Gut Acid Secretion Pharmacology

Gastric acid secretion is a tightly regulated process involving multiple cell types in the gastric glands. Chief cells at the gland base secrete digestive enzymes as inactive zymogens; parietal (oxyntic) cells in the mid-gland secrete hydrochloric acid into canaliculi, which flows into the gastric lumen. Mucous neck cells secrete a thick glycoprotein mucus layer containing bicarbonate (HCO3−) that neutralises acid penetrating the mucus and protects the epithelium.

Parietal cells drive acid secretion through the H+/K+ ATPase (proton pump), which exchanges intracellular H+ for luminal K+, acidifying the gastric lumen to a pH of approximately 1–2. This pump is activated by histamine (H2 receptors), gastrin (CCK-B receptors), and acetylcholine (M3 receptors) via intracellular cAMP and calcium pathways.

Drug classes targeting gastric acidity include antacids, proton pump inhibitors (PPIs), H2 receptor antagonists, prostaglandin analogues, mucosal strengtheners, and muscarinic antagonists.

Antacids (aluminium hydroxide, magnesium hydroxide, calcium carbonate) contain basic anions that directly neutralise gastric H+, providing rapid but short-lived relief. Simethicone is often co-formulated as a surfactant to reduce surface tension of gas bubbles, relieving bloating.

Proton pump inhibitors, exemplified by omeprazole, are the first-line agents for sustained acid suppression. They are administered as inactive prodrugs in enteric-coated capsules that dissolve at the higher pH of the small intestine. Absorbed systemically, PPIs enter parietal cell canaliculi where the acidic environment converts them to a highly reactive closed-ring form that irreversibly inhibits H+/K+ ATPase. New pump protein must be synthesised to restore function, giving a duration of action of 24–48 hours despite a plasma half-life of only 0.5–2 hours. PPIs are metabolised by hepatic CYP2C19 and CYP3A4; approximately 23% of Asians carry a CYP2C19 slow-metaboliser genotype, resulting in heightened efficacy.

Misoprostol, a prostaglandin E1 analogue, acts on EP3 receptors on parietal cells (Gi-coupled, reducing cAMP and proton pump activity) and on superficial epithelial cells (increasing mucus and HCO3− secretion). It is contraindicated in pregnancy because prostaglandins increase uterine contractility.

Sucralfate, a mucosal strengthener, consists of sucrose octasulfate conjugated to aluminium hydroxide. Under acidic conditions (pH less than 4) it cross-links into a viscous polymer that adheres to damaged epithelial cells, forming a protective barrier. It must not be co-administered with antacids within 30 minutes because raising pH prevents its activation.

H2 receptor antagonists (cimetidine, ranitidine) competitively block histamine H2 receptors on parietal cells, reducing acid secretion, but they are less efficacious than PPIs and cannot be combined with PPIs because H2 blockade reduces canalicular acidity, preventing PPI activation. Muscarinic M1 antagonists reduce basal acid secretion by 40–50% but are rarely used due to anticholinergic side effects.