Muscle and Glycogen Breakdown

[betterfuture]

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So if glycogen synthesis occurs in the skeletal muscles, why is it that during the postabsorptive state, glycogen is not broken down in muscles?

 

[aldol16]

It is broken down, but only for muscle contraction. Your skeletal muscle cells lack the machinery to export glucose into the blood.

 

[NextStepTutor_2]

So if glycogen synthesis occurs in the skeletal muscles, why is it that during the postabsorptive state, glycogen is not broken down in muscles?

Muscle cells are not designed to release their glucose, as their primary role is for E consumption and force generation.
The rate at which glycolysis proceeds depends on the rate of glycogen breakdown.

In resting muscle, little glycogen is broken down, so the rate of glycolysis is limited by muscle glucose uptake. However, during exercise, glycogen breakdown is greatly accelerated, and glycogen, not glucose, is the major precursor for glycolysis. For instance, during steady-rate exercise at 65% of glycogen breakdown can exceed glucose uptake by four to five times. Thus, glycolysis is under “feed-forward” control. Skeletal muscle glycolysis is heavily dependent on the intramuscular glycogen. During heavy exercise, glycogen may supply most of the immediate glucosyl residues for glycolysis, 80% or more of the carbon for glycolysis in muscle comes from the glycogen in muscle, and not from blood glucose.

For the MCAT, the AAMC has outlined that you should know that glucose 6-phosphate derived from glycogen can:

(1) be used as a fuel for anaerobic or aerobic metabolism as in muscle

(2) be converted into free glucose in the liver and subsequently released into the blood

(3) be processed by the pentose phosphate pathway (on the new MCAT) to generate NADPH or ribose in a variety of tissues

Glycogenolysis does occur in the muscle cells but there is a difference between what occurs immediately after a meal:

and then during fasting.

Hope this helps, good luck!

 

[betterfuture]

I see. So how is it that amino acids can come from muscles for gluconeogenesis? Are the amino acids coming from protein breakdown (muscle breakdown)? Why does that happen during postabsorptive state?

 

[aldol16]

I see. So how is it that amino acids can come from muscles for gluconeogenesis? Are the amino acids coming from protein breakdown (muscle breakdown)? Why does that happen during postabsorptive state?

In the post-absorptive state (soon after eating), you're not really undergoing gluconeogenesis. You undergo gluconeogenesis when you're fasting and your body is starving for glucose. The AAs you use during the post-absorptive state come from food. They're used to build up muscle. Then, when you're starving, those AAs from your muscle are then metabolized for glucose. This is a last resort because the body has no way to make amino acids de novo. By de novo, I mean a total synthesis. So if you starve for a long period of time, you're going to lose muscle mass and losing muscle mass inevitably results in death if uncorrected.

 

[betterfuture]

So I don't know why but I got that statement from Kaplan. They state that during the postabsorptive state(I am guessing when you sleep?), glucagon and other counterregulatory hormones release and stimulate gluconeogenesis in the skeletal muscles. The fuel that the three tissues(liver, adipose, and skeletal muscle) rely on then is fatty acid. Can you confirm this for me.

 

[aldol16]

So I don't know why but I got that statement from Kaplan. They state that during the postabsorptive state(I am guessing when you sleep?), glucagon and other counterregulatory hormones release and stimulate gluconeogenesis in the skeletal muscles. The fuel that the three tissues(liver, adipose, and skeletal muscle) rely on then is fatty acid. Can you confirm this for me.

 

[betterfuture]

So then what is the postprandial state? I thought that was right after one ate?

 

[aldol16]

There's "right after somebody eats," "a few hours after somebody eats," and "starving." In the late post-absorptive state, you can maintain blood glucose using glycogenolysis. Only in fasting does it make sense to do gluconeogenesis.

 

[geykey]