glucose transporter

 The cells of the body use glucose for energy, and the brain must have glucose to function. Glucose attracts water, but because cell membranes are made of lipids, or fats, they do not let substances pass through them and enter the cell if those substances attract water. To pass through the cell membrane, glucose needs a protein to carry it. These are the glucose transporters. There are 13 of them, but scientists have a good understanding only about the first four.

	特性	NOTE
GLUT 1	-widely distributed in fetal tissues. -adult, RBC (mc), also endothelial cells of barrier tissues, BBB. -is responsible for low-level of basal glucose uptake required to sustain respiration in all cells.	低血糖 -> ↑ GLUT1 高血糖 -> ↓ GLUT1
GLUT 2	-a bidirectional transporter, allowing glucose to flow in 2 directions. -expressed by 腎小管 cells, 小腸上皮cells (also present in the basolateral membrane), 肝 cells and 胰臟beta cells. -Bidirectionality is required in liver cells to uptake glucose for glycolysis, and release of glucose during gluconeogenesis. -In 胰臟 beta cells, free flowing glucose is required so that the intracellular environment of these cells can accurately gauge the serum glucose levels. -小腸借此吸收入all three monosaccharides (glucose, galactose and fructose)	-high-capacity / low-affinity isoform. There is some evidence that GLUT 1 and 3 are actually the functional transporters in beta cells.
GLUT 3	neurons (mc)(the main glucose transporter isoform), placenta.	-high-affinity isoform, -允許腦在低血糖下仍然可uptake葡萄糖
GLUT 4	脂肪組織 striated muscle (骨骼肌 / 心肌).	Is insulin-regulated. -for insulin-regulated glucose storage.

GLUT 1

Glucose transporter protein 1, or GLUT 1, is in all of the tissues of the body, but especially in the red blood cells and in the layer of flat cells that line the blood vessels of the brain, according to Patricia Molina, M.D., Ph.D., head of the department of physiology at the Louisiana State University Health Sciences Center. This transporter protein has a strong attraction for glucose. Because of its strong attraction, it can make sure that the red blood cells and the brain get the glucose that they need to be able to function.
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GLUT 2

GLUT 2 does not have a strong attraction for glucose. It transports glucose only when the glucose level in the bloodstream is high. The levels are usually high after people eat. GLUT 2 is the main transporter for the cells in the kidneys, intestines and liver. When the glucose level in the blood increases, these three organs get more glucose, as explained by Umesh Masharani, M.B., clinical professor of medicine at the University of California, in "Greenspan's Basic & Clinical Endocrinology."


 GLUT 3

Like GLUT 1, this transporter protein is in all of the tissues of the body and has a strong attraction for glucose. But GLUT 3 is especially in the nerve cells. It has the job of making sure that the nerve cells have the amount of glucose that they need to properly function. Dr. Molina writes in her book "Endocrine Physiology" that GLUT 3 and GLUT 1 are very important in making sure that the nerve cells of the brain have glucose.

GLUT 4

Fat tissue and skeletal muscle need a glucose transporter protein and insulin to move glucose from the bloodstream into their cells. Most transporter proteins are on the cell membranes. But approximately 90 percent of GLUT 4 is inside of the fat tissue and skeletal muscle, according to Dr. Molina. So, the pancreas releases insulin, which goes to the fat tissue and skeletal muscle. The insulin attaches to special proteins on the cell membrane. These proteins tell GLUT 4 to move from inside of the cell to the membrane. Then GLUT 4 can move glucose into the cell