Biochemistry · MU Pleven

56. Mechanism of action of hormones binding cell membrane receptors. Hormone- receptor interactions. Role of Second Messengers: DAG and Inositol triphosphate.

Covers how water-soluble hormones act through cell-surface receptors and G-proteins, and how receptor number and binding strength shape the response, including up- and down-regulation with insulin resistance as an example. It then traces the phospholipase C pathway, where PIP₂ splits into DAG and IP₃, and describes how IP₃ releases calcium while DAG activates protein kinase C. A closing list gives hormones that use this pathway, such as vasopressin, oxytocin and angiotensin II.

Inside: 4 sections · 1 labelled picture · 20 flashcards · 3 min

Topic 56 of 76 in the MU Pleven syllabus for Biochemistry (Medicine, Year 2). The free AceMed note covers it in 4 sections, about 385 words (3 min of reading), with 20 flashcards.

In this note

  1. Mechanism of action of hormones binding cell membrane receptors.
  2. Mechanism of Action of Hormones Binding to Cell Membrane Receptors
  3. Hormone-Receptor Interactions
  4. Role of Second Messengers: DAG and Inositol Triphosphate (IP₃)

Mechanism of Action of Hormones Binding to Cell Membrane Receptors

  • This pathway applies to water-soluble hormones such as many peptide hormones and some catecholamines.
  • Because these hormones cannot cross the lipid cell membrane, they bind to receptors located on the cell surface.
  • Once the hormone binds, the receptor changes conformation and activates a G-protein on the inner side of the membrane.
  • This G-protein acts like a “switch” that can turn intracellular signaling pathways on or off, depending on which subunit is activated.

Hormone-Receptor Interactions

  • The hormone fits the receptor in a highly specific way (lock-and-key model).
  • The strength of binding and the number of receptors determine how strong the cellular response will be.
  • Cells can up-regulate receptors (increase sensitivity) or down-regulate them (reduce sensitivity).
  • This is important in conditions like insulin resistance, where fewer receptors reduce the cell’s responsiveness.
  • Once the receptor is activated, it transmits the signal without the hormone entering the cell.

Role of Second Messengers: DAG and Inositol Triphosphate (IP₃)

This signaling pathway is called the Phospholipase C (PLC) Pathway.

How the signal proceeds inside the cell

  • Activated receptor stimulates a Gq protein inside the membrane.
  • Gq protein activates the enzyme Phospholipase C (PLC).
  • PLC acts on a membrane phospholipid called PIP₂ (phosphatidylinositol 4,5-bisphosphate).
  • PIP₂ is split into two second messengers:
  • DAG (Diacylglycerol)
  • IP₃ (Inositol Triphosphate)

These two then produce different effects inside the cell.

Function of IP₃

IP₃ diffuses into the cytoplasm and binds to IP₃ receptors on the endoplasmic reticulum.

This causes release of Ca²⁺ (calcium ions) into the cytosol.

The increase in Ca²⁺:

  • Activates various Ca²⁺-dependent enzymes
  • Helps control muscle contraction, secretion, metabolism, and signaling

Calcium acts as a potent amplifier of the hormonal signal.

Function of DAG

DAG remains in the cell membrane.

Its main function is to activate Protein Kinase C (PKC).

PKC then:

  • Phosphorylates (modifies) internal proteins
  • Alters activity of enzymes
  • Changes cell function quickly

This leads to a coordinated, targeted response inside the cell.

Combined Effect of IP₃, Ca²⁺, and DAG

These molecules work together to produce:

  • Enzyme activation
  • Hormone release (exocytosis)
  • Smooth muscle contraction
  • Changes in gene expression (in some longer-term actions)

This system allows one hormone molecule to create a large amplified response.

Examples of Hormones Using the IP₃–DAG Pathway

  • Vasopressin (V1 receptors)
  • Angiotensin II
  • Oxytocin
  • α₁-adrenergic agonists (norepinephrine on α₁ receptors)
  • TRH (thyrotropin-releasing hormone)

These actions are usually rapid and crucial in:

  • Smooth muscle contraction (e.g., blood vessels)
  • Glandular secretion (e.g., pituitary, pancreas)

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