Question

Which of the following substances does not stimulate H+ secretion by gastric parietal cells? A. GIP B. Ach C. Histamine D. Gastrin

Short answer

A. GIP

Step-by-step solution

- Understand the Question

Identify that the question is asking which substance does not stimulate hydrogen ion (H+) secretion in gastric parietal cells.

- Identify Substances that Stimulate H+ Secretion

Know that Acetylcholine (Ach), Histamine, and Gastrin are substances known to stimulate H+ secretion by gastric parietal cells.

- Consider GIP

Gastric Inhibitory Peptide (GIP) does not stimulate H+ secretion; rather, it inhibits gastric functions.

- Compare and Conclude

Since Acetylcholine, Histamine, and Gastrin all stimulate H+ secretion, the correct answer is the substance that does not, which is GIP.

Key concepts

H+ secretion

The process of H+ secretion in the stomach is crucial for digestion. This process occurs mainly in the gastric parietal cells, which are located in the lining of the stomach. These cells secrete hydrochloric acid (HCl), creating a highly acidic environment that helps break down food.
H+ ions, or protons, are secreted into the stomach's lumen via a protein called the H+/K+ ATPase pump. This pump exchanges intracellular H+ ions for extracellular K+ ions. This exchange is driven by ATP, the energy currency of the cell. As a result, HCl is produced, maintaining the acidic environment necessary for digestion and providing defense against pathogens.

Acetylcholine

Acetylcholine (Ach) is a neurotransmitter that plays a key role in stimulating H+ secretion in the stomach. When the vagus nerve is activated, it releases acetylcholine.
This neurotransmitter binds to receptors on the surface of parietal cells known as muscarinic receptors. Upon binding, a cascade of intracellular events is triggered, leading to the activation of the H+/K+ ATPase pump. This, in turn, increases the secretion of H+ ions into the stomach lumen.
Acetylcholine not only stimulates direct H+ secretion but also interacts with other cells to enhance overall gastric acid production.

Histamine

Histamine is another critical stimulant of H+ secretion. It is released by enterochromaffin-like (ECL) cells in the stomach lining.
Once released, histamine binds to H2 receptors on the parietal cells. This binding activates adenylate cyclase, an enzyme that converts ATP to cyclic AMP (cAMP). The increase in cAMP activates protein kinase A (PKA), which then stimulates the H+/K+ ATPase pump, resulting in increased H+ secretion.
Histamine's role in gastric acid secretion is so significant that drugs like H2 receptor antagonists are used to treat conditions like ulcers and GERD by blocking histamine's effects.

Gastric Inhibitory Peptide (GIP)

Gastric Inhibitory Peptide (GIP) has a unique role compared to acetylcholine and histamine. Rather than stimulating H+ secretion, GIP actually inhibits it. GIP is produced by K cells in the small intestine in response to nutrients like fats and carbohydrates.
When GIP is released, it acts on the stomach to reduce gastric motility and secretion. This helps in slowing down the digestion process, ensuring that nutrients are adequately absorbed in the intestine.
This inhibitory action of GIP makes it the correct answer to the exercise question, as it does not stimulate but rather inhibits H+ secretion in gastric parietal cells.