Does Muscle Respond to Glucagon?

[SuperSneaky24]

I was doing random questions and this popped up:
http://www.mcatquestion.com/findquestion.php?arg1=417

Of the tissues listed below, which ones break down glycogen when stimulated by glucagon?
I. Liver
II. Muscle
III. Brain

A. I only
B. I and II
C. III only
D. I, II, and III

I am very confused why the answer is D (highlight the empty space to get the answer). I chose A pretty quickly, thinking I was right.

So apparently brains have glucagon receptors, but that's not the important point. I could have sworn from a practice test that muscles didn't have any glucagon receptors and thus did not respond to glucagon. Is this website wrong, or is my understanding wrong?

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[Volvulus10]

Muscle responds to glucagon but does not export it's glucose from myocytes. If your muscle did not respond to glucagon, you'd be in big trouble. The enzyme glycogen phosphorylase is deficient in a glycogen storage disease known as McArdles Disease. There is severe muscle cramping, myoglobinuria (myoglobin in urine), and rhabdomyolysis (muscle breakdown) classically seen during exercise. I only give the disease info to help you remember why muscle needs glucagon!

 

[tdod]

the question is correct; the muscle also stores glycogen.

http://en.wikipedia.org/wiki/Glycogen

 

[SuperSneaky24]

Muscle responds to glucagon but does not export it's glucose from myocytes. If your muscle did not respond to glucagon, you'd be in big trouble. The enzyme glycogen phosphorylase is deficient in a glycogen storage disease known as McArdles Disease. There is severe muscle cramping, myoglobinuria (myoglobin in urine), and rhabdomyolysis (muscle breakdown) classically seen during exercise. I only give the disease info to help you remember why muscle needs glucagon!

Thanks for the response, but I know that muscles need to break down glycogen for obvious purposes. I just don't get how if there aren't any glucagon receptors in muscles (but there are insulin receptors), how they respond to glucagon at all. I've always thought the insulin would build up glycogen storages in muscle, but then it would be muscle contraction that broke down the glycogen, but not glucagon (and that the liver would be the only organ to respond to glucagon).

the question is correct; the muscle also stores glycogen.

http://en.wikipedia.org/wiki/Glycogen

Well I know muscles store glycogen, as they obviously need a storage of glucose. My question was whether they respond to glucagon or not. I know they respond to insulin, but thought they had no glucagon receptors.

Edit: Okay, a little more googling told me that some smooth muscle have glucagon receptors. I've always thought skeletal muscles have no glucagon receptors and thus don't respond to glucagon (but they do respond to insulin and have glycogen in them). Can anyone confirm this?

 

[Volvulus10]

Thanks for the response, but I know that muscles need to break down glycogen for obvious purposes. I just don't get how if there aren't any glucagon receptors in muscles (but there are insulin receptors), how they respond to glucagon at all. I've always thought the insulin would build up glycogen storages in muscle, but then it would be muscle contraction that broke down the glycogen, but not glucagon (and that the liver would be the only organ to respond to glucagon).


Well I know muscles store glycogen, as they obviously need a storage of glucose. My question was whether they respond to glucagon or not. I know they respond to insulin, but thought they had no glucagon receptors.

Edit: Okay, a little more googling told me that some smooth muscle have glucagon receptors. I've always thought skeletal muscles have no glucagon receptors and thus don't respond to glucagon (but they do respond to insulin and have glycogen in them). Can anyone confirm this?

Don't forget about epinephrine - a stimulator of glycogenolysis in skeletal muscle. http://www.ncbi.nlm.nih.gov/books/NBK22429/

 

[SuperSneaky24]

Right, but that's not my main question. I just included the insulin fact to show how they get the glucose to store into glycogen in the first place.

 

[mpeach]

I was doing random questions and this popped up:
http://www.mcatquestion.com/findquestion.php?arg1=417

Of the tissues listed below, which ones break down glycogen when stimulated by glucagon?
I. Liver
II. Muscle
III. Brain

A. I only
B. I and II
C. III only
D. I, II, and III

I am very confused why the answer is D (highlight the empty space to get the answer). I chose A pretty quickly, thinking I was right.

So apparently brains have glucagon receptors, but that's not the important point. I could have sworn from a practice test that muscles didn't have any glucagon receptors and thus did not respond to glucagon. Is this website wrong, or is my understanding wrong?

You are correct! There are no glucagon receptors on skeletal muscle! Here is why... muscle glycogen stores are not for the purpose of replenishing blood glucose levels, they are strictly for internal energy consumption only, hence the reason muscle has no Glucose-6-phosphatase, it does not need it. The enzyme Glycogen Phosphorylase IS active when phosphorylated and this IS a result of the beta-adrenergic receptor but only as a response from epinephrine, not glucagon. See Lippincott's Biochemistry Review, 6th Ed, page 314 for confirmation of muscles lack of glucagon receptor.

 

[mpeach]

Muscle responds to glucagon but does not export it's glucose from myocytes. If your muscle did not respond to glucagon, you'd be in big trouble. The enzyme glycogen phosphorylase is deficient in a glycogen storage disease known as McArdles Disease. There is severe muscle cramping, myoglobinuria (myoglobin in urine), and rhabdomyolysis (muscle breakdown) classically seen during exercise. I only give the disease info to help you remember why muscle needs glucagon!

There are no glucagon receptors on skeletal muscle. The beta-adrenergic response is due to epinephrine. McArdles disease is only a problem when the muscle needs energy, not when blood glucose levels are low. Glycogen in the muscles is strictly consumed by the muscle, not released into the blood stream, therefor the muscle has no use for glucagon receptors and in fact contains none.

 

[manohman]

You are correct! There are no glucagon receptors on skeletal muscle! Here is why... muscle glycogen stores are not for the purpose of replenishing blood glucose levels, they are strictly for internal energy consumption only, hence the reason muscle has no Glucose-6-phosphatase, it does not need it. The enzyme Glycogen Phosphorylase IS active when phosphorylated and this IS a result of the beta-adrenergic receptor but only as a response from epinephrine, not glucagon. See Lippincott's Biochemistry Review, 6th Ed, page 314 for confirmation of muscles lack of glucagon receptor.

Wait but dont glycogen phosphorylase and g6p-ase do the same thing in converting glyocgen to glucose? Or is it that glycogen phosphorylase does it direct and g6p-ase is near the end of a different process starting from glyxogen and ending in glucose?

 

[mpeach]

Wait but dont glycogen phosphorylase and g6p-ase do the same thing in converting glyocgen to glucose? Or is it that glycogen phosphorylase does it direct and g6p-ase is near the end of a different process starting from glyxogen and ending in glucose?

Glycogen phosphorylase breaks the a-1,4 bonds in glycogen creating glucose-1-P and limit dextrin. G-1-P is converted to G-6-P by phosphoglucomutse. In the muscle, G-6-P goes into glycolysis. In the liver, G-6-P is converted to glucose via G-6-Pase and released in to the blood stream. G-6-Pase is also used in gluconeogenesis to release glucose in to the blood stream hence the two pathways converge here in their common (hepatic!) function: to elevate blood glucose levels. In the liver, glycogen phosphorlyase is activated indirectly by glucagon. In skeletal muscle, which is responding to increased energy demand, glycogen phosphorylase is activated indirectly by epinephrin and calcium and directly activated by AMP. Hope this helps...

 

[manohman]

Glycogen phosphorylase breaks the a-1,4 bonds in glycogen creating glucose-1-P and limit dextrin. G-1-P is converted to G-6-P by phosphoglucomutse. In the muscle, G-6-P goes into glycolysis. In the liver, G-6-P is converted to glucose via G-6-Pase and released in to the blood stream. G-6-Pase is also used in gluconeogenesis to release glucose in to the blood stream hence the two pathways converge here in their common (hepatic!) function: to elevate blood glucose levels. In the liver, glycogen phosphorlyase is activated indirectly by glucagon. In skeletal muscle, which is responding to increased energy demand, glycogen phosphorylase is activated indirectly by epinephrin and calcium and directly activated by AMP. Hope this helps...

Holy cow thank you. That cleare it up a lot! Thanks!

 

[mpeach]

No problem! I'm new to this forum but if there is a way to do this, you can ask me questions about biochem anytime.