Acetylcholine Receptor Antagonists

Acetylcholine Receptor Antagonists

Muscarinic Receptor Antagonists

Muscarinic receptor antagonists (blockers) compete with acetylcholine for its receptors at parasympathetic neuroeffector junctions and thereby inhibit the effects of parasympathetic nerve stimulation. Hence, these drugs are sometimes called parasympatholytic drugs.

The muscarinic blockers include the belladonna alkaloids obtained from plants as well as a number of semisynthetic and synthetic drugs. These agents have similar effects on target organs, but they differ in their pharmacokinetic properties and clinical uses.

Belladonna Alkaloids

The belladonna alkaloids are extracted from various solanaceous plants found in temperate climates around the world, including Atropa belladonna (the deadly nightshade), Datura stramonium (jimson weed), and Hyoscyamus niger. Belladonna, which is an Italian expression meaning “beautiful lady,” refers to the pupillary dilatation (mydriasis) produced by ocular application of extracts from these plants to women, which was considered cosmetically attractive during the Renaissance.

Atropine, scopolamine, and hyoscyamine are examples of belladonna alkaloids. The belladonna alkaloids can be highly toxic and are sometimes the cause of accidental or intentional poisonings (Box 7-1). In fact, atropine was named after Atropos, one of the Fates in Greek mythology, who was known for cutting the thread of life.

Box 7-1   A Case of Dilated Pupils and Hallucinations

Case Presentation

A 16-year-old boy was brought to the emergency department by his friends after he became highly agitated and experienced visual hallucinations, claiming that one of his friends had a mailbox for a head. His examination showed dry skin and mucous membranes, absent bowel sounds, and sinus tachycardia. His pupils were dilated, and his vision was blurred. The boy had ingested some seeds from plants growing in a vacant lot, but he denied use of alcohol or other substances. The plant material was collected and later identified as Datura stramonium. His laboratory findings were normal, and his blood alcohol level was zero. Gastric lavage was performed, and activated charcoal was administered to remove any unabsorbed substances. The patient became more agitated and delusional over time, and he was given an intravenous infusion of physostigmine. This treatment was repeated after 20 minutes, and his symptoms gradually subsided. Twelve hours later, the patient was much improved. He continued to improve over the next 36 hours and was discharged from hospital with normal vital signs and mental status.

Case Discussion

Jimson weed or locoweed (D. stramonium) is a hallucinogenic plant containing belladonna alkaloids that is found throughout the United States. Ingestion or inhalation of any part of the plant can result in anticholinergic toxicity, with the clinical presentation resembling that seen in cases of atropine poisoning. Some fatalities have occurred from ingestion of this plant. Treatment is aimed at removing plant material from the gastrointestinal tract, keeping the patient safe, and counteracting severe anticholinergic effects with physostigmine, a cholinesterase inhibitor. Physostigmine increases levels of acetylcholine in peripheral tissues and the brain and thereby counteracts manifestations of atropine toxicity. Physostigmine should be reserved for persons with serious central nervous toxicity such as hallucinations and seizures.

Atropine and Scopolamine

Pharmacologic Effects

The muscarinic blockers inhibit the effects of parasympathetic nerve stimulation and thereby relax smooth muscle, increase heart rate and cardiac conduction, and inhibit exocrine gland secretion. As shown in Figure 7-1, as the dose of atropine increases, the severity of its effects increases. The signs of atropine toxicity are expressed by the mnemonic “dry as a bone, blind as a bat, red as a beet, and mad as a hatter.”


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Jul 23, 2016 | Posted by in PHARMACY | Comments Off on Acetylcholine Receptor Antagonists

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