Physiological mechanisms regulating hydrochloric acid secretion
Hydrochloric acid production in the stomach is carried out by parietal cells. Their stimulation involves several cellular elements and receptors:
- Parietal cell: produces HCl at the final stage.
- ECL cells: release histamine, which acts on $H_2$ histamine receptors of the parietal cell via the adenylate cyclase pathway (cAMP).
- Antral G cells: produce gastrin, which triggers the intracellular calcium pathway ($Ca^{2+}$).
- Vagus nerve ($N. Vagus$): provides parasympathetic stimulation through direct activation of parietal cell $M_3$ muscarinic receptors, which also triggers the calcium pathway.
All intracellular signaling pathways converge on the activation of $H^+/K^+$-ATPase (proton pump). This enzyme ensures the transport of hydrogen ions ($H^+$) into the gastric lumen in exchange for potassium ($K^+$) with the simultaneous efflux of chloride ions ($Cl^-$).
Pharmacological map: sites of drug action
Pharmacological agents act on various links of gastric secretion regulation:
- Anticholinergics: pirenzepine blocks $M_1$ receptors at the level of ganglia and ECL cells, while atropine blocks $M_3$ receptors directly on the parietal cell.
- $H_2$-antihistamines: drugs such as ranitidine and famotidine act as competitive histamine antagonists.
- Gastrin antagonists: proglumide blocks gastrin receptors.
- Prostaglandin analogues: misoprostol stimulates PG receptors, inhibiting cAMP synthesis and secretion.
- Proton pump inhibitors: omeprazole blocks $H^+/K^+$-ATPase activity, exerting the most potent antisecretory effect at the final stage.
Non-selective anticholinergics and the atropine group
Non-selective drugs reduce hydrochloric acid secretion by blocking $M_3$ receptors. The main problem with this group is their low selectivity of action.
Due to atropine-like systemic side effects, such agents are rarely used in peptic ulcer disease. Systemic manifestations include:
- dry mouth (xerostomia);
- mydriasis (pupillary dilation);
- cycloplegia (paralysis of accommodation);
- tachycardia.
Selective M1 muscarinic antagonists (Pirenzepine)
Selective blockers lack many of the disadvantages of non-selective analogues due to their targeted effect on $M_1$ muscarinic receptors located on intramural ganglia, ECL cells, and G cells.
Chain of effects of pirenzepine:
- Blockade of $M_1$ receptors.
- Reduction of histamine release from ECL cells and gastrin from G cells.
- Lack of stimulation of corresponding receptors on the parietal cell.
- Secondary suppression of HCl secretion.
Due to the drug's poor penetration across histohematic barriers (including the blood-brain barrier), systemic side effects are practically absent. The only frequent local manifestation remains dry mouth.
Briefly: answers to topical questions
Which drugs belong to the group of $H_2$-antihistamines?
The group of $H_2$-antihistamines includes five main antisecretory drugs.
- Cimetidine — a CYP2D6 isoenzyme inhibitor that can trigger acute drug-induced interstitial nephritis.
- Ranitidine — a competitive histamine antagonist at the level of the gastric parietal cell.
- Famotidine — an antisecretory agent that suppresses hydrochloric acid production.
- Nizatidine — a drug that blocks $H_2$ histamine receptors.
- Roxatidine — an antisecretory agent from the $H_2$-blocker group.
What proton pump inhibitors exist besides omeprazole?
In addition to omeprazole, the proton pump inhibitor group includes four more drugs.
- Lansoprazole — used at a daily dose of 30 mg.
- Pantoprazole — the recommended daily dose is 40 mg.
- Rabeprazole — prescribed at a dose of 20 mg per day.
- Esomeprazole — used at a daily dose of 20 mg.
These medications irreversibly block $H^+/K^+$-ATPase. Despite their short half-life, they provide a prolonged effect because the recovery of secretion requires the synthesis of new enzyme molecules.
What are the contraindications to prescribing pirenzepine?
For anticholinergics, including pirenzepine, sources indicate glaucoma as a contraindication. Blockade of muscarinic receptors causes mydriasis and relaxation of the ciliary muscle, which disrupts aqueous humor outflow and can increase intraocular pressure. Pirenzepine penetrates poorly through histohematic barriers; systemic side effects are virtually absent, though a local manifestation of dry mouth may occur.
What are the indications for anticholinergics in gastroenterology besides peptic ulcer disease?
In addition to peptic ulcer disease, anticholinergics are used in gastroenterology to treat gastritis, spasms, and colic.
- Gastritis — prescription of the selective $M_1$ receptor blocker pirenzepine is indicated.
- GI spasms — non-selective drugs (atropine, platyphylline, methacin) are used due to their pronounced spasmolytic effect.
- Biliary colic — atropine, platyphylline, and methacin are used to relax smooth muscle.
In addition, combination drugs (aerron) and histamine/muscarinic blockers (meclozine) are used to manage vomiting of vestibular origin.
Pirenzepine seals off ($M_1$) stimulants (gastrin and histamine), while atropine hits the target directly ($M_3$) but affects everything indiscriminately.
Class and exam questions
Why are non-selective antimuscarinics currently rarely used for peptic ulcer disease?
Due to their low selectivity of action, they cause pronounced systemic side effects such as dry mouth, mydriasis, accommodation paralysis, and tachycardia.
What is the mechanism of action of pirenzepine on hydrochloric acid secretion?
Pirenzepine selectively blocks $M_1$ receptors on ECL and G cells, reducing histamine and gastrin release, which leads to secondary suppression of hydrochloric acid production by parietal cells.
Which drug blocks the final stage of hydrochloric acid secretion?
Omeprazole, which belongs to proton pump inhibitors and blocks $H^+/K^+$-ATPase activity.
What else to study on this topic
- Intracellular signaling pathways of the parietal cell ($Ca^{2+}$ and cAMP)
- Pharmacokinetic features of pirenzepine and crossing barriers
- Comparative characteristics of $H_2$-blockers and proton pump inhibitors
- The role of prostaglandins in the regulation of gastric secretion
- Mechanisms of action of synthetic prostaglandin analogues (misoprostol)
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