Exercise glucose transport into muscle

Glucose Transport into Muscle during Exercise A Contradiction ... 

Glucose transport into muscle cells can be stimulated during exercise because of the activity of the AMP-activated protein kinase Fatty acid uptake into exercising muscle is dependent on the levels of circulating fatty acids, which are increased by epinephrine release. 

GLUT4, which has a AT value of 5 mM, transports glucose into muscle and fat cells. The presence of insulin, which signals the fed state, leads to a rapid increase in the number of GLUT4 transporters in the plasma membrane. Hence, insulin promotes the uptake of glucose by muscle and fat. The amount of this transporter present in muscle membranes increases in response to endurance exercise training. 

Once the fatty acids are in the cytoplasm of the muscle cells, they are transported across the mitochondrial membrane by carnitine palmityol transferases I and II for P-oxidation and energy production. During exercise, total fat oxidation rates can increase more than ten-fold during the transition from rest to moderate-intensity exercise. The availability of glucose, rather than that of fatty acids, controls the rate of fatty acid oxidation because glucose can decrease the oxidation of long-chain fatty acids by inhibiting their transport into the mitochondria. 

Muscle GLUT4 transporters also can be induced by AMP levels (see Chapter 47), through the actions of the AMP-activated protein kinase. Thus, if energy levels are very low, and AMP levels increase, glucose can still be transported from the circulation into the muscle to provide energy. This most often occurs during exercise. 

In animals, lactic acid is transported out of the muscles to the liver, where it is reconverted into glucose or amino acids. Lactic acid buildup is a source of the muscle stiffness that occurs after vigorous exercise. In yeast, pyruvate is converted to acetaldehyde and CO2 then the acetaldehyde is reduced to ethanol. 

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