Epinephrine (Adrenaline)
Epinephrine, also known as adrenaline, was the first hormone to be discovered. It is produced continuously in small amounts by the adrenal glands, which are endocrine glands located on the kidneys. Triggered by anxiety, danger, or other stress, the brain sends acetylcholine messages to the glands, which respond by increasing epinephrine production. This in turn increases alertness, energy level, heart rate, blood pressure, and strength. This body state is known as "fight or flight," in which a person's physical strength increases making him able to combat the problem at hand ("fight") or escape the situation quickly ("flight").
The power of adrenal extracts was first observed by the British physiologist Edward Sharpey-Schafer. 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 and Sharpey-Schäfer's colleague isolated epinephrine in 1901, based on preliminary work done in 1897 by the American pharmacologist John Jacob Abel. Epinephrine was soon available for medical purposes such as reviving persons suffering from hemorrhage and shock.
However, it wasn't until 1905 that the British physiologists William Bayliss and Ernest Starling introduced the concept of a hormone--a substance 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 soldiers. Cannon was born in Prairie du Chien, Wisconsin, and earned a medical degree at Harvard University. 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 1950s, the American pharmacologist Earl Sutherland discovered that epinephrine does not act directly on cells, but stimulates production of cyclic AMP, a second messenger that regulates cell activity.
Epinephrine binds with specific receptor sites, either alpha or beta. The binding of epinephrine to these sites is what causes the body to eventually produce its excitatory response. A class of drugs called "beta blockers" compete with epinephrine for the beta receptor sites. These sites are located mainly in the heart, lungs, kidneys, and blood vessels. When they are blocked from the action of epinephrine, blood pressure is lowered, heart rate is slowed, and vascular spasms are controlled. Beta blockers are therefore used for the treatment of hypertension (high blood pressure), cardiac arrhythmias, migraine headaches, and to prevent further damage after a heart attack.
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