Sodium-glucose transport proteins

Sodium-dependent glucose cotransporters (SGLT) are a family of glucose transporter found in the intestinal mucosa (enterocytes) of the small intestine (SGLT1) and the proximal tubule of the nephron (SGLT2 in PCT and SGLT1 in PST). They contribute to renal glucose reabsorption. In the kidneys, 100% of the filtered glucose in the glomerulus has to be reabsorbed along the nephron (98% in PCT, via SGLT2). In case of too high plasma glucose concentration (hyperglycemia), glucose is excreted in urine (glucosuria); because SGLT are saturated with the filtered monosaccharide. One must know that glucose is never secreted by the nephron.

Types
The two most well known members of SGLT family are SGLT1 and SGLT2, which are members of the SLC5A gene family.

Including SGLT1 and SGLT2, there are total seven members in the human protein family SLC5A, several of which may also be sodium-glucose transporters.

Function
First, the Na+/K+ ATPase pump on the basolateral membrane of the proximal tubule, cell actively (requires ATP) transports sodium from this cell into the peritubular capillary. This creates a downhill sodium gradient inside the proximal tubule cell. The SGLT proteins use the energy from this downhill sodium gradient created by the ATPase pump to transport glucose across the apical membrane against an uphill glucose gradient. Therefore, these co-transporters are an example of secondary active transport. (The GLUT uniporters then transport the glucose across the basolateral membrane, into the peritubular capillaries.) Both SGLT1 and SGLT2 are known as symporters, since both sodium and glucose are transported in the same direction across the membrane.

Discovery of sodium-glucose cotransport
In August 1960, in Prague, Robert K. Crane presented for the first time his discovery of the sodium-glucose cotransport as the mechanism for intestinal glucose absorption.

Crane's discovery of cotransport was the first-ever proposal of flux coupling in biology.