Antacids, H2 antagonists & PPIs.

Chapter 1

H₂ receptor antagonist

Open Question

Pharmacology


Stomach contains hydrochloric acid (HCl). This is produced by the parietal cells.


1) What are the 3 receptors that regulate HCl production by parietal cells?

2) Which neurotransmitters stimulate these receptors?

Open Question

What would result from blocking any one of the receptors?

The parietal cell secretes HCl. This is regulated by gastrin, histamine and muscarinic receptors.


If you blocked any one of them, this would result in reduced HCl. However, this effect is particularly prominent with antihistamine blockade.


This is because although gastrin and acetylcholine exert their effects on the gastrin and muscarinic receptors on the parietal cells, their main effect is on enterochromaffin-like cells. These  are the cells which release histamine. So in effect, the main effect of gastrin and acetylcholine is release of histamine, not direct stimulation of HCl secretion. Therefore the most effective receptor to block, would be the histamine receptors!


This is how drugs like ranitidine, cimetidine, famotidine, and nizatadine work.


Diagram adapted from Fig 10.2. Dawson, Taylor, Reide. Pharmacology.  Crash Course 2nd Edition, Mosby.

Mutliple Choice Question

Indication


In which of the following situations, is H₂ receptor antagonist not normally used?

Open Question

Adverse Effects


Cimetidine causes an increase in plasma prolactin, and has a modest affinity for androgen receptors. What effect do you think this would have?

Open Question

Interactions


Cimetidine interacts with other drugs in two ways:-


1) Inhibits oxidative hepatic drug metabolism by binding to microsomal cytochrome P450.

What effect do you think this would have on other drugs? Which drugs would be affected by this effect?


2) Reduces liver blood flow.What effect do you think this would have on other drugs? Which drugs would be affected by this effect?