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Am J Physiol. 1993 Feb;264(2 Pt 1):E215-20.

Effect of prolonged exercise on muscle citrate concentration before and after endurance training in men.

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  • 1Section of Applied Physiology, Washington University School of Medicine, St. Louis, Missouri 63110.


It has been hypothesized that endurance training reduces carbohydrate utilization during exercise via citrate-mediated inhibition of phosphofructokinase (PFK). To test this hypothesis, vastus lateralis muscle biopsy samples were obtained from eight men before and immediately (approximately 10 s) after 2 h of cycle ergometer exercise at 60% of pretraining peak O2 uptake, both before and after 12 wk of endurance exercise training (3 days/wk running, 3 days/wk interval cycling). Training increased muscle citrate synthase (CS) activity from 3.69 +/- 0.48 (SE) to 5.30 +/- 0.42 protein-1 and decreased the mean respiratory exchange ratio during exercise from 0.92 +/- 0.01 to 0.88 +/- 0.01 (both P < 0.001). Muscle citrate concentration at the end of exercise correlated significantly with CS activity (r = 0.70; P < 0.005) and was slightly but not significantly higher after training (0.80 +/- 0.19 vs. 0.54 +/- 0.19 mmol/kg dry wt; P = 0.16). Muscle glucose 6-phosphate (G-6-P) concentration at the end of exercise, however, was 31% lower in the trained state (1.17 +/- 0.10 vs. 1.66 +/- 0.27 mmol/kg dry wt; P < 0.05), in keeping with a 36% decrease in the amount of muscle glycogen utilized (133 +/- 22 vs. 209 +/- 19 dry wt-1.2 h-1; P < 0.01). The lower G-6-P concentration after training suggests that the training-induced reduction in carbohydrate utilization results from attenuation of flux before the PFK step in glycolysis and is not due to citrate-mediated inhibition of PFK.

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