Explain how glucose metabolism is regulated during exercise
Muscle glucose uptake is a three-step process consisting of muscle glucose delivery, membrane transport, and intracellular phosphorylation. With exercise, 70 to 85 percent of the cardiac output goes to muscle. This hyperemia increases delivery of glucose to muscle. In response to muscle contraction, GLUT4 is translocated from the intracellular storage vesicle to the sarcolemma. This makes the sarcolemma highly permeable to glucose, and it can accommodate the high muscle glucose influx during exercise. Exercise and insulin are the two major stimuli for GLUT4 translocation to the sarcolemma. A number of pathways may play a role in contraction-stimulated muscle glucose uptake. Muscle contraction requires cytosolic calcium influx from the sarcoplasmic reticulum, which can then act through a number of calcium-sensitive signaling pathways. Activation of AMPK is thought to increase glucose transport in muscle. Inside the cell, glucose is irreversibly phosphorylated by hexokinase. Exercise increases glucose phosphorylation capacity by causing the hexokinase II isozyme to bind to mitochondria, where it would have access to ATP, a substrate in the reaction it catalyzes.
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