Mechanism of Hormone action

Hormones are the chemical messenger secreted directly in the blood stream by endocrine gland. They are target specific and bind to the specific receptor. On the basis of binding of hormone on their specific receptor, the mechanism of hormonal action is categorized into two group. They are-

Fixed membrane receptor mechanism

The hormones that are protein or amines in compositions such as Growth hormone, ADH, oxytocin, Insulin, Adrenaline, FSH, TSH etc shows this mechanism of action.
These hormones are water soluble and cannot passes through the lipid membrane and they have their target receptor on the cell membrane. The receptor are fixed on the cell membrane, so hormone can bind on the specific receptor.
Binding of hormone on specific receptor on target cell activates the enzyme Adenylcyclase in the cell membrane and causes production of cyclic AMP (cAMP).
cAMP act as secondary messenger. It diffuse through the cell membrane and activates (Protein Kinase) various enzymatic reaction to cause biochemical changes. After the target cell responded to the changes, cAMP is deactivated by a group of enzyme Phosphodiesterase

Figure: Fixed membrane receptor mechanism of hormone action( Eg. FSH)

The lipid soluble hormones such as steroid hormones and Fatty acids hormones can easily passes through the plasma membrane.
They have their receptor inside the cell, freely floating in the cytoplasm. Binding of hormone to the specific receptor activates the enzymatic activity of the cell for biochemical changes.
Some hormones (testosterone, progesterone, estrogen, cortisol, thyroxine) have their receptor localized inside the nucleus, the hormone-receptor complex are carried inside the nucleus.
The hormone-receptor complex initiate transcription of the DNA to form specific mRNA.
mRNA initiate protein synthesis in the cytoplasm. The protein (enzyme) causes biochemical changes in the cell.Figure: Mobile receptor mechanism of hormone action (Oestrogen)