Glucose transporters (GLUTs) are classified into three groups based on sequence similarity, with a total of 14 members.
[7] GLUT4 is an insulin-responsive glucose transporter located in the heart, skeletal muscle, brain, and adipose tissue.
Facilitated diffusion can occur between the bloodstream and cells as the concentration gradient between the extracellular and intracellular environments is such that no ATP hydrolysis is required.
However, in the kidney, glucose is reabsorbed from the filtrate in the tubule lumen, where it is at a relatively low concentration, passes through the simple cuboidal epithelia lining the kidney tubule, and into the bloodstream where glucose is at a comparatively high concentration.
Hence reabsorption of glucose is dependent upon the existing sodium gradient which is generated through the active functioning of the Na+/K+-ATPase.
As the cotransport of glucose with sodium from the lumen does not directly require ATP hydrolysis but depends upon the action of the ATPase, this is described as secondary active transport.
[11] SGLT1 transporters are found close to the loop of Henle and in the distal convoluted tubule of the nephron where much glucose has been reabsorbed into the bloodstream.
[11] Functioning in conjunction, these two secondary active transporters ensure that only negligible amounts of glucose are wasted through excretion in the urine.