91. Effective oral rehydration therapy requires the presence of both Na+ and glucose because the intestinal
epithelial cells express a:
1. Na+ -glucose symporter on their basolateral membrane
2. Na+ -glucose antiporter on their basolateral membrane
3. Na+ -glucose symporter on their apical membrane
4. Na+ -glucose antiporter on their apical membrane

Introduction: The Mechanism Behind Oral Rehydration Therapy (ORT)

Oral Rehydration Therapy (ORT) is a simple, cost-effective, and widely used treatment for dehydration caused by diarrhea, particularly in developing countries. It works by restoring the balance of water and electrolytes in the body. The key to its effectiveness lies in the absorption of water and electrolytes from the intestines into the bloodstream.

An essential component of ORT is the presence of both sodium ions (Na+) and glucose in the rehydration solution. This combination is crucial because it enables optimal absorption of water and electrolytes from the intestinal lumen into the bloodstream. But what is the underlying mechanism that allows this process to occur?

The Role of the Na+-Glucose Symporter

The intestinal epithelial cells play a critical role in the absorption of nutrients and electrolytes. The specific mechanism that allows the absorption of both sodium and glucose is through a Na+-glucose symporter. This symporter is a type of cotransporter protein that moves Na+ ions and glucose simultaneously across the cell membrane.

Mechanism of Na+-Glucose Symporter:

• The Na+-glucose symporter is located on the apical membrane of the intestinal epithelial cells (the side of the cell facing the lumen of the intestine).

• When glucose and Na+ are present in the lumen of the intestine, the symporter uses the energy stored in the Na+ concentration gradient (which is maintained by the sodium-potassium pump on the basolateral membrane) to drive the active uptake of glucose along with Na+ into the cells.

• As Na+ is absorbed into the cell, it creates an osmotic gradient that helps pull water into the cells, facilitating the absorption of water from the intestinal lumen into the bloodstream.

Why Both Na+ and Glucose Are Essential

The presence of both Na+ and glucose is crucial for effective rehydration:

• Sodium (Na+) helps to create an osmotic gradient, which drives water absorption.

• Glucose acts as a carrier for sodium through the Na+-glucose symporter, and its presence increases the efficiency of sodium absorption, which in turn enhances water uptake.

Without both components, the rehydration process would be less efficient. This is why ORT solutions often contain a balance of sodium and glucose, such as in oral rehydration salts (ORS), to ensure effective treatment for dehydration.

Correct Answer

The correct answer is:

3. Na+ -glucose symporter on their apical membrane

This symporter is located on the apical membrane of the intestinal epithelial cells and is essential for the coupled absorption of Na+ and glucose, which facilitates water absorption and rehydration.

In summary, oral rehydration therapy works effectively due to the action of the Na+-glucose symporter on the apical membrane of intestinal epithelial cells, which allows the coupled absorption of Na+ and glucose, promoting water uptake.