Carnitine and acylcarnitine metabolism during exercise in humans. Dependence on skeletal muscle metabolic state.

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Abstract
Carnitine metabolism has been previously shown to change with exercise in normal subjects, and in patients with ischemic muscle diseases. To characterize carnitine metabolism further during exercise, six normal male subjects performed constant-load exercise on a bicycle ergometer on two separate occasions. Low-intensity exercise was performed for 60 min at a work load equal to 50% of the lactate threshold, and high-intensity exercise was performed for 30 min at a work load between the lactate threshold and maximal work capacity for the individual. Low-intensity exercise was not associated with a change in muscle (vastus lateralis) carnitine metabolism. In contrast, from rest to 10 min of high-intensity exercise, muscle short-chain acylcarnitine content increased 5.5-fold while free carnitine content decreased 66%, and muscle total carnitine content decreased by 19% (all P less than 0.01). These changes in skeletal muscle carnitine metabolism were present at the completion of 30 min of high-intensity exercise, and persisted through a 60-min recovery period. With 30 min of high-intensity exercise, plasma short-chain and long-chain acylcarnitine concentrations increased by 46% and 23%, respectively. Neither exercise state was associated with a change in the urine excretion rates of free carnitine or acylcarnitines. Thus, alterations in skeletal muscle carnitine metabolism, characterized by an increase in acylcarnitines and a decrease in free and total carnitine, are dependent on the work load and, therefore, the metabolic state associated with the exercise, and are poorly reflected in the plasma and urine carnitine pools.