Muscle carnitine metabolism during incremental dynamic exercise in humans

The changes in muscle content of carnitine and acetylcarnitine have been studied during incremental dynamic exercise. Six subjects exercised for 10 min on an ergometer at 40 and 75% of their maximal oxygen uptake (VO2 max) and to fatigue at 100% of VO2 max (about 4 min). Muscle samples were taken from the quadriceps femoris muscle at rest and after exercise. Muscle content of free carnitine was (means +/- SE) 15.9 +/- 1.7 mmol kg-1 d.wt (dry weight) at rest and remained unchanged after exercise at low intensity but decreased to 5.9 +/- 0.6 and 4.6 +/- 0.5 mmol kg-1 d.wt after exercise at 75 and 100% of VO2 max respectively. Acetylcarnine content at rest was 6.9 +/- 1.9 mmol kg-1 d.wt and increased during exercise in correspondence with the decrease in free carnitine. Muscle content of pyruvate and lactate was unchanged after exercise at 40% of VO2 max but increased at the higher intensities. The parallel increases in acetylcarnitine, pyruvate and lactate indicate that formation of acetylcarnitine is augmented when the availability of glycolytic three-carbon metabolites is high and is consistent with the idea that acetylcarnitine provides a sink for pyruvate and acetyl CoA. This could be of importance for the maintenance of an adequate level of CoA and thus function of the tricarboxylic acid cycle.