13.13:
Glucose Absorption Into the Small Intestine
Glucose is absorbed into the small intestine via transcellular transport, which takes place in two stages — import into the intestinal epithelial cells from the apical side and subsequent export through the basolateral side.
Import begins with the binding of two sodium ions from the intestinal lumen to the sodium-glucose symporter — SGLT1. This allows one glucose molecule to bind against its concentration gradient.
After releasing the bound sodium ions and glucose molecule inside the cell, the transporter reverts to its original conformation.
The intracellular glucose is exported out of the cell via facilitated transport through the GLUT2 uniporter located on the basolateral membrane.
To maintain a low cytosolic sodium concentration, the sodium/potassium pump on the basolateral side actively pumps out three sodium ions from the cell in exchange for two potassium ions.
In order to regulate the intracellular potassium concentration, the potassium channels present on the basolateral domain continuously pump the excess potassium ions out of the cell.
In this way, glucose crosses the epithelial cells and reaches the bloodstream for circulation.
13.13:
Glucose Absorption Into the Small Intestine
Complex carbohydrates consumed cannot be absorbed into the small intestine in their original form. First, they must be hydrolyzed to a monosaccharide form such as glucose or galactose. These monosaccharides are then transported across the intestinal membrane and into the blood via transcellular transport. The intestinal epithelial cells allow the movement of these monosaccharides with a defined 'entry' through membrane transporter proteins present on their apical membrane and 'exit' via the basolateral membrane proteins.
The classical pathway for this absorption across the intestinal membrane is mediated by a symporter, sodium-glucose linked transporter 1 (SGLT1). SGLT1 is present on the apical membrane of intestinal epithelial cells and couples the transport sodium ions and the monosaccharides l(glucose or galactose) into the cell.
The absorption of glucose in the small intestinal epithelium is electrogenic, depending on the membrane potential of the intestinal epithelial cells that regulate the activity of SGLT1. The maintenance of membrane potential depends on the activities of the channels and transporters. In the small intestine's epithelial cells, the potassium channels provide the driving force required for sodium-dependent uptake of glucose into the intestinal epithelial cells. The glucose uptake is further driven by the sodium transmembrane gradient and membrane potential maintained by the sodium-potassium pump. Thus, the sodium-potassium pump and potassium channel play a vital role in the glucose movement into the cell. The accumulated glucose is transported via GLUT2 transporter protein on the basolateral membrane.
Metabolic disorders like diabetes show increased expression of SGLT1, contributing to increased glucose absorption in the small intestine. Therefore, reducing the SGLT1-mediated transport of glucose appears to be one of the therapeutic targets for diabetes treatment.
Suggested Reading
- Chen, Lihong, Biguang Tuo, and Hui Dong. "Regulation of intestinal glucose absorption by ion channels and transporters." Nutrients 8, no. 1 (2016): 43.
- Fung, Karen YY, Gregory D. Fairn, and Warren L. Lee. "Transcellular vesicular transport in epithelial and endothelial cells: Challenges and opportunities." Traffic 19, no. 1 (2018): 5-18.
- Gromova, Lyudmila V., Serguei O. Fetissov, and Andrey A. Gruzdkov. "Mechanisms of Glucose Absorption in the Small Intestine in Health and Metabolic Diseases and Their Role in Appetite Regulation." Nutrients 13, no. 7 (2021): 2474.