Duodenal regulation (intestinal inhibition)
The inhibition process is closely linked to the movement of the food bolus into the small intestine. When chyme enters the duodenum, its receptors respond to increased acidity and the appearance of nutrient breakdown products. In response, endocrine cells of the intestinal mucosa release specific hormones into the bloodstream — secretin and cholecystokinin (CCK).
These substances exert a dissociated (divergent) effect on gastric glands:
- Inhibit the activity of parietal cells, sharply reducing hydrochloric acid production.
- Stimulate chief cells, enhancing the secretion of the enzyme pepsinogen.
In addition, if the environment in the duodenum becomes excessively acidic, hydrochloric acid secretion is inhibited via two pathways simultaneously: through the release of duodenal hormones and via reflex neural influences.
Humoral inhibitors of secretion
In addition to secretin and cholecystokinin, the body produces a wide range of substances capable of suppressing gastric gland activity. These humoral factors are delivered to target cells via the bloodstream and include the following compounds:
- Intestinal and pancreatic peptides: glucagon, gastric inhibitory peptide (GIP), vasoactive intestinal peptide (VIP), neurotensin.
- Tissue hormones and mediators: serotonin, somatostatin, gastrones (including bulrogastrone and cologastrone).
- Pituitary and thyroid hormones: vasopressin (antidiuretic hormone), oxytocin, thyrotropin-releasing hormone, calcitonin.
- Prostaglandins: specifically $PGE_2$, which not only suppresses acid production but also plays a key role in cytoprotection.
These substances provide fine-tuning of the digestive process, preventing the excessive release of aggressive gastric juice.
Autoregulation mechanisms and receptor apparatus
Hydrochloric acid production is controlled by a strict negative feedback mechanism. The main endogenous stimulator is the hormone gastrin, which is synthesized by G-cells located primarily in the mucosa of the pyloric (antral) part of the stomach.
Gastrin release depends on the acidity level (pH-dependent autoregulation):
- When the pH in the antral zone drops to 3.0, gastrin secretion begins to be inhibited.
- If the pH drops to 1.0, hormone release stops completely, leading to a sharp decrease in glandular activity.
Clinical significance: surgical removal of the pyloric part of the stomach deprives the body of the main pool of G-cells, resulting in a radical and persistent decrease in gastric secretion.
An important role in paracrine control is played by histamine, released by mast cells. It acts on $H_2$-receptors of parietal cells, causing the release of acidic juice that is poor in enzymes. Interestingly, high doses of histamine can exert an inhibitory effect on pepsinogen production by chief cells.
Protection of the gastric mucosa
The inhibition of aggressive factors always works in tandem with protective factors. The gastric mucosa is protected from digestion by its own enzymes and acid by a robust barrier. This barrier is primarily composed of insoluble mucus and bicarbonate ions ($HCO_3^-$), which create a neutralizing layer on the epithelial surface.
The key stimulators of bicarbonate and mucus production are prostaglandins ($PGE_2$ and $PGI_2$).
In clinical practice, this physiological mechanism is often disrupted by the use of nonsteroidal anti-inflammatory drugs (NSAIDs). These drugs block the enzyme cyclooxygenase-1 (COX-1), leading to a drop in prostaglandin levels. As a result, the protective bicarbonate-mucus barrier thins, acid secretion increases, and conditions become favorable for ulcer formation.
To remember the dissociated effect of duodenal hormones (CCK and secretin), use the phrase: "Acid on the brakes, enzymes on the gas." They inhibit hydrochloric acid, but stimulate pepsinogen.
Class and exam questions
Once in the duodenum, fats stimulate endocrine cells to release cholecystokinin. This hormone reaches the stomach via the bloodstream and suppresses parietal cell activity, reducing hydrochloric acid synthesis.
Once in the duodenum, fats stimulate endocrine cells to release cholecystokinin. This hormone reaches the stomach via the bloodstream and suppresses parietal cell activity, reducing hydrochloric acid synthesis.
The vagus nerve ($N. vagus$) itself stimulates secretion through the release of acetylcholine. However, the parasympathetic nervous system also participates in duodenal reflex inhibition during excessive acidification of the duodenum.
The vagus nerve ($N. vagus$) itself stimulates secretion through the release of acetylcholine. However, the parasympathetic nervous system also participates in duodenal reflex inhibition during excessive acidification of the duodenum.
Secretion will drop sharply. The pyloric part contains the bulk of G-cells that produce gastrin, the main stimulator of hydrochloric acid secretion.
Secretion will drop sharply. The pyloric part contains the bulk of G-cells that produce gastrin, the main stimulator of hydrochloric acid secretion.
What else to study on this topic
- Mechanisms of intracellular regulation of chief cells (the role of calcium ions, cAMP, and cGMP)
- Neural regulation of gastric secretion: cholinergic mechanisms
- Paracrine regulation via mast cells and H2 receptors
- Secretory function of parietal cells: the operation of the proton pump
- Influence of the sympathetic nervous system on pepsinogen production
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