Gastric Acid Secretion in Dogs and Cats

What is Digestion?

Digestion refers to the process of mechanical and chemical breakdown of food from large chunks into progressively smaller pieces. Once food has been broken down to the microscopic, it can be absorbed along the intestinal tract.

The first step of digestion starts in the mouth: chewing food helps to break it down into smaller pieces, increasing the surface area and making it softer. The next step occurs in the stomach, where a combination of gastric (stomach) acid and muscular mixing/churning help to further break down food. Partially digested food leaving the stomach is called “chyme,” and as it enters the small intestine it will be mixed with enzymes produced by the pancreas that will complete digestion.

Gastric Acid

Gastric acid is produced from glands, called gastric or oxyntic (acid producing) glands, that are found all along the lining of the stomach. Specialized cells within the glands called parietal or oxyntic cells secrete hydrogen and chloride to produce hydrochloric acid (HCl). Other components of gastric acid include potassium (an electrolyte), pepsinogen (an enzyme to start digesting proteins), and intrinsic factor (an enzyme that helps with vitamin B12 absorption).

In addition to oxyntic cells, other cells involved in gastric acid production include G cells and enterochromaffin-like (ECL) cells.

There are many triggers for gastric acid production. One of the most powerful is the sight and smell of food, which sends signals from the brain and central nervous system to the stomach. Another powerful trigger for gastric acid production is when food enters the stomach, and the stomach starts to fill-up (distend).

A typical pathway for gastric acid production looks something like this:

1. Smelling and/or seeing food triggers a signal to the stomach along the vagus nerve
2. The signal arrives in the stomach causing release of acetylcholine (a neurotransmitter)
3. Acetylcholine stimulates nearby ECL cells to release histamine (a hormone)
4. Histamine stimulates G cells to release gastrin (a hormone)
5. Gastrin stimulates the oxyntic cells to produce HCl
   

Acid Reduction

There are two main types of medications that are routinely used to decrease gastric acid production: histamine receptor agonists and proton pump inhibitors. Common reasons to reduce gastric acid include lowering the risk of regurgitation associated with anesthesia, and treating conditions associated with excess gastric acid such as gastric reflux (GERD), stomach ulcers, or bilious vomiting syndrome.

Histamine receptor agonists (HRAs) work by blocking the effects of the hormone histamine. This causes a decrease in the production of the hormone gastrin, which in turns reduces the amount of HCl. A common HRA is called famotidine, the generic form of Pepcid, which is available over the counter. HRAs can work well when used intermittently or for about 1-2 weeks of continuous use.  Eventually the ECL cells will start to make increasing amounts of histamine, which will override the effects of the medication.

Proton pump inhibitors (PPIs) work by directly blocking HCl production from the acid-producing oxyntic cells. A common PPI is called omeprazole, the generic form of Prilosec, which is also available over the counter. Because not all oxyntic cells are active at once, it can take 2-4 days to completely block them and to see the full therapeutic effect. Since PPIs work directly on the acid producing cells, they are generally considered to be a more effective form of acid reducing treatment compared to HRAs. PPIs are also less likely to lose efficacy over time compared to HRAs and are better for longer term use.

Regardless of the type of acid reducing medication, if they are used chronically (for 3 or more weeks) it is very important to gradually stop them. An abrupt stop can cause a very sudden and sharp spike in acid production (rebound hyperacidity) which can worsen GI signs (gastric reflux, vomiting, poor appetite) and can cause stomach ulcers; in rare cases it can create a hole in the stomach wall (perforation) which is life threatening without emergency surgery. For this reason, it is recommended to only use these medications with the guidance of a veterinarian.