Epinephrine quick review

• Hormone description: a catecholamine hormone, a sympathomimetic monoamine derived from the amino acids phenylalanine and tyrosine.

• Biological functions: relaxes smooth muscle in the bronchi and the iris, antagonizes the effects of histamine, increases glycogenolysis, and raises blood sugar.

• Health benefits: used as a stimulant in cardiac arrest, a vasoconstrictor in shock, a bronchodilator and antispasmodic in bronchial asthma, and to lower intra-ocular pressure in the treatment of glaucoma.

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Epinephrine (adrenaline)

Epinephrine (INN) or adrenaline (BAN) is a hormone and a neurotransmitter. Chemically, epinephrine is a catecholamine hormone, a sympathomimetic monoamine derived from the amino acids phenylalanine and tyrosine. The chemical formula of epinephrine is

C9H13NO3. It is only very slightly soluble in water, but it readily forms water soluble salts (e.g., HCl) when combined with acids. Both the commercial products and endogenous epinephrine are in the levo form, which is about 15 times more active than the dextro-isomer. Epinephrine is a hormone which, together with norepinephrine, is produced by the body in response to emotional or physical stress. Epinephrine is marketed under the trade names Adrenalin, Bronkaid Mist, Primatene Mist and Sus-Phrine to treat several conditions including asthma; high levels of epinephrine have been reported in anxiety disorders, although it is not yet clear whether this is a cause or effect of those disorders.

Whilst epinephrine is the currently accepted International Non-Proprietary Name (INN) and United States Approved Name (USAN), it is widely known as adrenaline in the rest of the world (outside the United States) which followed the British Approved Name (BAN) as listed in the British Pharmacopoeia.The monograph name in the European Pharmacopoeia remains adrenaline, despite the adoption of INN names for all other agents, and European Union (EU) countries continue to use either adrenaline or dual-labelling of both names on products. This is because of the confusion that the name epinephrine would cause to patients in EU countries, and resistance to the perceived Americanisation of drug names.

The basis for the name epinephrine in the United States was out of necessity - the name adrenalin (without a final "e") was registered as a trademark by Parke, Davis & Co. In other countries where this trademark was not registered, the name adrenaline was adopted at the insistence of the British pharmacologist Henry Hallett Dale. Resistance to the adoption of epinephrine has even resulted in some dispute as to the validity of the name (Aronson, 2000).

Pharmacology and pharmacokinetics

Epinephrine's actions are mediated through adrenergic receptors (sometimes referred to as adrenoceptors). It binds to α1 receptors of liver cells, which activate inositol-phospholipid signaling pathway, signaling the phosphorylation of insulin, leading to reduced ability of insulin to bind to its receptors. Epinephrine also activates β-adrenergic receptors of the liver and muscle cells, which activates the adenylate cyclase signaling pathway, which will in turn increase glycogenolysis. Epinephrine is an endogenous adrenergic agent that has both alpha and beta activity. It relaxes smooth muscle in the bronchi and the iris, antagonizes the effects of histamine, increases glycogenolysis, and raises blood sugar. If given by rapid IV injection it causes direct stimulation of the heart (increased heart rate and contractility), and increases systolic blood pressure. If given slowly IV, it usually produces a modest rise in systolic pressure and a decrease in diastolic blood pressure. Total peripheral resistance is decreased because of beta effects.

Epinephrine is well absorbed following IM or SQ administration. IM injections are slightly faster absorbed than SQ administration; absorption can be expedited by massaging the injection site. Epinephrine is rapidly metabolized in the GI tract and liver after oral administration and is not effective via this route. Following SQ injection, the onset of action is generally within 5-10 minutes. The onset of action following IV administration is immediate and intensified. Epinephrine does not cross the blood-brain barrier, but does cross the placenta and is distributed into milk. Epinephrine's actions are ended primarily by the uptake and metabolism of the drug into sympathetic nerve endings. Metabolism takes place in both the liver and other tissues by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT) to inactive metabolites.

Functions and usages

Epinephrine is used medicinally as a stimulant in cardiac arrest, as a vasoconstrictor in shock, as a bronchodilator and antispasmodic in bronchial asthma, and to lower intra-ocular pressure in the treatment of glaucoma. Epinephrine plays a central

role in the short-term stress reaction—the physiological response to conditions that threaten the physical integrity of the body. It is secreted by the adrenal medulla. When released into the bloodstream, epinephrine binds to multiple receptors and has numerous effects throughout the body. It increases heart rate and stroke volume, dilates the pupils, and constricts arterioles in the skin and gut while dilating arterioles in leg muscle. It elevates the blood sugar level by increasing hydrolysis of glycogen to glucose in the liver, and at the same time begins the breakdown of lipids in fat cells.

Epinephrine is used as a drug to promote peripheral vascular resistance via alpha-stimulated vasoconstriction in cardiac arrest and other cardiac disrhythmias resulting in diminished or absent cardiac output, such that blood is shunted to the body's core. This beneficial action comes with a significant negative consequence, increased cardiac irritability, which may lead to additional complications immediately following an otherwise successful resuscitation. Alternatives to this treatment include Vasopressin, a powerful diuretic which also promotes peripheral vascular resistance leading to blood shunting via vasoconstriction, but without the attendant increase to myocardial irritability.

Epinephrine is also used as a vasoconstrictor in anaphylaxis and sepsis, and as a bronchodilator for asthma if specific beta-2-adrenergic agonists are unavailable or ineffective. Allergy patients undergoing immunotherapy can get an epinephrine rinse before their allergen extract is administered. Adverse reactions to epinephrine include palpitations, tachycardia, anxiety, headache, tremor, hypertension, and acute pulmonary edema.

Epinephrine is widely used as an additive in local anesthetics to improve the depth and duration of the anesthesia, as well as to reduce bleeding in the operative field. Epinephrine counteracts the anesthetic's localized vasodilator effects in subcutaneous and submucosal vessels, thereby reducing the risk of anesthetic toxicity by decreasing the rate of systemic absorption from the site of injection. Epinephrine is the drug of choice for treating an anaphylactic reaction. Anaphylaxis, the most severe type of allergic reaction, refers to a collection of symptoms affecting multiple systems of the body. The most dangerous symptoms include breathing difficulties and a drop in blood pressure or shock, which are potentially fatal.