Platyphylline: Pharmacodynamics and Systemic Effects
Chemically, platyphylline is a tertiary amine compound. This structural property provides excellent gastrointestinal absorption and allows it to cross the blood-brain barrier (BBB) without hindrance.
The drug exhibits a unique dual mechanism of action:
- Muscarinic antagonist effect: platyphylline blocks muscarinic receptors, though this effect is significantly weaker than that of the classic reference drug, atropine.
- Direct myotropic antispasmodic effect: the ability to directly relax the smooth muscle of internal organs and blood vessels.
Through these combined effects, the drug causes vasodilation and leads to a moderate reduction in blood pressure.
In clinical practice, platyphylline hydrotartrate is used for:
- Spasms of smooth muscle organs in the abdominal cavity, including renal colic.
- Peptic ulcer disease of the stomach and duodenum.
- Cerebral and peripheral vascular spasms.
Pirenzepine: Mechanism of Antisecretory Action
Pirenzepine (also known as Gastrozepin) is characterized by high pharmacological selectivity. It is a selective blocker of $M_1$ muscarinic receptors located in the intramural ganglia of the stomach. Notably, at standard therapeutic doses, the drug does not affect $M_2$ and $M_3$ receptors.
The primary target of pirenzepine is enterochromaffin-like (ECL) cells. The mechanism of reduced gastric secretion involves a cascade of reactions:
- Inhibition of histamine release by enterochromaffin-like cells.
- Decreased histamine-mediated stimulation of parietal cells.
- Reduction in overall hydrochloric acid (HCl) production.
- Additional effect: blockade of the stimulatory influence of the vagus nerve on gastric secretion.
The primary clinical application of the drug is as an antisecretory agent in peptic ulcer disease.
Advantages of Pirenzepine Selectivity
Due to its selective action exclusively on $M_1$ receptors, pirenzepine lacks many of the systemic side effects typical of non-selective anticholinergics.
The drug does not affect:
- Heart rate (HR) and atrioventricular conduction.
- Pupil size and visual accommodation.
- Intestinal smooth muscle tone (does not disrupt normal peristalsis).
Furthermore, the drug does not cross the blood-brain barrier (BBB) and therefore exerts no central nervous system depression or stimulation.
Adverse effects of pirenzepine are rare and generally limited to mild inhibition of salivary gland secretion (manifesting as dry mouth). Occasionally, mild blurry vision, diarrhea, increased appetite, or allergic reactions may occur.