Hormones

I INTRODUCTION 

Hormone, chemical that transfers information and instructions between cells in animals and plants. Often described as the body’s chemical messengers, hormones regulate growth and development, control the function of various tissues, support reproductive functions, and regulate metabolism (the process used to break down food to create energy). Unlike information sent by the nervous system, which is transmitted via electronic impulses that travel quickly and have an almost immediate and short-term effect, hormones act more slowly, and their effects typically are maintained over a longer period of time.

Hormones were first identified in 1902 by British physiologists William Bayliss and Ernest Starling. These researchers showed that a substance taken from the lining of the intestine could be injected into a dog to stimulate the pancreas to secrete fluid. They called the substance secretin and coined the term hormone from the Greek word hormo, which means to set in motion. Today more than 100 hormones have been identified.

Hormones are made by specialized glands or tissues that manufacture and secrete these chemicals as the body needs them. The majority of hormones are produced by the glands of the endocrine system, such as the pituitary, thyroid, adrenal glands, and the ovaries or testes. These endocrine glands produce and secrete hormones directly into the bloodstream. However, not all hormones are produced by endocrine glands. The mucous membranes of the small intestine secrete hormones that stimulate secretion of digestive juices from the pancreas. Other hormones are produced in the placenta, an organ formed during pregnancy, to regulate some aspects of fetal development.

Hormones are classified into two basic types based on their chemical makeup. The majority of hormones are peptides, or amino acid derivatives that include the hormones produced by the anterior pituitary, thyroid, parathyroid, placenta, and pancreas. Peptide hormones are typically produced as larger proteins. When they are called into action, these peptides are broken down into biologically active hormones and secreted into the blood to be circulated throughout the body. The second type of hormones are steroid hormones, which include those hormones secreted by the adrenal glands and ovaries or testes. Steroid hormones are synthesized from cholesterol (a fatty substance produced by the body) and modified by a series of chemical reactions to form a hormone ready for immediate action.

II HOW HORMONES WORK  
Most hormones are released directly into the bloodstream, where they circulate throughout the body in very low concentrations. Some hormones travel intact in the bloodstream. Others require a carrier substance, such as a protein molecule, to keep them dissolved in the blood. These carriers also serve as a hormone reservoir, keeping hormone concentrations constant and protecting the bound hormone from chemical breakdown over time.

Hormones travel in the bloodstream until they reach their target tissue, where they activate a series of chemical changes. To achieve its intended result, a hormone must be recognized by a specialized protein in the cells of the target tissue called a receptor. Typically, hormones that are water-soluble use a receptor located on the cell membrane surface of the target tissues. A series of special molecules within the cell, known as second messengers, transport the hormone’s information into the cell. Fat-soluble hormones, such as steroid hormones, pass through the cell membrane and bind to receptors found in the cytoplasm. When a receptor and a hormone bind together, both the receptor and hormone molecules undergo structural changes that activate mechanisms within the cell. These mechanisms produce the special effects induced by the hormone.

Receptors on the cell membrane surface are in constant turnover. New receptors are produced by the cell and inserted into the cell wall, and receptors that have reacted with hormones are broken down or recycled. The cell can respond, if necessary, to irregular hormone concentrations in the blood by decreasing or increasing the number of receptors on its surface. If the concentration of a hormone in the blood increases, the number of receptors in the cell wall may go down to maintain the same level of hormonal interaction in the cell. This is known as downregulation. If concentrations of hormones in the blood decrease, upregulation increases the number of receptors in the cell wall.

Some hormones are delivered directly to the target tissues instead of circulating throughout the entire bloodstream. For example, hormones from the hypothalamus, a portion of the brain that controls the endocrine system, are delivered directly to the adjacent pituitary gland, where their concentrations are several hundred times higher than in the circulatory system.

III HORMONAL EFFECTS  
Hormonal effects are complex, but their functions can be divided into three broad categories. Some hormones change the permeability of the cell membrane. Other hormones can alter enzyme activity, and some hormones stimulate the release of other hormones.

Recent studies have shown that the more lasting effects of hormones ultimately result in the activation of specific genes. For example, when a steroid hormone enters a cell, it binds to a receptor in the cell’s cytoplasm. The receptor becomes activated and enters the cell’s nucleus, where it binds to specific sites in the deoxyribonucleic acid (DNA), the long molecules that contain individual genes. This activates some genes and inactivates others, altering the cell’s activity. Hormones have also been shown to regulate ribonucleic acids (RNA) in protein synthesiss.

A single hormone may affect one tissue in a different way than it affects another tissue, because tissue cells are programmed to respond differently to the same hormone. A single hormone may also have different effects on the same tissue at different times in life. To add to this complexity, some hormone-induced effects require the action of more than one hormone. This complex control system provides safety controls so that if one hormone is deficient, others will compensate.

IV TYPES OF HORMONES  
Hormones exist in mammals, including humans, as well as in invertebrates and plants. The hormones of humans, mammals, and other vertebrates are nearly identical in chemical structure and function in the body. They are generally characterized by their effect on specific tissues.

A Human Hormones  
Human hormones significantly affect the activity of every cell in the body. They influence mental acuity, physical agility, and body build and stature. Growth hormone is a hormone produced by the pituitary gland. It regulates growth by stimulating the formation of bone and the uptake of amino acids, molecules vital to building muscle and other tissue.

Sex hormones regulate the development of sexual organs, sexual behavior, reproduction, and pregnancy. For example, gonadotropins, also secreted by the pituitary gland, are sex hormones that stimulate egg and sperm production. The gonadotropin that stimulates production of sperm in men and formation of ovary follicles in women is called a follicle-stimulating hormone. When a follicle-stimulating hormone binds to an ovary cell, it stimulates the enzymes needed for the synthesis of estradiol, a female sex hormone. Another gonadotropin called luteinizing hormone regulates the production of eggs in women and the production of the male sex hormone testosterone. Produced in the male gonads, or testes, testosterone regulates changes to the male body during puberty, influences sexual behavior, and plays a role in growth. The female sex hormones, called estrogens, regulate female sexual development and behavior as well as some aspects of pregnancy. Progesterone, a female hormone secreted in the ovaries, regulates menstruation and stimulates lactation in humans and other mammals.

Other hormones regulate metabolism. For example, thyroxine, a hormone secreted by the thyroid gland, regulates rates of body metabolism. Glucagon and insulin, secreted in the pancreas, control levels of glucose in the blood and the availability of energy for the muscles. A number of hormones, including insulin, glucagon, cortisol, growth hormone, epinephrine, and norepinephrine, maintain glucose levels in the blood. While insulin lowers the blood glucose, all the other hormones raise it. In addition, several other hormones participate indirectly in the regulation. A protein called somatostatin blocks the release of insulin, glucagon, and growth hormone, while another hormone, gastric inhibitory polypeptide, enhances insulin release in response to glucose absorption. This complex system permits blood glucose concentration to remain within a very narrow range, despite external conditions that may vary to extremes.

Hormones also regulate blood pressure and other involuntary body functions. Epinephrine, also called adrenaline, is a hormone secreted in the adrenal gland. During periods of stress, epinephrine prepares the body for physical exertion by increasing the heart rate, raising the blood pressure, and releasing sugar stored in the liver for quick energy.

Hormones are sometimes used to treat medical problems, particularly diseases of the endocrine system. In people with diabetes mellitus type 1, for example, the pancreas secretes little or no insulin. Regular injections of insulin help maintain normal blood glucose levels. Sometimes, an illness or injury not directly related to the endocrine system can be helped by a dose of a particular hormone. Steroid hormones are often used as anti-inflammatory agents to treat the symptoms of various diseases, including cancer, asthma, or rheumatoid arthritis. Oral contraceptives, or birth control pills, use small, regular doses of female sex hormones to prevent pregnancy.

Initially, hormones used in medicine were collected from extracts of glands taken from humans or animals. For example, pituitary growth hormone was collected from the pituitary glands of dead human bodies, or cadavers, and insulin was extracted from cattle and hogs. As technology advanced, insulin molecules collected from animals were altered to produce the human form of insulin.

With improvements in biochemical technology, many hormones are now made in laboratories from basic chemical compounds. This eliminates the risk of transferring contaminating agents sometimes found in the human and animal sources. Advances in genetic engineering even enable scientists to introduce a gene of a specific protein hormone into a living cell, such as a bacterium, which causes the cell to secrete excess amounts of a desired hormone. This technique, known as recombinant DNA technology, has vastly improved the availability of hormones.

Recombinant DNA has been especially useful in producing growth hormone, once only available in limited supply from the pituitary glands of human cadavers. Treatments using the hormone were far from ideal because the cadaver hormone was often in short supply. Moveover, some of the pituitary glands used to make growth hormone were contaminated with particles called prions, which could cause diseases such as Creutzfeldt-Jakob disease, a fatal brain disorder. The advent of recombinant technology made growth hormone widely available for safe and effective therapy.

B Invertebrate Hormones  
In invertebrates, hormones regulate metamorphosis (the process in which many insects, crustaceans, and mollusks transform from eggs, to larva, to pupa, and finally to mature adults). A hormone called ecdysone triggers the insect molting process, in which these animals periodically shed their outer covering, or exoskeletons, and grow new ones. The molting process is delayed by juvenile hormone, which inhibits secretion of ecdysone. As an insect larva grows, secretion of juvenile hormone declines steadily until its concentrations are too low to prevent the secretion of ecdysone. When this happens, ecdysone concentrations increase until they are high enough to trigger the metamorphic molt.

In insects that migrate long distances, such as the locust, a hormone called octopamine increases the efficiency of glucose utilization by the muscles, while adipokinetic hormone increases the burning of fat as an energy source. In these insects, octopamine levels build up in the first five minutes of flight and then level off as adipokinetic hormone takes over, triggering the metabolism of fat reserves during long distance flights.

Hormones also trigger color changes in invertebrates. Squids, octopuses, and other mollusks, for example, have hormonally controlled pigment cells that enable the animals to change color to blend in with their surroundings.

C Plant Hormones  
Hormones in plants are called phytohormones. They regulate most of the life cycle events in plants, such as germination, cell division and extension, flowering, fruit ripening, seed and bud dormancy, and death (see Plant: Growth and Differentiation). Plant biologists believe that hormones exert their effects via specific receptor sites in target cells, similar to the mechanism found in animals. Five plant hormones have long been identified: auxin, cytokinin, gibberellin, abscisic acid, and ethylene. Recent discoveries of other plant hormones include brassinosteroids, salicylates, and jasmonates.

Auxins are primarily responsible for protein synthesis and promote the growth of the plant's length. The most common auxin, indoleacetic acid (IAA), is usually formed near the growing top shoots and flows downward, causing newly formed leaves to grow longer. Auxins stimulate growth toward light and root growth.

Gibberellins, which form in the seeds, young leaves, and roots, are also responsible for protein synthesis, especially in the main stem of the plant. Unlike auxins, gibberellins move upward from the roots. Cytokinins form in the roots and move up to the leaves and fruit to maintain growth, cell differentiation, and cell division. Among the growth inhibitors is abscisic acid, which promotes abscission, or leaf fall; dormancy in buds; and the formation of bulbs or tubers, possibly by preventing the synthesis of protein. Ethylene, another inhibitor, also causes abscission, perhaps by its destructive effect on auxins, and it also stimulates the ripening of fruit.

Brassinosteroids act with auxins to encourage leaf elongation and inhibit root growth. Brassinosteroids also protect plants from some insects because they work against some of the hormones that regulate insect molting. Salicylates stimulate flowering and cause disease resistance in some plants. Jasmonates regulate growth, germination, and flower bud formation. They also stimulate the formation of proteins that protect the plant against environmental stresses, such as temperature changes or droughts.

V COMMERCIAL USE OF HORMONES  
Hormones are used for a variety of commercial purposes. In the livestock industry, for example, growth hormones increase the amount of lean (non-fatty) meat in both cattle and hogs to produce bigger, less fatty animals. The cattle hormone bovine somatotropin increases milk production in dairy cows. Hormones are also used in animal husbandry to increase the success rates of artificial insemination and speed maturation of eggs.

In plants, auxins are used as herbicides, to induce fruit development without pollination, and to induce root formation in cuttings. Cytokinins are used to maintain the greenness of plant parts, such as cut flowers. Gibberellins are used to increase fruit size, increase cluster size in grapes, delay ripening of citrus fruits, speed up flowering of strawberries, and stimulate starch break down in barley used in beer making.

In addition, ethylene is used to control fruit ripening, which allows hard fruit to be transported without much bruising. The fruit is allowed to ripen after it is delivered to market. Genetic engineering also has produced fruits unable to form ethylene naturally. These fruits will ripen only if exposed to ethylene, allowing for extended shipping and storage of produce.

Contributed By:
Gad B. Kletter