Question

1. Describe the Na⁺-glucose transporter. Describe how glucose is transported from the intestinal lumen to the extracellular fluid in the gut lining.

Answer 1

Na+ -GLUCOSE TRANSPORTER-

Sodium-glucose transporters also said to be Na+-glucose cotransporters or symporters (SGLTs).

Sodium-glucose cotransporters (SGLTs) are mediators of glucose uptake across apical cell membranes.

SGLT1mediates almost all sodium-dependent glucose uptake in the small intestine,

In the kidney SGLT2, and to a less extent SGLT1, account for more than 90% and nearly 3%, respectively, of glucose reabsorption from the glomerular ultrafiltrate.

GLUCOSE TRANSPORTER FROM INTESTINAL LUMEN TO EXTRA CELLULAR FLUID IN THE GUT LINING-

The main apical transporter for active glucose uptake in small intestinal epithelial cells is the sodium-dependent glucose co transporter (SGLT).

SGLT1unidirectionally mediates glucose absorption from the intestinal lumen into epithelial cells. SGLT-1 co transports glucose and sodium.

The transporter is initially facing into the lumen and is capable of binding sodium but not glucose. Sodium binds and induces a change that opens the glucose-binding pocket.

The transporter then reorients in the membrane the pockets holding sodium and glucose are moved inside the cell. Sodium dissociates into the cytoplasm due to the conformal change which occurs causing glucose binding to destabilize and dissociate into the cytoplasm. This transcellular pathway is powered by a downhill gradient of Na⁺ across the apical membrane, maintained by the basolateral Na⁺/K⁺ GLUT2 in the basolateral membrane.

The GLUT-2 transports glucose, providing a common exit pathway into the blood.

Fructose is transported across the apical membrane by a specific facilitative transporter, GLUT5. (GLUT5)is a facilitative transporter not an active transport system.

The basolateral transporter GLUT-2 facilitates diffusive transport of intracellular glucose into the interstitium and bloodstream.