In mitochondrial myopathy, severely impaired muscle oxidative capacity poses a dilemma for metabolic regulation in exercise. We inquired whether fuel mobilization during exercise in mitochondrial myopathy is adjusted to the reduced capacity to oxidize substrate, or if fuel is mobilized in excess of oxidative capacity. Hormonal and metabolic responses to 20 minutes of cycle exercise were studied in 4 patients with mitochondrial myopathy working at near maximal effort and in 4 healthy matched controls. On 2 separate days, controls were studied at the same absolute (A) workload (9 +/- 3 W) and the same relative (R) workload (77 +/- 9 W) as performed by the patients. During exercise, average glucose production was higher in patients (28 +/- 5 micromol min(-1) kg(-1)) than in controls at both workloads (A, 12 +/- 1; R, 18 +/- 2 micromol min(-1) kg(-1)). Exercise-induced increases in plasma glucose, growth hormone, epinephrine, norepinephrine, corticotropin, and lactate, and decreases in plasma insulin and pH were also larger in patients compared with findings in controls at both workloads. In conclusion, mitochondrial myopathies are associated with excessive neuroendocrine responses and mobilization of glucose during exercise. These responses augment ATP synthesis but result in progressive accumulation of nonoxidized substrates. Apparently, substrate mobilization and neuroendocrine responses in exercise are linked to oxidative demand rather than to oxidative capacity in working muscle.
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