Adrenaline releases glucose from the liver during sport or if starved. This glucose goes to the blood through GLUT2 transporter. But how does it get transported into the muscle cells? GLUT4 is the glucose transporter into muscles, but it is activated (translocation to plasma membrane) by insulin, but when exercising/starved, there is no insulin. So is the glucose transported through some other transporter?
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GLUT4 is the major glucose transporter in muscle even in exercise. However, during exercise, it is translocated to the cell surface by contraction-triggered, insulin-independent mechanisms, such as AMP-activated protein kinase (AMPK) activation.
For more, you could read this review.
Note:
'No insulin' is too strong a phrase; exercise and fasting are associated with low but detectable insulin levels. The reference range for fasting insulin in healthy individuals is $<174 \ \text{pmol L}^{-1}$ (contrast this with levels upto $1917 \ \text{pmol L}^{-1}$ one hour after glucose ingestion). Exercise studies have shown levels around $60-80 \ \text{pmol L}^{-1}$.
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1$\begingroup$ A nice concise answer, and a most useful reference. However you might comment on the poster’s statement that “there is no insulin”. The concentration of insulin may be decreased, but surely not to zero. Do you know any figures? $\endgroup$– DavidCommented Apr 18, 2020 at 17:58
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$\begingroup$ A valuable contribution to the site. Hope to see more posts from you. $\endgroup$– DavidCommented Apr 20, 2020 at 19:29