#glut4

***Occasionally, we'll drop in here with some Sciencey/Nursey stuff***

November 14th and GLUT4

The healthy human body is really good at regulating blood glucose (sugar) levels within a normal range. Why? I dunno. You might want to ask a philosopher that question. But, this regulation is significant for many reasons. First, human nerve cells (like the cells that make up your brain) rely on glucose exclusively for energy. A lack of blood glucose and your brain cells will starve and die. Second, but not lastly, muscle cells also rely on glucose for energy, although they do have the ability to use other fuels. During physical activity (i.e., exercise), muscle becomes more and more reliant on glucose/glycogen for energy production, however. Short story: glucose is kinda a big deal and, therefore, blood sugar levels are kinda important too.

But, how does blood glucose get into the cells as needed? There are a lot of players and steps involved here. I want to focus, though, on the glucose transporter isoforms. They go by short hand GLUT1 and GLUT4, for example. Certain cells, like brain cells contain GLUT1 (short for glucose transporter #1). GLUT1 is pretty awesome because it can work nearly independently at the cell membrane and shuttle in all the glucose it wants from the blood to the inside of the cell. Skeletal muscle cells, however, don't so much have the awesomeness of GLUT1. Skeletal muscle cells mostly rely on its cousin GLUT4 (you guessed it, glucose transporter #4).

  Briefly, GLUT4 can also shuttle glucose across the cell membrane. But, GLUT4 lives within muscle cells and relies on some kind of a specific signal in order to get it to do its job. Most people are very familiar with the first signal: Insulin.  The body uses insulin as a signal to transport glucose from the blood and into many different types of cells. We'll stick to (skeletal) muscle here. Insulin is a hormone that binds with its receptor on the muscle cell membrane. This union of insulin to receptor triggers a second messenger system that springs GLUT4 into action. Thus, blood sugar decreases and muscle glycogen increases. Here is where we insert most of our common notions of eating "carbs" and stimulating large rises of blood glucose and insulin! This process is largely thought of as a bad thing. But, biologically speaking, it is physiology that is necessary for survival.  It is also oversimplification. 

Besides insulin, GLUT4 has another major "signal". There is another natural process that starts the same cascade of events as the binding of insulin to its receptor. What is it? Muscle contraction.  That's right. Muscle contraction (as happens during exercise) also signals GLUT4 to do its job and transport glucose from the blood into muscle cells. This transport is insulin independent. What's even cooler is that some awesome research over the last 50 years suggests that regular physical exercise increases the gene expression of GLUT4 and that even insulin sensitivity may be increased for several hours after the end of a single bout of exercise.

Department of Physiology and Biophysics Symposium (Part 2 of 3)