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Paradoxically enhanced glucose production during exercise in humans with blocked glycolysis caused by muscle phosphofructokinase deficiency

From the Department of Neurology (Dr. Haller), University of Texas, Southwestern Medical Center, and Neuromuscular Center of the Institute for Exercise and Environmental Medicine, Presbyterian Hospital, Dallas, TX; and the Department of Neurology (Dr. Vissing), and the Copenhagen Muscle Research Centre (Dr. Galbo), National University Hospital, Rigshospitalet, and the Department of Medical Physiology (Dr. Galbo), the Panum Institute, University of Copenhagen, Copenhagen, Denmark.
Supported by the Muscular Dystrophy Association, the Department of Veteran Affairs Merit Review, the Danish Medical Research Council (12-1362-1), and the Danish Hospital Foundation for Medical Research (Region of Copenhagen, the Faroe Island, and Greenland).
Received December 28, 1995. Accepted in final form March 13, 1996.
Address correspondence and reprint requests to Dr. John Vissing, Department of Neurology N, National University Hospital, Rigshospitalet, Blegdamsvej 9, DK-2100 Copenhagen, Denmark.

Muscle phosphofructokinase deficiency (PFKD) is characterized by exercise intolerance due to the enzymatic block in muscle glycolysis.Glucose infusion increases exertional fatigue in these patients, probably by decreasing the availability of free fatty acids (FFA) and ketones, which play a crucial role in ATP production during exercise in PFKD. This suggests that a lower than normal hepatic glucose production would be appropriate during exercise in PFKD. To investigate glucoregulation in PFKD, we measured glucose turnover and hormonal and metabolic responses to 20 minutes of cycle exercise at near maximal effort in three patients with PFKD and in healthy matched controls studied at the same absolute (A, 15 to 30 Watts) and relative (R, 35 to 80 Watts, matched heart rates) work load as the patients. During exercise, mean glucose production was higher in all patients versus controls (30 +/- 4 versus A: 18 +/- 2 and R: 20 +/- 1 micro mol centered dot min sup -1 centered dot kg sup -1). Mean glucose utilization during exercise was similar in patients and controls working at the same relative work load and higher than in controls at the low work load. Exercise-induced increases in arterialized blood were higher in all patients for glucose, FFA, growth hormone, glucagon, and norepinephrine. Plasma alanine and lactate always decreased during exercise in patients and consistently increased in controls. In conclusion, an enhanced neuroendocrine response and a paradoxically exaggerated mobilization of glucose occurs during exercise in PFKD. The responses are probably initiated by neural feedback elicited by disturbances in local muscle metabolism. The responses promote delivery of oxidizable fat to muscle, but at the expense of accumulation and futile cycling of glucose.

NEUROLOGY 1996;47: 766-771

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