Epinephrine, also known as adrenaline, is a hormone secreted by the medulla (inner part) of the adrenal glands, located on the kidneys. The adrenal glands are one of the body's endocrine glands (glands producing substances that are distributed by way of the bloodstream).
Epinephrine was the first hormone to be discovered. Hormones are substances produced by body cells that circulate in body fluid and influence the activity of cells in another part of the body. In the 1950s, the American pharmacologist Earl Sutherland (1915-1974) discovered that epinephrine does not act directly on cells, but stimulates production of cyclic AMP, a second messenger that regulates cell activity.
Epinephrine is produced continuously in small amounts by the adrenal glands, but when the body is threatened in times of excitement, danger, or emotional stress, the brain sends messages to the adrenal glands, which respond by increasing epinephrine production.
This increase in epinephrine stimulates the heart, raises blood pressure, constricts small blood vessels, releases sugar stored in the liver, and relaxes certain involuntary muscles while it contracts others. These changes in the body prepare it for "fight or flight," meaning the body is more alert, physically stronger, and has greater energy. The person is now better prepared to face the danger at hand (fight) or escape from the danger or stress (flight).
Early Research and Use
The power of adrenal extracts was first observed by the British physiologist Edward Sharpey-Schafer (1850-1935). In 1894 he injected an adrenal extract into an experimental animal, causing its blood vessels to narrow and forcing an increase in blood pressure. Japanese American chemist Jokichi Takemine (1854-1922) isolated epinephrine in 1901, based on preliminary work done in 1897 by American pharmacologist John Jacob Abel (1857-1938).
Epinephrine was soon available for medical purposes such as reviving persons suffering from hemorrhage and shock. It was once prepared using adrenal glands of animals, but is now produced synthetically.
Hormones and the Body
In 1905 the British physiologists William Bayliss (1860-1924) and Ernest Starling (1866-1927) introduced the concept of a hormone, a sub-stance that is produced by one organ and carried by the blood to another organ, where it influences its functions. Only then did scientists realize that epinephrine was a hormone.
The significance of epinephrine and other hormones in the body's operations was discovered by the American physiologist Walter Bradford Cannon (1871-1945), after he worked with injured World War I (1914-1918) soldiers. Other scientists had already studied the body as an internal environment and the interrelation of metabolism, hormones, and the immune system. In 1926 Cannon developed the concept of homeostasis (an organism's ability to remain stable internally, even when the surrounding environment exerts great stress upon it, such as hunger, thirst, and sudden danger). Homeostasis in turn led to such ideas as biofeedback (the interaction of internal and external signals and responses in the body).
Epinephrine is one of several structurally related compounds in the body called catecholamines. These compounds help regulate the sympathetic nervous system, which is part of the autonomic nervous system. The autonomic nervous system helps the body maintain homeostasis. The autonomic nervous system makes rapid adjustments to changes in environment by freeing chemical agents that act as they are released.
The endocrine system acts more slowly by releasing agents over periods of hours or days. Because it releases hormones but acts so quickly, the adrenal medulla cannot be strictly classified as part of the nervous system or part of the endocrine system. The neurohumoral theory may explain how the two act as one in many cases.
Other catecholamines are norepinephrine (also called noradrenaline or levoarterenol) and dopamine. The general function of norepinephrine seems to be the maintenance of normal blood circulation. It is also the chemical agent that is responsible for transmission of nerve impulses in the sympathetic nervous system. When a person has certain tumors of the adrenal glands, large amounts of epinephrine and norepinephrine are produced, causing a great increase in blood pressure. Dopamine is also a nerve impulse transmitter.
Synthetic (synthesized) catecholamines are important in medicine as heart stimulants and vasoconstrictors (substances that cause blood vessels to narrow), as well as relaxants of the bronchial and other muscles.
Epinephrine is one of the most powerful vasopressor (causing a rise in blood pressure) drugs known. It increases the strength of heart muscle contractions as well as the heart rate, and it constricts blood vessels and veins. Because it is a powerful heart stimulant, it is used in emergency medicine to restore heart rhythm in cases of shock and in certain cases of cardiac arrest (heart attack). The most common use of epinephrine in medicine is to relieve breathing distress in patients with asthma, bronchitis, and emphysema. The synthetic catecholamine isoproterenol is also used in the treatment of these diseases.
Epinephrine is a powerful bronchodilator, meaning it relaxes bronchial muscle. It also constricts pulmonary vessels (in the lung), and inhibits the release of histamines triggered by allergic reactions. As a bronchodilator it is most often inhaled by mouth as a spray or through another breathing apparatus. Epinephrine is also used on the skin or mucous membranes to control bleeding of wounds because it constricts blood vessels. It is sometimes used for the same reason during surgery of the nose, throat, and larynx, where it also shrinks mucosa (membranes that secrete slime), making surgery easier.
Epinephrine increases metabolism, accelerates blood coagulation, and lowers pressure inside the eye in some types of glaucoma.