Source of Control
Stimulates the liver to break down glycogen and convert noncarbohydrates into glucose; stimulates breakdown of fats
Blood glucose concentration
Promotes formation of glycogen from glucose, inhibits conversion of noncarbohydrates into glucose, and enhances movement of glucose through adipose and muscle cell membranes, decreasing blood glucose concentration; promotes transport of amino acids into cells; enhances synthesis of proteins and fats
Blood glucose concentration
Helps regulate carbohydrates
Other Endocrine Glands
Additional organs that produce hormones and, therefore, are parts of the endocrine system include the pineal gland, thymus gland, reproductive glands, and certain glands of the digestive tract, the heart, and the kidneys.
The pineal gland (pin'e-al gland) is a small, oval structure located deep between the cerebral hemispheres, where it attaches to the upper portion of the thalamus near the roof of the third ventricle. It largely consists of specialized pineal cells and supportive neuroglial cells (see fig. 11.20b).
The pineal gland secretes a hormone, melatonin, which is synthesized from serotonin. Varying patterns of light and dark outside the body control the gland's activities with environmental information arriving by means of nerve impulses. In the presence of light, nerve impulses originating in the retinas of the eyes travel to the hypothalamus. From the hypothalamus, they enter the reticular formation and then pass downward into the spinal cord. In the spinal cord, the impulses travel along sympathetic nerve fibers back into the brain, and finally they reach the
Life for a person with type I (insulin-dependent) diabetes mel-litus (IDDM) means constant awareness of the illness —usually several insulin injections a day, frequent finger punctures to monitor blood glucose level, a restrictive diet, and concern over complications, which include loss of vision, leg ulcers, and kidney damage. The many symptoms of this form of diabetes mellitus reflect disturbances in carbohydrate, protein, and fat metabolism.
In Latin, diabetes means "increased urine output," and mellitus means "honey," referring to urine's sugar content. Lack of insulin decreases movement of glucose into skeletal muscle and adipose cells, inhibiting glycogen formation. As a result, blood glucose concentration rises (hyperglycemia). When blood glucose reaches a certain level, the kidneys begin to excrete the excess, and glucose appears in the urine (glycosuria). Water follows the glucose by osmosis, causing excess urinary water loss. The person becomes dehydrated and very thirsty.
Untreated diabetes mellitus decreases protein synthesis, causing tissues to waste away as glucose-starved cells use protein as an energy source. Weight falls and wounds cannot heal. Fatty acids accumulate in the blood as a result of decreased fat synthesis and storage. Ketone bodies, a by-product of fat metabolism, also build up in the blood. They are excreted in the urine as sodium salts, and a large volume of water follows by osmosis, intensifying dehydration and lowering sodium ion concentration in the blood. Accumulation of ketones and loss of sodium ions lead to metabolic acidosis, a condition that lowers the pH of body fluids. Aci-dosis and dehydration adversely affect brain neurons. Without treatment (insulin replacement), the person becomes disoriented and may enter a diabetic coma and die.
Type I diabetes is also called juvenile-onset diabetes, because it usually appears before age twenty. About 15% of people with diabetes mellitus have this form. It is an autoimmune disorder in which the immune system attacks pancreatic beta cells, ultimately destroying them and halting insulin secretion. Treatment for type I diabetes is administering enough insulin to control carbohydrate metabolism. Insulin is typically injected two or three times a day, or provided continually by an implanted insulin pump. Several insulin aerosols are in development, which can replace one or two of the daily injections—a person still needs one shot of long-acting insulin a day.
A milder form of diabetes, type II or noninsulin-dependent diabetes mellitus (NIDDM), begins gradually, in people over forty. Usually, cells lose insulin receptors and therefore cannot respond to insulin. Heredity and a lifestyle of overeating and underexercising are risk factors for developing type II diabetes. People who develop it are often overweight. Treatment includes careful control of diet to avoid foods that stimulate insulin production, exercising, drugs such as Glucophage, and maintaining desirable body weight.
The glucose-tolerance test is used to diagnose both major types of diabetes mellitus. The patient ingests a known quantity of glucose, and blood glucose concentration is measured at intervals to determine glucose utilization. If the person has diabetes, blood glucose concentration rises greatly and remains elevated for several hours. In a healthy person, glucose rise is less dramatic, and the level returns to normal in about one-and-a-half hours. ■
pineal gland. In response to impulses that light triggers, melatonin secretion from the pineal gland decreases.
In the absence of light, nerve impulses from the eyes are decreased, and secretion of melatonin increases. Melatonin secretion is part of the regulation of circadian rhythms, which are patterns of repeated activity associated with the environmental cycles of day and night (Clinical Application 13.5). Circadian rhythms responding to light and dark include sleep/wake rhythms and seasonal cycles of fertility in many mammals.
Melatonin inhibits secretion of gonadotropins from the anterior pituitary gland and helps regulate the female reproductive cycle (menstrual cycle). It may also control onset of puberty (see chapter 22, p. 909).
The role of the pineal gland in humans is not well understood, possibly because we alter natural light-dark cycles with artificial lighting. Some researchers hypothesize that mood swings are linked to abnormal mela-tonin secretion patterns, particularly a form of depression called seasonal affective disorder (SAD). Exposing such individuals to thirty to sixty minutes of bright light in the morning elevates their mood.
If something sounds too good to be true, it usually is. So it may be with melatonin, a hormone that the pineal gland of vertebrates (animals with backbones) produces and millions of people take in pill form. It is marketed in the United States as a "food supplement," so a physician's advice is not required to take melatonin. Table 13A lists the conditions that melatonin has been claimed to treat—so far, insomnia is the only valid entry. Taking melatonin is widely reported to overcome the fatigue of jet lag, but in fact, melatonin only resets the body's clock by an hour. In species that are seasonal breeders, such as deer and hamsters, additional melatonin disturbs reproduction — we don't yet know it's effects on human reproduction.
Melatonin was discovered, as a pineal gland hormone, in the 1950s, but research stalled until interest in circadian rhythms resurged in the 1970s. The pineal gland synthesizes melatonin during periods of darkness, but not in the light. Biologists hypothesize that animals use mela-tonin levels to distinguish day from night and to tell the season by determining relative lengths of day and night. The hormone may cue species other than humans when to molt, mate, migrate, and hibernate.
Melatonin binds to two types of receptors on brain neurons, one that is very abundant and one that is relatively uncommon. The major recep-
tors are found on cells of the suprachi-asmatic nucleus, a region that regulates the circadian clock. Binding to the second, less-abundant receptors, however, induces sleepiness.
Endocrinologists still have much to learn about melatonin's role in the human body. Compelling questions that remain include:
• What are melatonin's effects on a human fetus?
• What are long-term effects of taking the hormone?
• Do different people react differently to the same dosage of melatonin?
• Does melatonin interact with any drugs?
• What are the consequences of taking too much melatonin or taking it at the wrong time of day? ■
Melatonin Has Been Claimed to Treat:
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