Muscle glucose-6-phosphate dehydrogenase (G6PD) deficiency is described in four clinically heterogeneous patients: an athlete who developed myoglobinuria after physical exercise; a 7-year-old, mildly mentally retarded boy, who had episodes of dark urine and high creatine kinase; and two brothers of Sardinian origin, the elder showing moderate exercise intolerance. Histochemical and biochemical studies showed a lack of G6PD activity in muscle biopsy specimens as well as in erythrocytes. G6PD characterization in erythrocytes classified these mutant enzymes as Mediterranean variant in all the patients. The deficiency was confirmed in the patients' myotubes and skin fibroblasts, where residual activity was present. Electrophoretic studies in tissue culture extracts showed that the residual muscle enzyme migrated as a single electrophoretic band like normal human muscle G6PD.

Monolayer cultures were established from explants of muscle obtained from 6 patients with Duchenne muscular dystrophy (DMD) and 9 controls. Electrophysiological studies were made after 3-4 weeks in vitro, when many myotubes had formed. An intracellular electrode was used to record cell membrane potential, and acetylcholine was applied by ionophoresis. The myotubes grown from Duchenne muscle showed greater acetylcholine sensitivity than controls.

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