Understanding Glucose transport and alternative pathways

Is Insulin the only 'driver' of glucose into cells?

By Veeraj Goyaram

The basics of glucose transport

Our muscles represent an important site where glucose from the blood stream is taken up to restore normal blood glucose levels in after a meal.  In scientific terms, the muscles is said to be an important site for glucose disposal because of its large size and ability to utilise and store glucose.

The insulin-dependent glucose transport system

After you eat a meal, insulin is released from the pancreas into the bloodstream.  This insulin travels to the muscles and then binds to receptors on the cell membrane (sarcoplasm).  This initiates a series of signals inside the cells to eventually activate special transporter molecules called glucose transporter 4 (GLUT4) to move to the cell surface to transport glucose, as shown below:

The Insulin-dependent system of Glucose uptake by muscles

The above pathway is called the "insulin-dependent" pathway because of its reliance on insulin.  This is the pathway which is the most commonly known in blood glucose regulation.  In fact this is the pathway that most people believe to be the sole existing pathway. This is what has always been taught in schools and even in university physiology, to medical doctors, much to the neglect of another very important and therapeutically promising pathway: THE INSULIN-INDEPENDENT PATHWAY. 

I have personally been surprised to see how this pathway is so under emphasized in teaching curricula. I have spoken to several medical students and come to realise that only a mere mention (compared to the large emphasis on the insulin-dependent pathway) is made of of this pathway which is really a pity because it has great importance and therapeutic potential. 

The Insulin-independent glucose uptake pathway

When you exercise, your muscles contract and in so doing induce many rapid changes in the muscle cells. For example an increase in cellular calcium levels occur as well as a decrease in energy (ATP) levels.  These act as "signals" to activate GLUT4 transporters to transport glucose into cells, thus maintaining normal glucose levels in blood.  It is interesting to note that the popular diabetes drug Metformin is an activator of AMPK and therefore does some of the same jobs as muscle contraction does.  Of course, Metformin is big $$$ business and that's all I can say for now. 

The take-home message of this article

I hope this gives you an appreciation of the benefits of exercise as a cheap remedy, accessible to most of us. Now the next question is, what exercise? How long? How intense? Our future posts will focus on these points.

In the next instalment I will explain how exercise not only increases the activity but also the AMOUNTS of these glucose transporters. The more transporters the better you are able to control blood sugar.

Thank you.