Understanding Intestinal Glucose Absorption Mechanism

The absorption of glucose in the small intestine primarily involves a sodium-dependent co-transport mechanism. At the apical membrane of intestinal epithelial cells (enterocytes), the sodium-glucose linked transporter 1 (SGLT1) uses the sodium gradient to drive glucose uptake from the intestinal lumen into the cell. This process is a form of secondary active transport because the energy to move glucose against its concentration gradient is derived from the sodium electrochemical gradient.

Maintaining this sodium gradient across the basolateral membrane of enterocytes is crucial. The enzyme responsible for this is the Na+-K+ ATPase, located on the basolateral side, which actively pumps sodium out from the cell into the interstitial space (and consequently into the bloodstream), while importing potassium into the cell. This action lowers intracellular sodium concentration, enabling continuous sodium influx at the apical membrane through SGLT1 and thus promoting effective glucose absorption.

Effect of Ouabain on Na+-K+ ATPase and Glucose Absorption

Ouabain is a well-known inhibitor of the Na+-K+ ATPase pump. By binding specifically to this pump, ouabain blocks its activity, preventing it from transporting sodium ions out of the enterocytes to the interstitial space.

When ouabain inhibits this pump:

• The intracellular sodium concentration rises because sodium is no longer effectively extruded.

• The sodium gradient across the apical membrane diminishes.

• Secondary active transport of glucose via SGLT1 slows down or stops because it depends on a favorable gradient of sodium moving into the cell.

• Consequently, intestinal glucose absorption is impaired.

Correct Explanation of Ouabain Action

Among the given options, the correct explanation is:

(3) The inhibitor has blocked the transport of Na+ from epithelial cells to interstitial space.

This is because the Na+-K+ ATPase actively transports sodium ions from the cytoplasm of epithelial cells into the interstitial space. The blocking effect of ouabain on this pump prevents sodium efflux, disrupting the sodium gradient essential for glucose co-transport at the apical membrane.

Summary

• Na+-K+ ATPase located on the basolateral membrane of intestinal epithelial cells pumps sodium out into the interstitial space, maintaining a low intracellular sodium concentration.

• This sodium gradient is essential for sodium-dependent glucose co-transport (SGLT1) into the enterocytes.

• Ouabain inhibits Na+-K+ ATPase, resulting in elevated intracellular sodium, reduced sodium gradient, and impaired glucose absorption.

• Therefore, ouabain blocks the transport of sodium from epithelial cells to interstitial space, disrupting glucose uptake.