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Endocrine Glands and Hormones 286

I. Hormones are chemicals that are secreted into the blood by endocrine glands.

A. The chemical classes of hormones include amines, polypeptides, glycoproteins, and steroids.

B. Nonpolar hormones, which can pass through the plasma membrane of their target cells, are called lipophilic hormones.

II. Precursors of active hormones may be classified as either prohormones or prehormones.

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.

Mechanisms of Hormone Action 292

I. The Lipophilic hormones (steroids and thyroid hormones) bind to nuclear receptor proteins, which function as ligand-dependent transcription factors.

A. Some steroid hormones bind to cytoplasmic receptors, which then move into the nucleus. Other steroids and thyroxine bind to receptors already in the nucleus.

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).

II. The polar hormones bind to receptors located on the outer surface of the plasma membrane. This activates enzymes that enlist second-messenger molecules.

A. Many hormones activate adenylate cyclase when they bind to their receptors. This enzyme produces cyclic AMP (cAMP), which activates protein kinase enzymes within the cell cytoplasm.

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

I. The pituitary gland secretes eight hormones.

A. The anterior pituitary secretes growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin.

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.

II. The release of posterior pituitary hormones is controlled by neuroendocrine reflexes.

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.

IV. Secretions of the anterior pituitary are also regulated by the feedback (usually negative feedback) exerted by target gland hormones.

V. Higher brain centers, acting through the hypothalamus, can influence pituitary secretion.

Adrenal Glands 305

I. The adrenal cortex secretes mineralocorticoids (mainly aldosterone), glucocorticoids (mainly cortisol), and sex steroids (primarily weak androgens).

A. The glucocorticoids help to regulate energy balance. They also can inhibit inflammation and suppress immune function.

B. The pituitary-adrenal axis is stimulated by stress as part of the general adaptation syndrome.

II. The adrenal medulla secretes epinephrine and lesser amounts of norepinephrine. These hormones complement the action of the sympathetic nervous system.

Thyroid and Parathyroid Glands 308

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.

II. The parathyroids are small structures embedded in the thyroid gland. They secrete parathyroid hormone (PTH), which promotes a rise in blood calcium levels.

Pancreas and Other Endocrine Glands 312

I. Beta cells in the islets secrete insulin, and alpha cells secrete glucagon.

A. Insulin lowers blood glucose and stimulates the production of glycogen, fat, and protein.

B. Glucagon raises blood glucose by stimulating the breakdown of liver glycogen. It also promotes lipolysis and the formation of ketone bodies.

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.

II. The pineal gland, located on the roof of the third ventricle of the brain, secretes melatonin.

A. Melatonin secretion is regulated by the suprachiasmatic nucleus of the hypothalamus, which is the major center for the control of circadian rhythms.

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.

III. The thymus is the site of T cell lymphocyte production and secretes a number of hormones that may help to regulate the immune system.

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.

B. Granulosa cells of the ovarian follicles secrete estrogen.

C. The corpus luteum of the ovaries secretes progesterone, as well as estrogen.

VI. The placenta secretes estrogen, progesterone, and a variety of polypeptide and protein hormones that have actions similar to some anterior pituitary hormones.

Autocrine and Paracrine Regulation 316

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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