Physiology of gastric secretion and defense
Hydrogen ion secretion is carried out by parietal cells. The regulation of this process occurs at the CNS level and via humoral factors. The main stimulation pathways are neuronal (acetylcholine via the vagus nerve), paracrine (histamine), and humoral (gastrin). The basolateral membrane of the cell features corresponding receptors: M3 muscarinic receptors, H2 histamine receptors, and CCK2 receptors.
The intracellular signaling cascade culminates in the activation of the H+, K+-ATPase enzyme (proton pump). It transports protons into the gastric lumen in exchange for potassium ions, creating a massive gradient: the intracellular pH is 7.8, while the extracellular pH drops to 0.8.
Mucosal defense (cytoprotection) is provided by mucus (hydrophilic glycoproteins), bicarbonate secretion, and active blood supply, which washes away penetrated hydrogen ions. The main regulators of defense are prostaglandins E2 and I2, which stimulate mucus production and suppress acid secretion.
Classification of antacids
Antacids are divided into three main groups depending on their ability to be absorbed into the systemic circulation and their mechanism of action:
- Absorbable (systemic): sodium bicarbonate (baking soda), calcium carbonate, magnesium oxide and carbonate.
- Non-absorbable (non-systemic): algeldrate, magnesium trisilicate, as well as combination agents (Almagel, Maalox).
- Adsorbing agents: bismuth subnitrate and preparations containing it (Vicalin, Vicair).
The standard dose of powder formulations is 0.5–1.0 g. Before administration, they must be dissolved in half a glass of water.
Absorbable drugs: features and disadvantages
Sodium bicarbonate is the fastest-acting agent, yet it has serious drawbacks. The acid neutralization reaction is accompanied by vigorous evolution of carbon dioxide. The gas distends the gastric walls, which triggers a secondary increase in secretion (rebound syndrome). Furthermore, frequent use of the drug causes systemic alkalosis.
Calcium carbonate (precipitated chalk) has moderate efficacy. It also releases carbon dioxide, causing belching and flatulence. The calcium chloride formed during the reaction is absorbed into the blood, and the drug itself has a pronounced antidiarrheal effect that can lead to constipation.
Common side effects of antacids include decreased appetite and dyspeptic disorders (belching, vomiting).
Non-absorbable and combination agents
Magnesium preparations (magnesium oxide) neutralize acid without gas evolution and act more effectively than soda. Upon entering the intestine, they exert a laxative effect. Magnesium trisilicate works slowly: it forms a colloid that envelops and protects the mucosa from pepsin and acid.
Aluminum preparations (algeldrate) also do not produce gas, possess an adsorbing effect, but cause constipation.
To balance the side effects on intestinal motility, combination drugs have been developed:
- Almagel (aluminum + magnesium + D-sorbitol) is available as a gel for even distribution over the mucosa. It has a choleretic and laxative effect. The Almagel A modification additionally contains benzocaine to relieve pain and nausea.
- Maalox (aluminum + magnesium + sorbitol + mannitol) has a unique cytoprotective property. It stimulates the synthesis of prostaglandin E2, increasing the production of protective mucus.
Systemic risks of aluminum-containing drugs
With prolonged use of non-absorbable aluminum-containing antacids, specific metabolic disorders occur. In the small intestine, insoluble aluminum phosphate salts are formed, which completely blocks the absorption of dietary phosphates.
Clinically, phosphate deficiency manifests as weakness, malaise, and suppression of vitamin D3 synthesis. In severe cases, osteoporosis, kidney damage, and encephalopathy (brain damage) develop. Due to these risks, the course of treatment with aluminum-containing drugs is strictly limited and should not exceed two weeks.
It is easy to remember the effect on intestinal motility: ALuminum — ALgorithm for stopping (constipation), MAGnesium — MAGistral open (laxative). In combination drugs, they balance each other out.
Class and exam questions
Why is sodium bicarbonate unsuitable for long-term treatment?
It causes a rapid release of carbon dioxide, which leads to gastric distension and the "rebound" syndrome (secondary hypersecretion). Additionally, its absorption threatens the development of systemic alkalosis.
What is the pharmacological advantage of Maalox?
In addition to acid neutralization and a balanced effect on the intestine, it possesses a cytoprotective action — stimulating the synthesis of prostaglandin E2, which protects the mucous membrane.
How does Helicobacter pylori survive in the harsh acidic environment of the stomach?
The bacterium forms a "cloud" of a neutral environment around itself by producing substances (e.g., urea). The neutralization process produces ammonium hydrochloride, which destroys the epithelium.
Why is it forbidden to take aluminum-containing antacids for longer than 2 weeks?
They bind phosphates in the intestine, preventing their absorption. This can lead to severe hypophosphatemia, osteoporosis, kidney damage, and encephalopathy.
What else to study on this topic
- Intracellular signaling cascade of H2 receptors (role of Gs protein and cAMP)
- Mechanism of action of H+, K+-ATPase (proton pump)
- Pathogenesis of peptic ulcer disease and GERD: imbalance of aggression and defense factors
- Role of enterochromaffin-like cells and M1 receptors in paracrine regulation of secretion
- Adsorbing antacids: pharmacodynamics of bismuth preparations
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