Endocrine Glands and Hormones 286
I. Hormones are chemicals that are secreted into the blood by endocrine glands.
A. Prohormones are relatively inactive precursor molecules made in the endocrine cells.
B. Prehormones are the normal secretions of an endocrine gland that must be converted to other derivatives by target cells in order to be active.
III. Hormones can interact in permissive, synergistic, or antagonistic ways.
IV. The effects of a hormone in the body depend on its concentration.
A. Abnormally high amounts of a hormone can result in atypical effects.
B. Target tissues can become desensitized by high hormone concentrations.
B. Each receptor binds to both the hormone and to a region of DNA called a hormone-response element.
C. Two units of the nuclear receptor are needed to bind to the hormone-response element to activate a gene; as a result, the gene is transcribed (makes mRNA).
B. Other hormones may activate phospholipase C when they bind to their receptors. This leads to the release of inositol triphosphate (IP3), which stimulates the endoplasmic reticulum to release Ca2+ into the cytoplasm, activating calmodulin.
C. The membrane receptors for insulin and various growth factors are tyrosine kinase enzymes that are activated by binding to the hormone. Once activated, the receptor kinase phosphorylates signaling molecules in the cytoplasm that can have many effects.
Pituitary Gland 299
B. The posterior pituitary releases antidiuretic hormone (also called vasopressin) and oxytocin, both of which are produced in the hypothalamus and transported to the posterior pituitary by the hypothalamo-hypophyseal tract.
III. Secretions of the anterior pituitary are controlled by hypothalamic hormones that stimulate or inhibit these secretions.
A. Hypothalamic hormones include TRH, CRH, GnRH, PIH, somatostatin, and a growth hormone-releasing hormone (GHRH).
B. These hormones are carried to the anterior pituitary by the hypothalamo-hypophyseal portal system.
V. Higher brain centers, acting through the hypothalamus, can influence pituitary secretion.
B. The pituitary-adrenal axis is stimulated by stress as part of the general adaptation syndrome.
I. The thyroid follicles secrete tetraiodothyronine (T4, or thyroxine) and lesser amounts of triiodothyronine (T3). A. These hormones are formed within the colloid of the thyroid follicles.
B. The parafollicular cells of the thyroid secrete the hormone calcitonin, which may act to lower blood calcium levels.
Pancreas and Other Endocrine Glands 312
C. The secretion of insulin is stimulated by a rise in blood glucose following meals. The secretion of glucagon is stimulated by a fall in blood glucose during periods of fasting.
B. Melatonin secretion is highest at night, and this hormone has a sleep-promoting effect. In many species, it also has an antigonadotropic effect and may play a role in timing the onset of puberty in humans, although this is as yet unproven.
IV. The gastrointestinal tract secretes a number of hormones that help to regulate digestive functions.
V. The gonads secrete sex steroid hormones.
A. Leydig cells in the interstitial tissue of the testes secrete testosterone and other androgens.
I. Autocrine regulators are produced and act within the same tissue of an organ, whereas paracrine regulators are produced within one tissue and regulate a different tissue of the same organ. Both types are local regulators—they do not travel in the blood.
II. Prostaglandins are special twenty-carbon-long fatty acids produced by many different organs. They usually have regulatory functions within the organ in which they are produced.
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