Effect of prolonged exercise on muscle citrate concentration before and after endurance training in men.
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 mol.h-1.kg 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 mmol.kg 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.
Coggan
A R
AR
Section of Applied Physiology, Washington University School of Medicine, St. Louis, Missouri 63110.
Spina
R J
RJ
Kohrt
W M
WM
Holloszy
J O
JO
D000328:Adult; D001786:Blood Glucose / Q000032:analysis; D002951:Citrates / Q000378:metabolism*; D019343:Citric Acid; D005230:Fatty Acids, Nonesterified / Q000097:blood; D006339:Heart Rate; D006801:Humans; D007773:Lactates / Q000097:blood; D019344:Lactic Acid; D008297:Male; D009132:Muscles / Q000378:metabolism*; D009994:Osmolar Concentration; D010101:Oxygen Consumption; D010806:Physical Education and Training; D010807:Physical Endurance; D005082:Physical Exertion; D011659:Pulmonary Gas Exchange; D013997:Time Factors
D001786:Blood Glucose; D002951:Citrates; D005230:Fatty Acids, Nonesterified; D007773:Lactates; D019343:Citric Acid; D019344:Lactic Acid