Mechanism of Hormone Action:
Hormones produce their effects on target tissues by binding to receptor proteins present in the target tissues.
On the basis of chemical nature, hormones are divided into:
- Peptide, polypeptide & protein hormones: Insulin, Glucagon, Pituitary hormones, Hypothalamic hormones.
- Steroids: Cortisol, Testosterone, Progesterone, Estrogen.
- Iodothyronines: Thyroid hormones.
- Amino acid derivatives: Epinephrine.
Mode of Hormone Action:
- Non-steroid hormones: They are amino acid, peptides & protein hormones. As they are water soluble & lipid insoluble, they cannot pass through the cell membrane. So, they act through second messengers.
- Steroid hormones: They are lipid soluble, and can pass through the cell membrane. So they directly enter the cell. The only exception being Thyroid hormones that are amine derivatives but are lipid soluble.
Non-steroid Hormone Action:
- These hormones produce their effect on target tissues by binding to the receptors.
- Receptors are protein molecules, specific for a hormone.
- These are extra cellular or surface receptors, i.e., present in the cell membrane.
- These hormones/ first messenger as they cannot enter the target cell produces Second Messengers with the help of receptors, that regulate cell metabolism.
- The second messengers are Cyclic AMP, Ca2+, IP3 (Inositol triphosphate).
- Present in the cell membrane are the receptors, G-protein & Adenyl cyclase. The G-proteins are associated with GDP (Guanosine diphosphate).
- The hormone forms a complex with the receptor protein, causing a change in shape due to which it comes in contact with G-protein. It interacts with G-protein changing GDP to GTP.
- The G-protein now activates Adenyl cyclase that converts ATP to Cyclic AMP (Adenosine monophosphate).
- The Cyclic AMP activates the Kinase enzyme which triggers intracellular biochemical changes like enzyme activation, secretion, ion channel changes etc. These biochemical changes result in physiological & developmental effects.
- Hormones are required in very small amounts as a single activated molecule produces a lot of Cyclic AMP, this is known as Signal Amplification.
Steroid Hormone Action:
- These hormones diffuse through the cell membrane of the target cell. They either the cytoplasm or they directly enter the nucleus.
- Their receptors are intracellular.
- They form a complex with the receptors. These complexes bind to the chromosomes and activate certain genes, thus affecting transcription of mRNA and translation of proteins. These proteins promote metabolic reactions in the cell.
- Actions of these hormones are slower but they last longer.