MOLECULAR MECHANISM OF HORMONE ACTION -

1. Once a hormone enters the bloodstream it can reach almost any cell in the body. However, each hormone affects only certain kinds of cells, which make up that hormone's target organ, Some hormones have more than one target organ.
2. All hormones do not act in the same way.
3. Catecholamine (epinephrine and norepinephrine), peptide and polypeptide (protein) hormones are not lipid-soluble, therefore, they cannot enter their target cells through the bilipid layer of plasma membrane (cell membrane), Thus these water soluble hormones inter act with a surface receptor, normally a glycoprotein and initiate a chain of events within it.
4. However, steroid hormones readily pass through the plasma membrane to enter the cytoplasm of the target cell. So molecular mechamsm of hormone action is described under two head-ings.
5. Mode of hormone action through the extracellular receptors and mode of hormone action through the intracellular receptors.