Question

Unlike liver cells, muscle cells do not secrete glucose into the blood. What about glucose-6-P prevents its secretion and how do liver cells overcome this problem?

McArdle’s disease (also called glycogen storage disease V) is a genetic disorder in which muscle cannot make functional phosphorylase. Speculate as to why McArdle’s patients show muscle fatigue (tiredness) with even moderate exercise.

McArdle’s disease is inherited as a recessive trait. Consider the activity of the phosphorylase enzyme and suggest an explanation for the recessive nature of this disease.

Answer 1

Skeletal muscles are unable to release glucose. Because muscles lack glucose 6-phosphatase enzyme. Muscles glycogen is mainly a local energy substrate for exercise, rather than an energy source to maintain blood glucose concentration during fasting.

But liver secretes sugar or glucose by turning glycogen into glucose (glycogenolysis). The liver also can manufacture necessary sugar or glucose by harvesting amino acids, waste products and fat byproducts (gluconeogenesis).

The inability of the skeletal muscles to metabolize glycogen into glucose with strenuous activity leads to an abrupt feeling of fatigue. Due to lack of phophorylase enzyme. Also fatigue in McArdle's disease is due to a failure in the muscle excitation-contraction (E-C) coupling.

McArdle disease is an autosomal recessive metabolic disorder caused by a deficiency of the muscle isoform of glycogen phosphorylase (myophosphorylase, PYGM).