No hormone travels in the bloodstream alone, and no cell is exposed to only one hormone. Rather, there are many hormones in the blood and tissue fluid at once. Cells ignore the majority of them because they have no receptors for them, but most cells are sensitive to more than one. In these cases, the hormones may have three kinds of interactive effects:
1. Synergistic effects, in which two or more hormones act together to produce an effect that is greater than the sum of their separate effects. Neither FSH nor testosterone alone, for example, can stimulate significant sperm production. When they act together, however, the testes produce some 300,000 sperm per minute.
2. Permissive effects, in which one hormone enhances the target organ's response to a second hormone that is secreted later. Estrogen stimulates the up-regulation of progesterone receptors in the uterus. The uterus would respond poorly to progesterone, if at all, had it not been primed by the first hormone. Estrogen thus has a permissive effect on progesterone action.
3. Antagonistic effects, in which one hormone opposes the action of another. For example, insulin lowers blood glucose level and glucagon raises it. During pregnancy, estrogen from the placenta inhibits the mammary glands from responding to prolactin; thus milk is not secreted until the placenta is shed at birth.
Before You Go On
Answer the following questions to test your understanding of the preceding section:
14. What are the three chemical classes of hormones? Name at least one hormone in each class.
15. Why do corticosteroids and thyroid hormones require transport proteins to travel in the bloodstream?
16. Explain how MIT, DIT, T3, and T4 relate to each other structurally.
17. Where are hormone receptors located in target cells? Name one hormone that employs each receptor location.
18. Explain how one hormone molecule can activate millions of enzyme molecules.
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