Effects of Graded Exercise on Leg Exchange of Energy Substrates in Malignant Hyperthermia Susceptible Subjects

It has been speculated that, in malignant hyperthermia-susceptible (MHS) individuals, an abnormality of sympathetic activity is seen during stressful situations, such as exercise. The authors investigated whether muscle metabolism in eight MHS subjects, at rest and during moderate and heavy short-term exercise, is different then that in normals. Leg exchange of energy substrates (glucose, lactate, and glycerol) was quantified by measuring leg blood flow and arterial-venous concentration differences. Muscle biopsies were also performed, and ATP, glycogen, and lactate were analyzed. Catecholamines and oxygen uptake were also measured. The study was performed at rest with subjects in the supine position and during two periods (40% and 80% of the subjects maximal oxygen uptake, respectively) on a bicycle ergometer. The principal finding of the study was that there was no major difference in oxygen uptake or leg exchange of glucose, lactate, and glycerol between MHS-subjects and previously standard normals during different grades of exercise. Furthermore, muscle metabolites and plasma catecholamines did not differ between the groups. This study indicates a normal sympathetic activity and muscle metabolism in MHS subjects during rest, as well as during moderate and severe exercise. The authors' results do not support the opinion that persons with positive in vitro tests for MH should restrict their physical activity.