Tricarboxylic Acid Cycle Intermediates during Incremental Exercise in Healthy Subjects and in Patients with McArdle's Disease

1. The importance of the level of tricarboxylic acid cycle intermediates (malate, citrate and fumarate) for energy transduction during exercise has been investigated in six healthy subjects and in two patients with muscle phosphorylase deficiency (McArdle's disease). 2. Healthy subjects cycled for 10 min at low (50 W), moderate [130 ± 6 W (mean ± SEM)] and high (226 ± 12 W) work rates, corresponding to 26, 50 and 80% of their maximal O2 uptake, respectively. Patients with McArdle's disease cycled for 11–13 min at submaximal (40 W) rates, and to fatigue at maximal work rates of 60–90 W. 3. In healthy subjects, phosphocreatine was unchanged during low work rates, but decreased to 79 and 32% of the initial level during moderate and high work rates. In patients with McArdle's disease, phosphocreatine decreased to 82 and 34% of the initial level during submaximal and peak exercise. Muscle lactate increased in healthy subjects during exercise at moderate and high work rates, but remained low in patients with McArdle's disease. 4. In healthy subjects, tricarboxylic acid cycle intermediates were similar at rest and at low work rates (0.48 ± 0.04 mmol/kg dry weight), but increased to 1.6 ± 0.2 mmol/kg dry weight and 4.0 ± 0.3 mmol/kg dry weight at moderate and high work rates. The tricarboxylic acid cycle intermediate level in patients with McArdle's disease was similar to that in healthy subjects at rest, but was markedly reduced during exercise when compared at the same relative intensity. The peak level of tricarboxylic acid cycle intermediates in patients with McArdle's disease was 22% of that in healthy subjects. However, when compared at the same absolute workload, tricarboxylic acid cycle intermediates were similar in patients with McArdle's disease and in healthy subjects. 5. The decrease in glutamate and increase in alanine suggest that the alanine aminotransaminase reaction was the major anaplerotic process in healthy subjects. However, in patients with McArdle's disease (n = 1), muscle alanine remained unchanged and the purine nucleotide cycle may instead be the route of a limited anaplerosis during maximal exercise. The muscle content of glutamate and glutamine (n = 1) was markedly reduced in patients with McArdle's disease. 6. It is concluded that the tricarboxylic acid cycle intermediate level is related to the work rate in healthy subjects, and that the attenuated peak level in patients with McArdle's disease may be a limitation for aerobic energy transduction.