Prolonged exposure to high concentrations of polypeptide hormones has been found to desensitize the target cells. Subsequent exposure to the same concentration of the same hormone thus produces less of a target tissue response. This desensitization may be partly due to the fact that high concentrations of these hormones cause a decrease in the number of receptor proteins in their target cells—a phenomenon called downregulation. Such desensitization and downregulation of receptors has been shown to occur, for example, in adipose cells exposed to high concentrations of insulin and in testicular cells exposed to high concentrations of luteinizing hormone (LH).
In order to prevent desensitization from occurring under normal conditions, many polypeptide and glycoprotein hormones are secreted in spurts rather than continuously. This pulsatile secretion is an important aspect, for example, in the hormonal control of the reproductive system. The pulsatile secretion of GnRH and LH is needed to prevent desensitization; when these hormones are artificially presented in a continuous fashion, they produce a decrease (rather than the normal increase) in gonadal function. This effect has important clinical implications, as will be described in chapter 20.
Test Yourself Before You Continue
1. Compare the four chemical classes of hormones with reference to hormones within each class.
2. Define prohormone and prehormone, and give examples of each of these molecules.
3. Describe the common characteristics of hormones and neurotransmitters.
4. List the terms used to describe hormone interactions and give examples of these effects.
5. Explain how the response of the body to a given hormone can be affected by the concentration of that hormone in the blood.
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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.