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When the muscle is exercising, and only anaerobic respiration is done, pyruvate -> lactate to regenerate NAD+. Lactate is then transported out of the muscle and into the liver, to regenerate glucose, which is then shuttled back to the muscle (the Cori cycle).

I was wondering whether once the muscle stops exercising, the muscle itself could regenerate pyruvate (using lactate dehydrogenase), or whether muscles always release their lactic acid to the liver? Or maybe there is no way of knowing this (how would this be tested experimentally...).

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  • $\begingroup$ What would you envisage the skeletal muscle doing with the pyruvate, assuming that the liver had not cleared the lactic acid? $\endgroup$ – David Apr 17 '20 at 19:03
  • $\begingroup$ Once there is enough oxygen, I would think that the pyruvate could be used for oxidative phosphorylation. $\endgroup$ – liliae Apr 18 '20 at 12:20
  • $\begingroup$ But the whole point is that the muscles that are used for exercise need an energy source primarily under anaerobic conditions, and have few mitochondria. There is no way of storing pyruvate and in very light aerobic exercise other substrates for mitochondria available. $\endgroup$ – David Apr 18 '20 at 12:36
  • $\begingroup$ Oh okay thank you. Would it be possible for you to explain or attach a link about these myocytes with few mitochondria? I always believed muscles had a lot of mitochondria.. Also, would the other substrates for mitochondria in aerobic exercise (which you mentioned) be fatty acids? Thank you very much for clarifying. $\endgroup$ – liliae Apr 18 '20 at 12:53
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    $\begingroup$ I never was a card carrying metabolism man, but I always understood that you could tell a muscle’s metabolism by its colour: red muscle has a good blood supply for oxidative metabolism — hence it would have the requisite complement of mitochondria; white muscle would be mainly anaerobic. Muscles working continuously, like heart and the flight muscle of migratory birds, would need to be capable of aerobic metabolism. And yes, fatty acids can be used by heart muscle, sparing glucose for tissues like the brain. (All directed by the liver.) $\endgroup$ – David Apr 18 '20 at 16:05

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