Insulin is said to be the "unlock" key to let glucose enter body cells.
Then why do stable Type 1 Diabetes patients have to inject less insulin in case of scheduled sport activities?
Don't muscles cells need insulin as well to tap glucose?
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No, certain cells like skeletal muscle cells (during exercise), liver cells, red blood cells and the brain do not need insulin for glucose uptake. During exercise, the glucose uptake by muscles is increased so glucose level in the blood drops down. Thus, less (or no) insulin is needed during exercise.
For more details, I found useful information in a biology lecture in Columbia University, New York: http://www.columbia.edu/cu/biology/courses/c2006/lectures11/lect16.11.html
I can summarize it like this: Insulin works on cell surface receptors. In resting skeletal muscle & adipose tissues insulin acts through the receptor GLUT4 which promotes the infusion of glucose to the cell cytoplasm.
In liver and brain: They can take up glucose without insulin -- they do not use GLUT4. They use different transporters (GLUT 1, 2 &/or 3) located permanently in the plasma membrane.
Insulin has no effect on glucose uptake in brain.
In Working skeletal muscle: Insulin is not required for the uptake of glucose because exercise mobilizes GLUT4 in skeletal muscle. I hope this is clear fo you.