Mechanisms of Hormone Action


  • Though hormones are able to reach where ever the blood circulates, they are only able to act upon specific cells that possessreceptors specific to that particular hormone.

  • Such cells are termed target cells (target tissues, or target organ).

  • Proteins, peptides and amines are not lipid soluble thus they cannot pass accross the plasma membranes of cells. The receptors for such hormones are present on the outside of the cell membrane. Binding of the hormone with its membrane receptor causes the production within the cell of a second messenger compound which then exerts the hormone's actions within the cell.

  • The second messenger modifies activities within the cell. These activities could include changing enzyme action, altering protein synthesis (through messenger RNA) or opening or closing membrane channels.

  • Examples of second messengers include:


cyclic adenosine monophosphate (cAMP),

cyclic guanosine monophosphate (cGMP),

inositol triphosphate (IP3) and

calcium ions.