Exercise-trained but not untrained rats maintain free carnitine reserves during acute exercise

Exercise training is known to induce physiological adaptations that improve exercise performance and alter patterns of energy substrate utilization to favour fatty acid oxidation. L-Carnitine is an essential cofactor for the oxidation of fatty acids under all physiological conditions, including exercise. This study evaluated the effect of acute exercise on carnitine concentrations in tissue and serum, liver carnitine palmitoyltransferase-I activity and expression, and serum lipids in both trained and untrained rats as compared to non-exercised rats. Serum acyl- and total carnitine was significantly higher in the trained animals, whether exercised or not, suggesting an exercise-induced increase in a renal threshold for carnitine. Untrained rats had significantly higher acylcarnitine in skeletal muscle and an acyl/free carnitine ratio of 0.63 +/- 0.06 compared with 0.31 +/- 0.16 in trained animals receiving an identical acute bout of exercise, demonstrating that untrained animals utilized a significantly higher percentage of free carnitine reserves during exercise. This study suggests that free carnitine reserves may be reduced during exercise in untrained rats, an effect that has the potential to impair both carbohydrate and fat metabolism during exercise.