Each hormone exerts its characteristic effects on target organs by acting on the cells of these organs. Hormones of the same chemical class have similar mechanisms of action. Lipid-soluble hormones pass through the target cell's plasma membrane, bind to intracellular receptor proteins, and act directly within the target cell. Polar hormones do not enter the target cells, but instead bind to receptors on the plasma membrane.This results in the activation of intracellular second-messenger systems that mediate the actions of the hormone.
Although each hormone exerts its own characteristic effects on specific target cells, hormones that are in the same chemical category have similar mechanisms of action. These similarities involve the location of cellular receptor proteins and the events that occur in the target cells after the hormone has combined with its receptor protein.
Hormones are delivered by the blood to every cell in the body, but only the target cells are able to respond to these hormones. In order to respond to any given hormone, a target cell must have specific receptor proteins for that hormone. Receptor protein-hormone interaction is highly specific. In addition to this property of specificity, hormones bind to receptors with a high affinity (high bond strength) and a low capacity. The latter characteristic refers to the possibility of saturating receptors with hormone molecules because of the limited number of receptors per target cell (usually a few thousand). Notice that the characteristics of specificity and saturation that apply to receptor proteins are similar to the characteristics of enzyme and carrier proteins discussed in previous chapters.
The location of a hormone's receptor proteins in its target cells depends on the chemical nature of the hormone. Since the lipophilic hormones (steroids and thyroxine) can pass through the plasma membrane and enter their target cells, the receptor proteins for lipophilic hormones are located within the cytoplasm and nucleus. Since the water-soluble hormones (cate-cholamines, polypeptides, and glycoproteins) cannot pass through the plasma membrane, their receptors are located on the outer surface of the membrane. In these cases, hormone action requires the activation of second messengers within the cell.
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.