Parasympatholytics, also known as anticholinergics, are a class of drugs that inhibit the actions of the neurotransmitter acetylcholine on parasympathetic nerve endings. These drugs can be classified based on their mode of action and target sites. This article will delve into the classification of parasympatholytics, individual drug pharmacological actions, clinical uses, adverse effects, atropine poisoning, contraindications, and other important information.
Classification of Parasympatholytics
Parasympatholytics can be broadly classified into two groups:
These agents block the action of acetylcholine on muscarinic receptors, which are found in smooth muscle, cardiac muscle, and glandular tissue. Examples include atropine, scopolamine, and glycopyrrolate.
These drugs act on nicotinic receptors present at autonomic ganglia and can inhibit both sympathetic and parasympathetic transmission. Examples include mecamylamine and hexamethonium.
Individual Drug Pharmacological Actions
Atropine is a naturally occurring alkaloid derived from plants like Atropa belladonna. It competitively antagonizes muscarinic receptors, blocking the actions of acetylcholine. Its effects include increased heart rate, bronchodilation, decreased secretions, and relaxation of smooth muscles in the gastrointestinal and genitourinary tracts.
Scopolamine, also known as hyoscine, is a belladonna alkaloid with similar properties to atropine but has a greater effect on the central nervous system. It is well known for its ability to prevent motion sickness and its use as an antiemetic.
Glycopyrrolate is a synthetic antimuscarinic agent that has similar effects as atropine but with a longer duration of action and fewer central nervous system side effects.
Mecamylamine is a non-competitive antagonist at nicotinic receptors, blocking both sympathetic and parasympathetic transmission at the ganglionic level.
Central Nervous System
Parasympatholytics can have various effects on the central nervous system (CNS), including sedation, amnesia, and decreased seizure threshold. The intensity of these effects depends on the specific drug and dosage.
Peripheral Nervous System
In the peripheral nervous system, parasympatholytics can inhibit the actions of acetylcholine on smooth muscles, glands, and the heart. This results in decreased gastrointestinal motility, bronchodilation, reduced secretion of salivary, bronchial and gastric glands, and increased heart rate.
Parasympatholytics are often used as a pre-treatment before anesthesia to reduce secretions and prevent bradycardia during surgery.
These drugs can cause mydriasis (pupil dilation) and cycloloplegia (paralysis of the ciliary muscle), making them useful for diagnostic and therapeutic procedures in ophthalmology.
Parasympatholytics can be used to treat various gastrointestinal and genitourinary spasms, such as irritable bowel syndrome and urinary incontinence.
These drugs, particularly atropine, can be used to treat bradycardia (slow heart rate) caused by increased vagal tone or certain medications.
Parasympatholytics can be lifesaving in cases of organophosphate poisoning by counteracting the excessive stimulation of cholinergic receptors.
Parasympatholytics are contraindicated in patients with narrow-angle glaucoma or a predisposition to acute angle-closure glaucoma, as they can increase intraocular pressure.
These drugs should be avoided in patients with prostatic hypertrophy, as they can worsen urinary retention.
Parasympatholytics are contraindicated in patients with intestinal obstruction or ileus, as they can further reduce gastrointestinal motility.
These drugs should be used with caution in patients with myasthenia gravis, as they can exacerbate muscle weakness.
Parasympatholytics should be avoided in patients with pre-existing tachycardia or a history of arrhythmias, as they can worsen these conditions.
Using parasympatholytics can lead to several side effects, commonly referred to as anticholinergic effects. These may include:
- Dry mouth
- Blurred vision
- Urinary retention
- Tachycardia (increased heart rate)
- Confusion or agitation
- Drowsiness or dizziness
Atropine poisoning, also known as anticholinergic poisoning or atropine overdose, can occur following excessive ingestion or administration of atropine or other antimuscarinic agents. Symptoms of atropine poisoning include:
- Dry, flushed skin
- Dilated pupils
- Rapid heart rate
- Delirium or hallucinations
Treatment for atropine poisoning includes supportive care, administration of activated charcoal if ingestion is recent, and the use of the antidote physostigmine, which is a cholinesterase inhibitor that increases the levels of acetylcholine.
Parasympatholytics are a diverse group of drugs that exert their effects by inhibiting the actions of acetylcholine on parasympathetic nerve endings. They have a wide range of clinical applications, but their use should be carefully considered due to potential adverse effects, contraindications, and the risk of atropine poisoning. Understanding the classification, pharmacological actions, and clinical uses of these drugs is essential for healthcare providers to ensure their safe and effective use.
Disclaimer: This article is for informational purposes only and should not be taken as medical advice. Always consult with a healthcare professional before making any decisions related to medication or treatment.