Mechanism of Hormone Action

Hormones act only on their target tissues, and they do so by binding to specific proteins known as hormone receptors, which are present in those target tissues alone. The binding of a hormone to its receptor produces a hormone-receptor complex, and it is this complex that sets off the tissue's response. Each receptor recognises just one hormone, so receptors are highly specific. Once the complex forms, it brings about biochemical changes in the target tissue, and through these changes the tissue's metabolism — and therefore its physiological functions — is regulated.

Two kinds of receptor

Receptors are located in two different places within a target cell, giving two categories:

• Membrane-bound receptors — found on the cell membrane, at the cell surface.
• Intracellular receptors — found inside the cell, and most of these are nuclear receptors present in the nucleus.

Hormones grouped by chemical nature

By their chemical make-up, hormones fall into four groups:

• Peptide, polypeptide and protein hormones — for example insulin, glucagon, the pituitary hormones and the hypothalamic hormones.
• Steroids — for example cortisol, testosterone, estradiol and progesterone.
• Iodothyronines, the thyroid hormones.
• Amino-acid derivatives — for example epinephrine.

How each type acts

The chemical nature of a hormone decides which receptor it uses and how it produces its effect:

• Hormones acting through membrane-bound receptors (typically the protein and peptide hormones) normally do not enter the target cell. Instead, their binding at the surface generates second messengers inside the cell — such as cyclic AMP, IP3 or calcium ions — and these second messengers then regulate the cell's metabolism. …