Because they travel in the blood, hormones are able to reach virtually all tissues. Yet the body's response to a hormone is not all-inclusive, but is highly specific, involving only the target cells for that hormone. The ability to respond depends upon the presence on (or in) the target cells of specific receptors for those hormones.
As emphasized in Chapter 7, the response of a target cell to a chemical messenger is the final event in a sequence that begins when the messenger binds to specific cell receptors. As described in that chapter, the receptors for peptide hormones and catecholamines are proteins located in the plasma membranes of the target cells. In contrast, the receptors for steroid hormones and the thyroid hormones are proteins located mainly inside the target cells.
Hormones can influence the ability of target cells to respond by regulating hormone receptors. Basic concepts of receptor modulation (up-regulation and down-regulation) were described in Chapter 7. In the context of hormones, up-regulation is an increase in the number of a hormone's receptors, often resulting from a prolonged exposure to a low concentration of the hormone. Down-regulation is a decrease in receptor number, often from exposure to high concentrations of the hormone.
Hormones can down-regulate or up-regulate not only their own receptors but the receptors for other hormones as well. If one hormone induces a loss of a second hormone's receptors, the result will be a reduction of the second hormone's effectiveness; in such cases, the one hormone is said to antagonize the action of the other. On the other hand, a hormone may induce an increase in the number of receptors for a second hormone. In this case the effectiveness of the second hormone is increased.
This latter phenomenon, in some cases, underlies the important hormone-hormone interaction known as permissiveness. In general terms, permissiveness means that hormone A must be present for the full strength of hormone B's effect. A low concentration of hormone A is usually all that is needed for this permissive effect, which is due to A's positive effect on B's receptors. For example (Figure 10-8), epinephrine causes a large release of fatty acids from adipose tissue, but only in the presence of permissive amounts of thyroid hormone. The major reason is that thyroid hormone facilitates the synthesis of receptors for epi-nephrine in adipose tissue and so the tissue becomes much more sensitive to epinephrine. It should be noted, however, that receptor alteration does not explain all cases of permissiveness; often the explanation is not known.
No fatty acids released
No fatty acids released
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.