Norepinephrine Versus Epinephrine
Epinephrine and norepinephrine are closely related substances but there are important differences in their structure, production sites, functions and medical uses.
Norepinephrine and epinephrine share much in common but also differ from one another in several ways. Certain types of nerve and adrenal cells produce norepinephrine and epinephrine, which are also known as noradrenaline and adrenaline, respectively. These powerful chemicals regulate many important reflexive process, including the fight-or-flight response that allows the body to immediately respond to a dangerous situation.
Structure
The production of both norepinephrine and epinephrine begins with tyrosine, an amino acid found in a variety of foods. Tyrosine is converted into a chemical called DOPA and then dopamine, which acts as a neurotransmitter -- a signaling chemical -- in the brain. In the production of norepinephrine and epinephrine, dopamine is converted into norepinephrine which can then be converted into epinephrine. Whether a given cell primarily produces dopamine, norepinephrine or epinephrine is largely controlled by the presence and relative concentration of the enzymes responsible for the final 2 steps in the production process.
Production Sites and Functions
Certain nerve cells and specialized cells located in the interior of the adrenal gland, or the adrenal medulla, serve as the main sites of norepinephrine and epinephrine production. Nerve cells predominantly produce norepinephrine, although some in the brain produce epinephrine. Peripheral nerves -- those outside the brain and spinal cord -- that produce norepinephrine are called adrenergic neurons. They are part of a branch of the involuntary nervous system. Norepinephrine and epinephrine produced by nerve cells function as neurotransmitters, signaling chemicals released by the terminal ends of the nerve cells that trigger specific effects in the innervated tissue.
The adrenal medulla produces both norepinephrine and epinephrine but the latter predominates, accounting for roughly 80 percent. When released from the adrenal glands, norepinephrine and epinephrine function as hormones -- substances produced by glands and released into the bloodstream where they travel to other organs and tissues and exert their effects. When functioning as hormones, the effects of norepinephrine and epinephrine last longer than when they function as neurotransmitters.
Effects
Norepinephrine and epinephrine from nerves and the adrenal glands work together in complementary ways. During exercise or when experiencing a fight-or-flight reaction, increased activity of the sympathetic branch of the involuntary nervous system leads to augmented release of neuronal norepinephrine. The augmented activity of the adrenergic neurons stimulates increased adrenal gland release of epinephrine -- and to a lesser extent norepinephrine -- into the bloodstream. These adrenal hormones perpetuate and broaden the response because they reach organs and tissues not directly innervated by adrenergic neurons, including the liver, airways of the lungs and fat tissue. The collective effects of norepinephrine from adrenergic neurons and epinephrine and norepinephrine from the adrenal glands in response to exercise or a fight-or-flight reaction include:
- Increased heart rate and blood pressure
- Increased heart pumping force
- Increased blood flow to the muscles
- Decreased blood flow to the digestive system
- Dilation of the airways and pupils
- Increased breathing rate
- Increased availability of energy fuels, including blood sugar and fatty acids
Medical Uses
Both epinephrine and norepinephrine are available as medications. Norepinephrine is primarily used to treat severe low blood pressure caused by conditions such as a bacterial infection of the bloodstream or a severe blood transfusion reaction. It is also sometimes used to maintain blood pressure after restarting the heart following a cardiac arrest.
Epinephrine is more widely used medicinally than norepinephrine. Medical uses include treatment of certain serious conditions, including:
- Severe allergic reaction, or anaphylaxis
- Severe asthma attack not responsive to other medications
- Cardiac arrest or near arrest
- Certain severe heart rhythm abnormalities
Small amounts of epinephrine are sometimes mixed with local numbing medicines to prolong the numbness by constricting the blood vessels in the area. A small amount is also sometimes used to control bleeding from a mucosal surface, such as the lining of the throat or esophagus.