Glucose arteriovenous differences across muscle in 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.

A new variant of late-onset myophosphorylase deficiency

McArdle disease classically presents in childhood or adolescence. Rarely does it become symptomatic for the first time in late adulthood, with the onset of progressive muscle wasting and weakness. Our patient is unusual in that despite a life of physical vigor, she developed immobilizing cramps, stiffness, and muscle swelling abruptly at age 60. She had no previous symptoms of muscle disease. The diagnosis was indicated by the ischemic forearm test, which produced muscle contracture and no rise in venous lactate levels, and confirmed by histochemical, electrophoretic, and biochemical studies that showed complete absence of myophosphorylase. This case defines a new variant of the late-onset type and raises important questions about compensatory mechanisms, inheritance patterns, and etiological factors in myophosphorylase deficiency.