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