**Question:** Which of the following inhibit gastric secretion by blocking H2 receptors: March 2013

**Core Concept:**
H2 receptors are histamine H2-receptors that are primarily found on the surface of gastric parietal cells. These receptors play a crucial role in the stimulation of gastric acid secretion. Inhibition of these receptors leads to reduced gastric acid secretion.

**Why the Correct Answer is Right:**
The correct answer, Cimetidine (Tagamet), belongs to a class of medications called histamine H2-receptor antagonists. By blocking the action of histamine on H2 receptors on gastric parietal cells, cimetidine reduces gastric acid secretion, leading to its anti-secretory effect.

**Why Each Wrong Option is Incorrect:**
A. Omeprazole (Prilosec) is a proton pump inhibitor (PPI) that works by blocking the H+/K+-ATPase enzyme in the gastric parietal cells, leading to reduced gastric acid secretion. Omeprazole is not a H2-receptor antagonist and therefore does not fit the description.

B. Ranitidine (Zantac) is another H2-receptor antagonist, similar to cimetidine. However, ranitidine was used as an example of a wrong option as it is not the correct answer.

C. Metoclopramide (Reglan) is a dopamine D2 receptor antagonist, not an H2-receptor antagonist. It is primarily used for its prokinetic effect and is not an anti-secretory agent.

D. Domperidone (Motilium) is a dopamine D2 receptor blocker, similar to metoclopramide. It is primarily used for its prokinetic effect and is not an anti-secretory agent.

**Clinical Pearl:**

Histamine H2-receptor antagonists (H2RAs), such as cimetidine and ranitidine, are commonly used for treating gastroesophageal reflux disease (GERD) and other conditions associated with excessive gastric acid secretion. They work by blocking the action of histamine at H2 receptors, reducing gastric acid secretion.

**Explanation:**

The correct answer, Cimetidine (Tagamet), is an H2-receptor antagonist that inhibits gastric acid secretion by blocking the action of histamine at H2 receptors on gastric parietal cells. This results in reduced gastric acid production, making it effective in treating conditions like GERD and other disorders associated with excessive gastric acid secretion.

**Why Each Wrong Option is Incorrect:**

Omeprazole (Pril) is a proton pump inhibitor (PPI), which inhibits the enzyme responsible for gastric acid production (H+/K+ ATPase) directly, leading to reduced gastric acid secretion. However, omeprazole is not an H2-receptor antagonist like cimetidine.

Metoclopramide (Reglan) is a dopamine D2 receptor antagonist that primarily acts as a prokinetic agent to improve gastrointestinal motility. It does not inhibit gastric acid secretion and is