Question

1. Duncan is a Caucasian male (age 14) that is brought to your attention because his family concerned about his inability to perform strenuous exercise. Duncan suffered severe muscle cramping during his P.E. classes but is normal if at doing routine physical activity or performing light to moderate exercise. A physical examination showed poorly developed muscle tissue, but his liver size was normal. A fasting glucose level check showed normal levels.
2. 1. You suspect that Duncan is suffering from a glycogen metabolism related disease and check his response to glucagon. You inject him with a large dose of the hormone and measure the glucose levels in his blood. After the injection, Duncan’s blood glucose levels rose sharply.

Why did you perform this test?

Is Duncan’s glucagon response what you would expect in a normal person? Explain briefly

2. To further investigate you analyze biopsies of Duncan’s muscle and liver. Although the structure of the glycogen in both tissues appear to be normal, there is an elevation in the glycogen level in the muscle tissue, while that in the liver is within normal range.

What is a hypothesis regarding Duncan’s condition?

Answer 1

Background information:

Glycogen stores in muscle and liver. The liver converts its stored glycogen to glucose when the blood level of glucose decreases while muscle converts when the requirement of ATP increases. Therefore, the storage level of glycogen in these two varies depending upon the demand of the body.

Glucagon is the hormone responsible for the breakdown of glycogen to glucose in the liver only. This hormone is not involved in the breakdown of glycogen in muscle, while epinephrine plays a vital role in the break down of glycogen in muscle as well as liver.

From the information given in the problem, it can be concluded that Duncan is suffering from McArdle’s Disease. In this disease, there is the malfunctioning of the glycogen phosphorylase enzyme of muscle tissue only. In this disease, the glycogen phosphorylase enzyme will be affected (figure) and thereby affect the glycogen catalysis in muscles.

Answers to the questions

Why did you perform this test?

Explanation: As mentioned above, the glycogen stores in the liver and muscles. Therefore it is essential to know does liver glycogen metabolism is normal? This information will also tell glycogen metabolism is affected in muscles only.

Is Duncan’s glucagon response what you would expect in a normal person? Explain briefly

Explanation: Yes, glucagon response will be similar to the normal person. Because glucagon will affect only glycogen metabolism in the liver, not in muscles, and the liver of the Duncan is functioning normally.

What is a hypothesis regarding Duncan’s condition?

Explanation: The absence of functional glycogen phosphorylase in muscles will prevent glycogen breakdown, and therefore amount will be increased in muscles.