When muscles require energy for short duration or strenuous movements, muscle cells default to anaerobic glycolysis to quickly produce abundant amounts of ATP. The byproduct of anaerobic glycolysis, lactate, diffuses into the blood and is taken up by the liver, where it is converted back into pyruvate by the enzyme lactate dehydrogenase. Pyruvate is then converted back into glucose via gluconeogenesis. The newly formed glucose is released into the blood to be used once again for energy by the red blood cells and muscle. Note that the Cori cycle is energy consuming (4 net ATP used per cycle; there is a 2 ATP gain in the anaerobic glycolysis of glucose and there is a consumption of 6 ATP in the production of glucose via gluconeogensis).
Its importance is based on the prevention of lactic acidosis in the muscle under anaerobic condition. The muscles are cramped when the lactate is accumulated. Lactate is the blind end of the biochemical reaction. The unwanted lactate can be only converted to pyruvate with lactate dehydrogenase.
For red blood cells (erythrocytes), the cycle is essential as the cells do not contain the mitochrondria; the significance is to preserve the oxygen transported by the cells. The NAD+ can be regenerated via the lactate fermentation. The Cori cycle is essential for maintaining glycolysis in the erythrocytes as the Cori cycle continue providing the glucose molecules as the fuel molecules.
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