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. more...
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 inhibitor of the muscarinic acetylcholine receptors. (Acetylcholine is the neurotransmitter used by the parasympathetic nervous system.) Therefore, it may cause swallowing difficulties and reduced secretions.
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 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.
Injections of atropine are used in the treatment of bradycardia (an extremely low heart rate) and asystole, which is a condition of 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.
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 brochoconstriction
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.
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 conjuntion 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.
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