Mechanism of action
Pirenzepine selectively blocks M1 muscarinic receptors located in the intramural ganglia, G-cells, and enterochromaffin-like (ECL) cells of the stomach. At therapeutic doses, the drug does not affect M2 and M3 receptors. The primary effect is achieved by inhibiting the release of histamine and gastrin, which secondarily reduces hydrochloric acid production by parietal cells.
Pharmacological effects
Due to its selectivity, pirenzepine lacks the systemic effects characteristic of atropine. It does not affect heart rate, atrioventricular conduction, pupil size, accommodation, or bowel motility. The drug practically does not cross the blood-brain barrier, which rules out any effect on the central nervous system.
Indications
The main indication is gastric and duodenal ulcer disease. The drug is used as an antisecretory agent to reduce the volume and acidity of gastric juice while maintaining a high safety profile for the patient.
Side effects
Despite high selectivity, local manifestations are possible: dry mouth (due to a slight decrease in salivary gland secretion). Mild near-vision disturbances, diarrhea, bulimia, or allergic reactions are less common.
Prescription details
The drug is taken orally before meals, 0.05 g 2–3 times a day. For parenteral administration (IV or IM), the dosage is 0.005–0.01 g 2–3 times a day. The contents of the ampoule are diluted in 15–20 ml of 0.9% NaCl solution and administered slowly intravenously over 3–4 minutes.
Briefly: answers to topical questions
What are the contraindications to prescribing pirenzepine?
A contraindication to the use of anticholinergics, to which pirenzepine belongs, is glaucoma. At the same time, the drug features a high safety profile, as at average therapeutic doses it does not block $M_2$ and $M_3$ receptors. Due to its selective action, the systemic side effects characteristic of non-selective atropine-like agents are practically absent. In particular, pirenzepine does not affect pupil size or accommodation, does not alter heart rate or atrioventricular conduction, and does not reduce intestinal motility or smooth muscle tone.
Pirenzepine — "Gastrozepin": blocks M1 so the stomach "sleeps" (reduces acid), but doesn't "touch" the heart and eyes.
Class and exam questions
Why is pirenzepine safer than atropine in the treatment of peptic ulcer disease?
Pirenzepine selectively blocks M1 receptors without affecting M2 and M3 receptors. This avoids the systemic side effects of atropine: tachycardia, mydriasis, and cycloplegia.
Does pirenzepine cross the blood-brain barrier?
No, the drug poorly penetrates histohematic barriers, including the blood-brain barrier, and therefore has no effect on the central nervous system.
How does pirenzepine affect hydrochloric acid production?
It blocks M1 receptors on ECL cells, reducing the release of histamine, which is a stimulant for parietal cells. As a result, the production of hydrochloric acid is secondarily decreased.
What else to study on this topic
- Comparative characteristics of selective M1 anticholinergics and H2 histamine receptor blockers.
- The role of ECL cells in the regulation of gastric secretion.
- Principles of differential diagnostics for antisecretory drugs.
- Pharmacodynamics of non-selective anticholinergics (atropine-like) and reasons for their restricted use in peptic ulcer therapy.
- Mechanisms of side effect development during M-cholinergic receptor stimulation.
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