Platyphylline: Pharmacodynamics and Systemic Effects
By chemical nature, platyphylline is a tertiary ammonium compound. This structural feature ensures its excellent absorption in the gastrointestinal tract and unhindered penetration across the blood-brain barrier (BBB).
The drug possesses a unique dual mechanism of action:
- M-anticholinergic (antimuscarinic) action: platyphylline blocks receptors, though this effect is significantly weaker than that of the classic reference standard of the group, atropine.
- Direct myotropic spasmolytic action: the ability to directly relax the smooth muscles of internal organs and blood vessels.
Due to this combined effect, the drug causes vasodilation and leads to a moderate decrease in blood pressure (BP).
In clinical practice, platyphylline hydrotartrate is used for:
- Spasms of the smooth muscle organs of the abdominal cavity, including renal colic.
- Peptic ulcer disease of the stomach and duodenum.
- Spasms of cerebral and peripheral blood vessels.
Pirenzepine: Mechanism of Antisecretory Action
Pirenzepine (also known as Gastrozepin) fundamentally differs by its high pharmacological selectivity. It is a selective blocker of $M_1$ muscarinic receptors located in the intramural ganglia of the stomach. It is important to note that at average therapeutic doses, the drug does not affect $M_2$ and $M_3$ receptors.
The primary target of pirenzepine is enterochromaffin-like cells. The mechanism of reduced gastric secretion is realized through a cascade of reactions:
- Inhibition of histamine release by enterochromaffin-like cells.
- Reduction of histamine-mediated stimulation of parietal cells.
- Decrease in the net production of hydrochloric acid (HCl).
- Additional effect: blockade of the stimulatory influence of the vagus nerve on gastric secretion.
The main clinical application of the drug is its use as an antisecretory agent in peptic ulcer disease of the stomach and duodenum.
Advantages of Pirenzepine Selectivity
Due to its selective action exclusively on $M_1$ muscarinic receptors, pirenzepine lacks many of the systemic effects characteristic of non-selective anticholinergics.
The drug does not affect:
- Heart rate (HR) and atrioventricular conduction.
- Pupil size and visual accommodation.
- Smooth muscle tone of the intestine (does not disrupt normal peristalsis).
Furthermore, the drug does not cross the blood-brain barrier (BBB) and therefore exerts no inhibitory or stimulatory effect on the central nervous system.
Side effects of pirenzepine are rare and usually limited to a minor decrease in salivary gland secretion (manifesting as dry mouth). Occasionally, mild near-vision impairment, diarrhea, bulimia, or allergic reactions may occur.
Platyphylline — Crosses the BBB, Exhibits a direct spasmolytic action. Pirenzepine — Specifically suppresses acidity (gastric M1 receptors) without entering the brain.
Class and exam questions
Why is platyphylline able to penetrate the central nervous system?
This is due to its chemical structure. Platyphylline is a tertiary ammonium compound, which makes it lipophilic, allowing it to easily cross the blood-brain barrier (BBB) and be well absorbed in the GI tract.
How does pirenzepine reduce hydrochloric acid production?
Pirenzepine blocks M1 receptors of the intramural ganglia, which inhibits histamine release by enterochromaffin-like cells. Without histamine, stimulation of parietal cells decreases, and they produce less hydrochloric acid (HCl).
Does pirenzepine cause tachycardia and mydriasis (pupil dilation)?
No. In average therapeutic doses, it selectively blocks only M1 receptors without affecting M2 (heart) and M3 (eyes, intestines), and therefore does not affect heart rate, pupil size, or accommodation.
What else to study on this topic
- Structural features of tertiary and quaternary ammonium compounds
- Physiology of enterochromaffin-like and parietal cells of the stomach
- Comparative characteristics of M-anticholinergics by degree of selectivity
- The role of the vagus nerve in the regulation of gastric secretion
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