Key Properties and Biological Role of Atropine
Atropine is a poisonous crystalline tropane alkaloid and is also used as anticholinergic medication. These are naturally found in Belladona (Atropa Belladonna) and in 1831 it was first extracted from this substance in a crystalline form. Since then many synthetic and non-synthetic substitutes of atropine have been developed and put into medical treatments despite the fact that it lacks therapeutic selectivity and adverse effects. Medically it is used to treat certain kinds of nerve agents and pesticide poisoning. It also manages the slow heart rates and is given to the patients during surgery in order to reduce saliva production. It is either directly pushed into the body through an intravenous route or is injected into the muscle. It is also available as an eye drop that dilates the pupil. This action of atropine starts within two minutes of application and the effect can last for upto an hour. It is basically used as an eye drop to treat uveitis and early amblyopia. The structure of atropine can be diagrammatically represented as follows.
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IUPAC name of atropine is benzene acetic acid, alpha-(hydroxymethyl)-8-methyl-azabicyclo {3.2.1} oct-3-yl ester endo-(±)-. The atropine structure shows that it is a tropane alkaloid. It is an enantiomeric mixture of d-hyoscyamine and l-hyoscyamine and most of its physiological effect is due to the presence of l-hyoscyamine. The atropine structure of the most commonly used atropine in medicine is given below.
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According to the above structure of atropine its chemical formula is designated as 1αH, 5αH - Tropane - 3 - α ol (±) - tropate (easter) and is commonly termed as atropine sulphate monohydrate. Chemically atropine is an antimuscarinic agent and most of its effects are generated as it binds itself with muscarinic acetylcholine receptors present in the body. In the human body, the level of CNS appears within 30 minutes to 1 hour of its intake and rapidly disappears with a half life within 2 hours from the bloodstream. About 60% of atropine is discharged with urine without any characteristic change and the rest 40% of it appears in the urine as conjugation and hydrolysis products. The major conjugation products are noratropine(24%), atropine-N-oxide (15%), tropic acid (3%) and tropine (2%).
Properties of Atropine
Atropines have some very distinctive properties that are significant to atropine action in the human body. Some of the physical and chemical properties are listed below.
Atropine Pharmacology
Atropin generally hinders the “rest and digest” activity carried out by the glands that are supported by parasympathetic nervous receptors. As atropine hinders acetylcholine which is the main neurotransmitter used by the parasympathetic nervous receptors thus, atropine becomes the competitive reversible antagonist of the muscarinic acetylcholine receptors.
In cardiac use, atropine behaves like a non-selective antagonist for muscarinic acetylcholinergic receptors. Thus, it increases the conduction of the atrioventricular nodes (NA) and rapids the firing of sinoatrial nodes (SN) of the heart and prevents the action of vagus nerves. This results in blockage of acetylcholine receptor sites and reduces bronchial secretion.
Atropine induced mydriasis in the eye that results in the contraction of the pupillary sphincter muscle which is normally stimulated by the secretion of acetylcholine. This in turn allows the iris to contract and dilate the pupil. Atropine helps in paralysing the ciliary muscle by inducing cycloplegia that allows proper refraction in children and relieves pain associated with iridocyclitis.
Uses of Atropine
There are many therapeutic uses of atropine in the medical field despite it being therupatively selective and have certain adverse effects. Therapeutic uses of atropine are mostly studied for the eyes, heart. It also plays an important role in inhibiting secretions from glands and as an antidote for organophosphate poisoning.
It is used as a mydriatic to dilate pupils and as cycloplegic to temporarily paralyze the accommodation reflex of the eye. Many studies show that atropine penalization is very effective as occlusion in improving visual accuracy.
Atropine is useful in the treatment of heart block of second degree and third degree with high AV escape nodal rhythm or purkinje. In order to treat symptomatic or unstable bradycardia, atropine is injected into the muscles.
Atropine inhibits the secretion of saliva from the salivary glands by acting as a protagonist of the parasympathetic nervous system. As it also has the same effect on the sympathetic nervous system, it inhibits the secretion of sweat.
Atropine helps in treating organophosphate poisoning by organophosphate insecticides and nerve agents by blocking the action of acetylcholine at muscarinic receptors.
As it can mimic the side effects of antidepressants and few other medications it is used as an active placebo in many drug trials.
Side Effects of Atropine
There are many side effects of atropine that have been observed over the years. For instance, excess doses of atropine sulphate cause difficulty in swallowing, dilated pupils, dizziness, fatigue, restlessness and even hinders coordination. If there is an extension of its dosage it may cause dryness of the mouth, fever, constipation, tachycardia, local allergy such as dermatitis, swelling of eyelids, conjunctivitis and sometimes the atropin drugs can cause atropine intox such as phenothiazines, antihistamines and TCA’s. As atropine has the ability to cross the blood brain barrier and because of its hallucinogenic properties, it causes hallucination and excitation especially among elderly people.
FAQs on Atropine: Chemistry, Actions & Applications Explained
1. What is atropine from a chemical perspective?
Atropine is a naturally occurring tropane alkaloid, an organic compound typically extracted from plants like the deadly nightshade (Atropa belladonna) and jimsonweed. From a chemical standpoint, it is a racemic mixture, meaning it is composed of equal amounts of two enantiomers (mirror-image isomers): d-hyoscyamine and l-hyoscyamine. It functions as an antimuscarinic agent by interacting with specific receptors in the nervous system.
2. How is atropine classified as a drug?
Atropine is broadly classified as an anticholinergic or parasympatholytic drug. This classification means it functions by blocking the action of acetylcholine, a key neurotransmitter in the parasympathetic nervous system. By inhibiting the signals transmitted by acetylcholine at muscarinic receptors, atropine effectively reduces or stops involuntary functions like salivation and slowing of the heart.
3. What is the primary mechanism of action of atropine?
The primary mechanism of action for atropine is its role as a competitive antagonist at muscarinic acetylcholine receptors (mAChRs). It physically binds to these receptors on cells without activating them. This occupation prevents the body's natural neurotransmitter, acetylcholine, from binding and triggering a response. Consequently, the effects of parasympathetic nerve stimulation are blocked.
4. What are the main medical applications of atropine?
Atropine has several crucial medical applications due to its ability to inhibit parasympathetic activity. Key uses include:
- Treating Bradycardia: It is used to increase a dangerously slow heart rate by blocking the vagus nerve's effect on the heart.
- Antidote for Poisoning: It serves as a vital antidote for poisoning caused by organophosphate nerve agents, pesticides, and certain types of mushrooms.
- Ophthalmology: In the form of eye drops, it is used to dilate the pupils (mydriasis) to allow for thorough examination of the retina and other internal eye structures.
- Preoperative Medication: It is administered before surgery to reduce secretions like saliva and mucus, lowering the risk of aspiration during anaesthesia.
5. What are the common side effects associated with atropine?
The side effects of atropine are a direct extension of its anticholinergic action. By suppressing the parasympathetic nervous system, it can cause:
- Dry mouth and throat
- Blurred vision and sensitivity to light
- An increased heart rate (tachycardia)
- Difficulty in urination
- Reduced sweating, which may lead to an increase in body temperature
6. Why is atropine's racemic nature significant for its function?
Atropine is a racemic mixture, containing both d-hyoscyamine and l-hyoscyamine isomers. This is highly significant because the biological activity is almost exclusively due to one isomer: l-hyoscyamine. This is the isomer that binds strongly to muscarinic receptors to produce the drug's effects. The d-hyoscyamine isomer is largely inactive. The chemical extraction and purification process often cause the natural l-form to partially convert to the d-form, resulting in the racemic mixture known as atropine.
7. How does atropine work as an antidote for organophosphate poisoning?
In organophosphate poisoning, a vital enzyme (acetylcholinesterase) is blocked, leading to a massive, toxic accumulation of the neurotransmitter acetylcholine. This causes overstimulation of muscarinic receptors. Atropine works as an antidote not by fixing the enzyme, but by acting as a shield. It competitively blocks the overstimulated muscarinic receptors, protecting organs like the heart, lungs, and glands from the acetylcholine excess and allowing the body's vital functions to stabilise.
8. From a chemistry standpoint, why is atropine administered as atropine sulphate?
Atropine is an alkaloid, which is a basic compound. Its free base form has low water solubility. To make it suitable for medical injections, it is reacted with sulphuric acid to form atropine sulphate. This salt form is highly soluble in water, allowing for the preparation of stable aqueous solutions for intravenous (IV) or intramuscular (IM) use. This enhanced solubility and stability are critical for its use in emergency medicine.
9. How can atropine be used to explain the difference between a pharmacological agonist and an antagonist?
An agonist is a substance that binds to a receptor and activates it, producing a response. The body's own acetylcholine is the natural agonist for muscarinic receptors. An antagonist binds to the same receptor but does not activate it; instead, it blocks the agonist from binding. Atropine is a perfect example of a competitive antagonist. It occupies the muscarinic receptor site, thereby preventing acetylcholine from binding and blocking its normal physiological actions.
