Atropine

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Atropine

Atropine is a tropane alkaloid extracted from the deadly nightshade (Atropa belladonna) and other plants of the family Solanaceae. It is a secondary metabolite of these plants and serves as a drug with a wide variety of effects. Being potentially deadly, it derives its name from Atropos, one of the three Fates who, according to Greek mythology, chose how a person was to die.

Physiological effects and uses

Generally, atropine lowers the "rest and digest" activity of all muscles and glands regulated by the parasympathetic nervous system. This occurs because atropine is a competitive antagonist of the muscarinic acetylcholine receptors. (Acetylcholine is the main neurotransmitter used by the parasympathetic nervous system.) Therefore, it may cause swallowing difficulties and reduced secretions.

Ophthalmic use

Topical atropine is used as a cycloplegic, to temporarily paralyze accommodation, and as a mydriatic, to dilate the pupils. Atropine degrades slowly, typically wearing off in 2 to 3 days, so tropicamide and phenylephrine is generally preferred as a mydriatic. In atropine-induced mydriasis, the mechanism of action involves blocking the contraction of the circular pupillary sphincter muscle which is normally stimulated by acetylcholine release, thereby allowing the radial pupillary dilator muscle to contract and dilate the pupil. Atropine is contraindicated in patients predisposed to narrow angle glaucoma.

Resuscitation

Injections of atropine are used in the treatment of bradycardia (an extremely low heart rate), asystole and pulseless electrical activity (PEA) in cardiac arrest. This works because the main action of the vagus nerve of the parasympathetic system on the heart is to slow it down. Atropine blocks that action and therefore may speed up the heart rate.

Atropine is also useful in treating first degree heart block, second degree heart block Type 1 Wenckebach, and also third degree heart block with a high Purkinje or AV-nodal escape rhythm. It is contraindicated in second degree heart block Type II Mobitz, and in third degree heart block with a low Purkinje or ventricular escape rhythm.

The main action of the parasympathetic nervous system is to stimulate the M2 muscarinic receptor in the heart, but atropine inhibits this action.

Secretions and bronchoconstriction

Atropine's actions on the parasympathetic nervous system inhibits salivary, sweat, and mucus glands. This can be useful in treating Hyperhidrosis and can prevent the death rattle of dying patients. Even though it has not been officially indicated for either of these purposes by the FDA, it has been used by physicians for these purposes.

Antidote for organophosphate poisoning

By blocking the action of acetylcholine at muscarinic receptors, atropine also serves as an antidote for poisoning by organophosphate insecticides and nerve gases. Troops who are likely to be attacked with chemical weapons often carry autoinjectors with atropine and obidoxime which can be quickly injected into the thigh. It is often used in conjunction with Pralidoxime chloride.

Some of the nerve gases attack and destroy acetylcholinesterase, so the action of acetylcholine becomes prolonged. Therefore, atropine can be used to reduce the effect of ACh.

Side effects and overdoses

Adverse reactions to atropine include ventricular fibrillation, supraventricular or ventricular tachycardia, giddiness, nausea, blurred vision, loss of balance, dilated pupils, photophobia, and possibly, notably in the elderly, confusion, hallucinations and excitation. These latter effects are due to the fact that atropine is able to cross the blood-brain barrier. Because of the hallucinogenic properties, some have used the drug recreationally, though this is very dangerous.

In overdoses, atropine is poisonous. Atropine is sometimes added to other potentially addictive drugs; abuse of those drugs is then prevented by the unpleasant effects of atropine overdose.

Atropine can cause heart rate slowing when given at very low doses, presumably as a result of a weak partial agonist effect at the cardiac muscarinic receptors.

The antidote to atropine itself is physostigmine or pilocarpine.

Chemistry and pharmacology

Atropine is a racemic mixture of D-hyoscyamine and L-hyoscyamine, with most of its physiological effects due to L-hyoscyamine. Its pharmacological effects are due to binding to muscarinic acetylcholine receptors.

The most common atropine compound used in medicine is atropine sulfate (C₁₇H₂₃N O₃)₂. H₂SO₄. H₂O, the full chemical name is 1α H, 5α H-Tropan-3-α ol (±)-tropate(ester), sulfate monohydrate.

History

Atropine extracts from the Egyptian henbane were used by Cleopatra in the last century B.C. to dilate her pupils, in the hope that she would appear more alluring. In the Renaissance, women used the juice of the berries of Atropa belladonna to enlarge the pupils of their eyes, for cosmetic reasons; "belladonna" is Italian for "beautiful lady".

Atropine and its mydriatic effects were discovered in 1833 by the German chemist Friedrich Ferdinand Runge (1795-1867).

Natural sources

Atropine is found in several members of the Solanaceae family. The most commonly found sources are Atropa belladonna, Datura innoxia, D. metel, and D. stramonium. Other sources include members of the Brugmansia and Hyoscyamus genera. The Nicotiana genus (including the tobacco plant, N. tabacum) is also found in the Solanaceae family, but these plants do not contain atropine or other tropane alkaloids.

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Alkaloids | Antidotes | Deliriants | Muscarinic antagonists

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