Sodium and Glucose uptake

An example of specific absorption is that of sodium in Figure 4.12. Sodium introduced, for example, through the diet and present in the intestinal lumen at a high concentration, can enter the cell by diffusion according to a concentration gradient through specific ion channels present only on the apical membrane (long upper arrow) if its intracellular concentration is kept low.

This is done by a Na⁺/K⁺ ATPase, present only on the basolateral membrane, which releases sodium (long lower arrow) by exchanging it for potassium (short upper arrow).

Figure 4.12. Schematic representation of the sodium absorption pattern in the intestine. See text for explanation

Potassium leaves the cell by gradient diffusion across the basolateral membrane using specific ion channels present only on that side of the membrane (short down arrow) and sodium enters the bloodstream by gradient diffusion across the basal membrane (black down arrow). This complex mechanism keeps the potassium concentration constantly high and the sodium concentration constantly low in the cell, but produces a continuous gradient of sodium flow from the lumen to the circulatory system.

Glucose uptake. Glucose is not able to cross the plasma membrane freely. A probable functional model of glucose uptake at the level of the intestinal epithelium is shown in Figure 4.13; a similar situation is also present in the proximal convoluted tubule of the renal nephron for reabsorption of glucose from the ultrafiltrate.

The cells that form the intestinal epithelium are polarized: they have an apical membrane facing the intestinal lumen and a basolateral membrane facing the basement membrane and blood capillaries (Figure 4.13).

Figure 4.13. Schematic representation of the glucose absorption pattern in the intestine. See text for explanation

In the apical membrane, there are carriers, that is, integral transporter proteins (Cgna) with a high-affinity binding site for glucose (G) and one for sodium (Na). The carrier continually changes conformation due to thermal agitation, but has a very high probability of exposing the two binding sites to the lumen. In the basolateral membrane, there are Na⁺/K⁺ ATPases and specific carriers for glucose (Cg).

If there is a high concentration of glucose in the lumen as a result of ingestion and digestion of sugars, there is a high probability of it binding to the carrier (Cgna in Figure 4.13). In the presence of carrier-bound glucose, the probability is also high that a sodium ion will bind to its specific site and induce a conformational change in the carrier that turns the two sites towards the interior of the cell. Its low concentration makes sodium release highly likely and induces glucose release.

Straddling the apical membrane of the epithelial cell, there is a strong electrochemical gradient for sodium produced by the Na⁺/K⁺ ATPase, which makes the concentration of intracellular sodium particularly low by exchanging a sodium ion (bottom long arrow) for a potassium ion (top short arrow). This results in a gradient-dependent flow of sodium (upper straight arrow), which also induces a gradient-dependent flow of glucose (upper curved arrow) from the lumen into the cell.

Potassium entering the cell through the Na⁺/K⁺ ATPase and glucose entering the cell through the carrier pass through the basement membrane via specific potassium channels (short downward arrow) and another carrier (lower curved arrow), respectively, and then diffuse by concentration gradient to the capillaries. In the capillaries, continuous drainage makes concentrations low, and potassium and glucose enter the blood stream (lower black arrows).

There is sodium and glucose co-transport and secondary active glucose transport also against the gradient. The energy required for glucose flow is provided by the sodium gradient created by the Na⁺/K⁺ ATPase (Data sheet present on the basolateral membrane, for gradient-facilitated transport (section 4.3.1) of glucose.

The overall result is that the continuous entry of sodium according to the gradient is associated with a continuous entry of glucose against the gradient, regulated only by the glucose concentration in the lumen and the difference in sodium concentration between the lumen and the epithelial cell interior.