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Tricarboxylic acid cycle intermediates in human muscle during prolonged exercise
1Department of Clinical Physiology, Karolinska Institute, Huddinge University Hospital.
The American Journal of Physiology
|November 1, 1990
Summary
Prolonged exercise to fatigue depletes muscle glycogen and impairs aerobic energy production by reducing tricarboxylic acid cycle intermediates (TCAI). Carbohydrate depletion may limit TCAI levels, impacting endurance performance.
Area of Science:
- Exercise Physiology
- Skeletal Muscle Metabolism
- Biochemistry
Background:
- Muscle glycogen is a primary fuel source during prolonged moderate-intensity exercise.
- Energy metabolism shifts during exercise, influencing substrate utilization and intermediate compound levels.
- Understanding metabolic changes at fatigue is crucial for optimizing athletic performance and training.
Purpose of the Study:
- To investigate the metabolic alterations in skeletal muscle during prolonged cycling exercise to fatigue.
- To examine the relationship between muscle glycogen depletion, tricarboxylic acid cycle intermediates (TCAI), and indicators of energy status at fatigue.
- To determine the impact of carbohydrate availability on aerobic energy production during sustained exertion.
Main Methods:
- Muscle biopsies were obtained from the quadriceps femoris of seven subjects at rest and during cycling exercise to fatigue (approx. 75% maximal oxygen uptake).
- Analysis included muscle glycogen, tricarboxylic acid cycle intermediates (TCAI), lactate, pyruvate, alanine, AMP, and IMP.
- Subjects cycled for approximately 75 minutes to reach volitional fatigue.
Main Results:
- Muscle glycogen decreased significantly from 445 to 50 mmol/kg dry wt at fatigue.
- TCAI levels increased initially but then decreased significantly by fatigue (P < 0.05 vs. 5 min).
- Fatigue was associated with increased AMP deamination (elevated IMP) and reduced lactate, pyruvate, and alanine, while acetylcarnitine remained elevated.
Conclusions:
- Prolonged exercise to fatigue at moderate workloads leads to glycogen depletion and energy deficiency.
- Reduced TCAI levels, despite maintained acetyl units, suggest carbohydrate depletion may impair aerobic energy production.
- These findings highlight the critical role of carbohydrate availability in sustaining TCAI function and aerobic metabolism during prolonged exercise.