Norepinephrine vs epinephrine: what's the difference?

Although norepinephrine and epinephrine are structurally related, they have differing effects. Noradrenaline has a more specific action working mainly on alpha receptors to increase and maintain blood pressure whereas epinephrine has more wide-ranging effects. Norepinephrine is continuously released into circulation at low levels while epinephrine is only released during times of stress.

Norepinephrine is also known as noradrenaline. It is both a hormone and the most common neurotransmitter of the sympathetic nervous system. Epinephrine is also known as adrenaline. It is mainly made in the adrenal medulla so acts more like a hormone, although small amounts are made in nerve fibers where it acts as a neurotransmitter.

Norepinephrine vs epinephrine: Synthesis and Actions in the body

Naturally occurring norepinephrine is mostly made inside nerve axons (the shaft of the nerve), stored inside vesicles (small fluid-filled sacs), then released when an action potential (an electrical impulse) travels down the nerve. Noradrenaline travels across the gap between two nerves where it binds to a receptor on the second nerve and stimulates that nerve to respond. This is norepinephrine acting as a neurotransmitter. Norepinephrine causes vasoconstriction (a narrowing of the blood vessels) so is useful for maintaining blood pressure and increasing it in times of acute stress.

Norepinephrine is also made in the adrenal medulla where it synthesized from dopamine and is released into the blood as a hormone.

Epinephrine is made from norepinephrine inside the adrenal medulla (the inner part of the adrenal gland, a small gland associated with the kidneys). Our adrenal medulla helps us to cope with physical and emotional stress. The synthesis of epinephrine increases during times of stress. Epinephrine acts on almost all body tissues, but its effects are different depending on the tissue, for example, epinephrine relaxes the breathing tubes, allowing easier breathing, but contracts the blood vessels (keeping blood pressure up and ensuring brain and heart are perfused with blood). Epinephrine also increases the heart rate and force of contraction, blood flow to the muscles and brain and aids the conversion of glycogen (a stored form of energy) into glucose in the liver.

Epinephrine diffuses through the adrenal medulla into the blood which perfuses the adrenal glands and is then carried throughout the body.

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Norepinephrine vs epinephrine: Epinephrine has a wider range of effects

Norepinephrine acts mostly on alpha receptors, although it does stimulate beta receptors to a certain degree. One of its most important roles is to increase the rate of contractions of the heart, and together with epinephrine, it underlies the fight-or-flight response.

Epinephrine is relatively nonspecific, stimulating both alpha, beta 1, beta 2, and beta 3 receptors more or less equally. By binding to these receptors epinephrine triggers a number of changes, all of which are aimed at either increasing energy use by the body or making more energy available to be used, for example, stimulating glucagon secretion from the alpha cells of the pancreas.

Norepinephrine vs epinephrine: Use in medicine

In medicine, norepinephrine is used to increase and maintain blood pressure in acute situations where low blood pressure is a feature (such as cardiac arrest, spinal anesthesia, septicemia, blood transfusions, drug reactions). It is usually used in addition to other agents.

Epinephrine is used in medicine to treat low blood pressure associated with septic shock, for the emergency treatment of allergic reactions, and in eye surgery to maintain dilation of the pupil. It is also available in an autoinjector for people with a history of severe allergic reactions.

Bottom-line

In medicine, norepinephrine is used to increase or maintain blood pressure during acute medical situations that cause low blood pressure and epinephrine is used in the emergency treatment of allergic reactions, to treat low blood pressure during septic shock, and in eye surgery to maintain dilation of the pupil.

Epinephrine is mainly produced by the adrenal medulla as a hormone, although small amounts are produced in the nerves and act as a neurotransmitter. Noradrenaline is mainly produced in the nerves, although small amounts are also produced in the adrenal medulla. Both norepinephrine and epinephrine are released during a fight-or-flight response.