McArdle's syndrome: phosphorylase-deficient myopathy

McArdle Disease: New Insights into Its Underlying Molecular Mechanisms

McArdle disease, also known as glycogen storage disease type V (GSDV), is characterized by exercise intolerance, the second wind phenomenon, and high serum creatine kinase activity. Here, we recapitulate PYGM mutations in the population responsible for this disease. Traditionally, McArdle disease has been considered a metabolic myopathy caused by the lack of expression of the muscle isoform of the glycogen phosphorylase (PYGM). However, recent findings challenge this view, since it has been shown that PYGM is present in other tissues than the skeletal muscle. We review the latest studies about the molecular mechanism involved in glycogen phosphorylase activity regulation. Further, we summarize the expression and functional significance of PYGM in other tissues than skeletal muscle both in health and McArdle disease. Furthermore, we examine the different animal models that have served as the knowledge base for better understanding of McArdle disease. Finally, we give an overview of the latest state-of-the-art clinical trials currently being carried out and present an updated view of the current therapies.

31-P NMR characterization of the metabolic anomalies associated with the lack of glycogen phosphorylase activity in human forearm muscle

Exercise-induced changes in phosphorus-containing metabolites and intracellular pH (pHi) have been studied in the finger flexor muscles of 3 patients with glycogen phosphorylase deficiency (McArdle's disease) in comparison to 14 healthy volunteers. At rest, no difference was observed for PCr/Pi ratio and pHi while patients exhibited a higher PCr/ATP ratio (5.91 +/- 0.98 vs 4.02 +/- 0.6). At end-of-exercise, PCr/Pi was abnormally low (0.51 +/- 0.19 vs 1.64 +/- 0.37) whereas no acidosis was observed. The slow recovery of PCr/Pi ratio indicates an impairment of oxidative capacity accompanying the defect in the glycogenolytic pathway. The failure to observe a transient Pi disappearance at the onset of recovery (an index of glycogen phosphorylase activity) can be used in conjunction with the lack of exercise acidosis as a diagnostic index of McArdle's disease.

A comparative study on glucagon effect between McArdle disease and Tarui disease

Pretreatment with glucagon relieved patients with McArdle disease from muscular symptoms during exercise and enhanced exercise performance, though it did not produce any improvement in patients with Tarui disease. The difference in glucagon effect between the two diseases was clearly demonstrated in the bicycle ergometer exercise tests. In addition, the semi-ischemic forearm exercise tests performed after glucagon injection showed that increased lactate production was significantly induced by exercise in McArdle disease, but it was not the case in Tarui disease. In McArdle disease, the augmentation in exercise-induced lactate production was also observed after administration of glucose, or glucose plus insulin, but it was neither observed after administration of insulin alone nor after arginine or epinephrine administration. These findings suggest that the beneficial effect of glucagon in McArdle disease is due to the enhanced utilization of circulating glucose through the muscular glycolytic pathway realized in the coexistence of hyperglycemia and hyperinsulinemia.

Lactate production in McArdle's disease

A case of McArdle's disease in a man is described in detail and a less complete study of his family is reported. This patient showed the classical features of McArdle's disease and the diagnosis was confirmed by muscle biopsy. Unlike other reported cases of this disorder, this case showed a normal rise in blood lactate levels on ischaemic exercise. This apparently paradoxical finding is discussed. It is suggested that a normal rise in the level of blood lactate on ischaemic exercise should not exclude myophosphorylase deficiency.

A CASE OF MCARDLE'S SYNDROME WITH A POSITIVE FAMILY HISTORY

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Skeletal muscle glycogenosis: an investigation of two dissimilar cases

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