Muscle citrate content and the regulation of metabolism in fed and fasted human skeletal muscle

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Abstract
Summary. Blood and muscle metabolite levels were measured in seven healthy young adult male subjects in a fed state and again following a 24‐hour fast. Skeletal muscle samples were obtained under local anaesthesia from m. vastus lateralis using a needle biopsy technique. The blood glucose concentration fell during fasting; the blood lactate concentration remained unchanged. Plasma free fatty acid concentrations rose, as did 3‐hydroxybutyrate and acetoacetate levels. There was no change in the amount of carbohydrate stored as glycogen in the muscle in response to fasting. The muscle content of phos‐phagens (adenosine triphosphate and phosphocreatine), glycolytic intermediates (glucose‐6‐phosphate, fructose‐6‐phosphate, fructose‐1, 6‐diphosphate, triose phosphates and lactate) and citrate was also not affected by fasting. There was a significant increase in the muscle content of 3‐hydroxybutyrate. These results give no indication as to the mechanism by which a decreased rate of carbohydrate degradation might occur in muscle in the fasted state. It is clear, however, that an intracellular accumulation of citrate and a consequent inhibition of glycolysis at the level of phosphofructokinase does not take place in fasting human skeletal muscle.

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