Epinephrine

Epinephrine, better known as adrenaline, is a hormone and neurotransmitter produced by the adrenal glands and is also used as a medication due to its various important functions. In general, parenteral epinephrine is most commonly used to relieve respiratory distress due to bronchospasm, to rapidly relieve hypersensitivity reactions to drugs, animal sera, and other allergens, and to prolong the effects of infiltration anesthetics. Epinephrine is also the primary drug used to induce cardiac arrest.

Epinephrine is administered parenterally through the vein, muscle or under the skin only.

The substance was first discovered in 1856 by the French physician Alfred Vulpian. Around 1900, the substance was isolated in pure form for the first time and named epinephrine. In 1904, the structure was finally elucidated. Epinephrine is on the World Health Organization's list of essential medicines.

Role in the organism

Adrenaline is produced mainly in the adrenal medulla and provides about 90% of circulating adrenaline. Little adrenaline is found in other tissues, mostly in scattered chromaffin cells and in a small number of neurons that use adrenaline as a neurotransmitter.

Adrenaline stimulates the adrenoceptors of the sympathetic nervous system. These include the α1, α2, β1, β2, and β3 subtypes, which are found in almost all body tissues. Its effects on different tissues depend on the type of tissue and on the types of adrenergic receptors that predominate there. For example, high levels of adrenaline cause smooth muscle relaxation in the airways but cause contraction of the smooth muscle lining most arterioles.

Adrenaline functions primarily as a stress hormone. It is released in animals and humans to trigger the so-called "fight-or-flight response." The release of adrenaline thus triggers specific physiological reactions that are intended to prepare the body to respond to particular stressful situations or threats.

These responses include:

• Stimulation of the liver to break down glycogen into glucose (to provide the body with energy quickly)
• Relaxation of smooth muscles in the lungs and airways to increase oxygen uptake and improve lung capacity
• Stimulation of beta-adrenergic receptors in the heart muscle to increase the heart's contractility and heart rate
• Contraction of arteries in the skin to redirect blood flow to internal organs
• Contraction of smooth muscles in the skin, causing hairs on the surface of the skin to stand upright (possibly to increase heat/energy conservation of the body

Application in medicine

In medicine, the more potent form L-adrenaline is used. Through its action on alpha-adrenergic receptors, epinephrine reduces the vasodilation and increased vascular permeability that occur during anaphylaxis, which can lead to a loss of intravascular fluid volume as well as hypotension. This prevents potential shock. Epinephrine relaxes bronchial and iris smooth muscle and is a histamine antagonist, making it useful in the treatment of allergic reactions and related conditions. Through its action on beta-adrenergic receptors, epinephrine causes relaxation of bronchial smooth muscle, which helps relieve bronchospasm, wheezing, and dyspnea that may occur during anaphylaxis. This effect is also exploited in an acute asthma attack. In cardiac arrest, epinephrine can restore spontaneous circulation in the heart. Epinephrine is often added to local anesthetics to prevent the local anesthetic from entering the bloodstream by contracting the blood vessels.

Adverse drug reactions

Adverse effects of epinephrine include:

• Palpitations
• Tachycardia
• Cardiac arrhythmias
• Anxiety
• Panic attacks
• Headaches
• Loss of appetite
• Tremor
• High blood pressure
• Acute pulmonary edema.

In rare cases, exposure to medically administered epinephrine may cause takotsubo cardiomyopathy.

Contraindications

Use is contraindicated in individuals taking non-selective β-blockers as severe hypertension and even cerebral hemorrhage may occur.