The mechanism of hormone action involves complex biochemical processes that allow hormones to regulate various physiological functions in the body.
Hormones are chemical messengers secreted by endocrine glands into the bloodstream, where they travel to target cells or organs to exert their effects.
The action of hormones can be broadly classified into two types based on their chemical nature and receptor location: lipid-soluble (steroid hormones and thyroid hormones) and water-soluble (protein/peptide hormones and amine hormones) hormones.
Here’s how these mechanisms work:
1. Lipid-soluble Hormones (Steroid and Thyroid Hormones)
Lipid-soluble hormones easily pass through the cell membrane of the target cell due to their lipophilic nature.
Their receptors are typically located inside the cell, either in the cytoplasm or the nucleus.
Mechanism:
Entry: The hormone diffuses through the plasma membrane of the target cell.
Binding: It binds to a specific intracellular receptor, forming a hormone-receptor complex.
Activation: The binding alters the receptor's shape, allowing it to enter the nucleus (if it's not already there) and bind to specific DNA sequences.
Transcription: This binding to DNA promotes or inhibits the transcription of specific genes, leading to an increase or decrease in the production of mRNA.
Translation: The mRNA is translated into specific proteins by ribosomes in the cytoplasm, leading to changes in cell function.
2. Water-soluble Hormones (Protein/Peptide and Amine Hormones)
Water-soluble hormones cannot easily cross the lipid bilayer of the cell membrane.
Thus, their receptors are located on the surface of the target cells.
Mechanism:
Binding: The hormone binds to its receptor on the cell surface, which is specific to that hormone.
Signal Transduction: This binding activates a signal transduction pathway inside the cell, often involving second messengers like cyclic AMP (cAMP), inositol triphosphate (IP3), or calcium ions.
Cascade Effect: The activation of the second messenger triggers a series of biochemical reactions inside the cell.
Effect: These reactions lead to various cellular responses, such as the activation of enzymes, opening of ion channels, or changes in gene expression, ultimately altering the cell’s function.