Effect of deficient muscular glycogenolysis on extramuscular fuel production in exercise

Hormonal, metabolic, and cardiovascular responses to 21 min of cycling in three saline- or glucose-infused men with McArdle's disease were compared with those of matched controls to elucidate whether mobilization of extramuscular fuel is enhanced to compensate for the lack of intramuscular glycogenolysis in patients with McArdle's disease. During exercise, all saline-infused patients compared with controls working at both the same absolute and at similar relative work rates had higher glucose production (31 ± 7 vs. 19 ± 5 and 26 ± 4 μmol · min^-1 · kg^-1) and utilization (34 ± 8 vs. 22 ± 2 and 28 ± 4 μmol · min^-1 · kg^-1); higher plasma glycerol (155 ± 19 vs. 75 ± 20 and 90 ± 22 μmol/l), free fatty acids (487 ± 175 vs. 295 ± 47 and 202 ± 52 μmol/l), growth hormone (7.7 ± 2.8 vs. 2.6 ± 1.1 and 3.6 ± 3.4 mU/l), and cortisol (530 ± 168 vs. 268 ± 8 and 367 ± 80 nmol/l), greater decrease in insulin (Δ57 ± 4 vs. Δ11 ± 8 and Δ11 ± 23 pmol/l), and similar glucose concentrations. Furthermore, norepinephrine, epinephrine, and adrenocorticotropic hormone levels were higher and heart rate and cardiac output were higher during exercise in all patients than in controls at the same absolute work rate. Glucose infusion induced hyperglycemia and hyperinsulinemia in patients and inhibited the exercise-induced increases in glucose production, glycerol, free fatty acids, catecholamines, growth hormone, cortisol, and heart rate. In conclusion, feedback from metabolism in contracting muscle enhances hormonal responses and extramuscular substrate mobilization during exercise in McArdle's disease.