Question

McArdle Disease (glycogen storage disease 5) is caused by mutations in the phosphorylase enzyme in muscle. The symptoms are muscle cramps, pain, and fatigue during strenuous exercise. An ischemic exercise test is often used as part of the diagnostic process. This test measures lactate in the blood but is not always specific for McArdle. Explain why lactate might be a good diagnostic measurement for this disease and how its production is regulated in muscle in a normal person versus a McArdle patient

Answer 1

McArdle Disease is a rare abnormality where a person will not be able to break down glycogen stored in their muscle cells because of the absence of an enzyme myophosphorylase. Lack of this enzyme will not let your muscles use stored gycogen.

Ischemic exercise test is used to diagnose McArdle Disease. The test will study the levels of lactate in the blood after the muscle has exercised ischemically. Normal increase in the levels indicate no disease. But if the exercise does not lead to increase in the levels of lactate in blood, it is indicative of the glycogen storage disease.

When a person is exercising strenuously, stored glycogen is used in the muscle cells and, under anaerobic conditions, lactate is produced. Anaerobic respiration occurs because of the lack of sufficient o2 reaching the muscle. The excessively released lactate is carried to liver along with blood where it is converted back to pyruvate. Pyruvate may be converted back to glycogen or it may be completely broken to form CO2 and H2O. So whenever a muscle is working hard, blood lactate levels need to increase. So it is a test for detecting glycogen storage disease.

Excess glucose in the body is converted in to glycogen and stored in muscles, liver etc in the body. Stored glycogen is used whenever excess energy is needed like during a strenuous work out.

Normal person produces Lactate during excessive exercise by using glycogen stored, where as the person with this disease can't generate lactate.