A protein produced in the pituitary gland that stimulates the liver to produce somatomedins, which stimulate growth of bone and muscle.
Concentrated chemical substances, or hormones, which control 10 to 12 functions in the body, have been obtained as extracts from the anterior pituitary glands of cattle, sheep, and swine. Eight hormones have been isolated, purified, and identified; all of them are peptides, that is, they are composed of amino acids. Growth hormone (GH), or the somatotropic hormone (STH), is essential for normal skeletal growth and is neutralized during adolescence by the gonadal sex hormones.
Research has shown that the hormonal activity of the anterior lobe is controlled by chemical messengers sent from the hypothalamus through tiny blood vessels to the anterior lobe. In the 1950s, the British neurologist Geoffrey Harris discovered that cutting the blood supply from the hypothalamus to the pituitary impaired the function of the pituitary. In 1964, chemical agents called releasing factors were found in the hypothalamus; these substances, it was learned, affect the secretion of growth hormone, a thyroid-stimulating hormone called thyrotropin, and the gonadotropic hormones involving the testes and ovaries.
In 1969 the American endocrinologist Roger Guillemin and colleagues isolated and characterized thyrotropin-releasing factor, which stimulates the secretion of thyroid-stimulating hormone from the pituitary. In the next few years his group and that of the American physiologist Andrew Victor Schally isolated the luteinizing hormone-releasing factor, which stimulates secretion of both LH and FSH, and somatostatin, which inhibits release of growth hormone. For this work, which proved that the brain and the endocrine system are linked, they shared the Nobel Prize in physiology or medicine in 1977. Human somatostatin was one of the first substances to be grown in bacteria by recombinant DNA.
The presence of the releasing factors in the hypothalamus helped to explain the action of the female sex hormones, estrogen and progesterone, and their synthetic versions contained in oral contraceptives, or birth-control pills. During a woman's normal monthly cycle, several hormonal changes are needed for the ovary to produce an egg cell for possible fertilization. When the estrogen level in the body declines, the follicle-releasing factor (FRF) flows to the pituitary and stimulates the secretion of the follicle-stimulating hormone. Through a similar feedback principle, the declining level of progesterone causes a release of luteal-releasing factor (LRF), which stimulates secretion of the luteinizing hormone. The ripening follicle in the ovary then produces estrogen, and the high level of that hormone influences the hypothalamus to shut down temporarily the production of FSH. Increased progesterone feedback to the hypothalamus shuts down LH production by the pituitary. The daily doses of synthetic estrogen and progesterone in oral contraceptives, or injections of the actual hormones, inhibit the normal reproductive activity of the ovaries by mimicking the effect of these hormones on the hypothalamus.