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REGULATION OF HORMONE SECRETION
Hormones are secreted by endocrine glands when there is a need for them, that is, for their effects on their target organs. The cells of endocrine glands respond to changes in the blood or perhaps to other hormones in the blood. These stimuli are the infor- mation they use to increase or decrease secretion of their own hormones. When a hormone brings about its effects, the stimulus is reversed, and secretion of the hormone decreases until the stimulus reoccurs. You may recall that this is a negative feedback mechanism, and the mechanism for thyrox-ine was depicted in Fig. 1–3. Let us use insulin as a dif-ferent example here.
Insulin is secreted by the pancreas when the blood glucose level is high; that is, hyperglycemia is the stimulus for secretion of insulin. Once circulating in the blood, insulin enables cells to remove glucose from the blood so that it can be used for energy production and enables the liver to store glucose as glycogen. As a result of these actions of insulin, the blood glucose level decreases, reversing the stim-ulus for secretion of insulin. Insulin secretion then decreases until the blood glucose level increases again.
In any hormonal negative feedback mechanism, information about the effects of the hormone is “fed back” to the gland, which then decreases its secretion of the hormone. This is why the mechanism is called “negative”: The effects of the hormone reverse the stimulus and decrease the secretion of the hormone. The secretion of many other hormones is regulated in a similar way.
The hormones of the anterior pituitary gland are secreted in response to releasing hormones (also called releasing factors) secreted by the hypothalamus. You may recall this. Growth hormone, for example, is secreted in response to growth hor-mone–releasing hormone (GHRH) from the hypo-thalamus. As growth hormone exerts its effects, the secretion of GHRH decreases, which in turn decreases the secretion of growth hormone. This is another type of negative feedback mechanism.
For each of the hormones to be discussed, the stimulus for its secretion will also be men-tioned. Some hormones function as anantagonistic pair to regulate a particular aspect of blood chemistry; these mechanisms will also be covered.
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