Light micrograph of the anterior pituitary gland (100x micrograph enlarged to 240x).
Growth hormone is secreted in pulses, especially during sleep. Two biochemicals from the hypothalamus control its secretion. They are released alternately, exerting opposite effects. Growth hormone-releasing hormone (GHRH) stimulates secretion of GH, and somatostatin (SS) inhibits secretion.
Nutritional state seems to play a role in control of GH. For example, more GH is released during periods of protein deficiency and abnormally low blood glucose concentration. Conversely, when blood protein and glucose concentrations increase, growth hormone secretion decreases. Apparently, the hypothalamus is able to sense changes in the concentrations of certain blood nutrients, and it releases (GHRH) in response to some of them.
Growth hormone can stimulate elongation of bone tissue directly, but its effect on cartilage requires a mediator substance. The liver releases a biochemical called insulin-like growth factor-I (IGF-I), a somatomedin, in response to GH. IGF I promotes growth of cartilage. Clinical Application 13.2 discusses some clinical uses of growth hormone.
Prolactin is a protein, and as its name suggests, it promotes milk production. In males, PRL decreases secretion of luteinizing hormone (LH). Because LH is necessary for production of male sex hormones (androgens), excess prolactin secretion may cause a male to produce too few sex hormones and become infertile.
Two biochemicals from the hypothalamus may regulate prolactin secretion. One of these, prolactin release-inhibiting hormone (PIH), restrains secretion of prolactin. The other, prolactin-releasing factor (PRF), is thought to stimulate its secretion, but PRF has not yet been identified.
Releasing hormone (Hormone 1)
Anterior pituitary hormone (Hormone 2)
Hormones from Hypothalamus
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