The purine-nucleotide cycle. Comparison of the levels of citric acid cycle intermediates with the operation of the purine nucleotide cycle in rat skeletal muscle during exercise and recovery from exercise.

Abstract

The contents of citric acid cycle compounds were measured in rat hind limb muscle in situ during rest, exercise, and recovery from exercise. The following changes in citric acid cycle intermediates were observed during exercise for 15 min. The contents of fumarate and malate increased fourfold over resting values. The contents of citrate, isocitrate, and succinate rose 68%, 48% and 87% respectively, but the increase in these intermediates was delayed relative to the increase in fumarate and malate. The content of 2‐oxoglutarate did not change significantly. The content of oxaloacetate increased from less than 1 nmol/g dry weight in resting muscle to 8 nmol/g dry weight during exercise. The orthophosphate content doubled. The sum of the contents of citric acid cycle intermediates doubled. The ratios of the contents of malate/fumarate and citrate/isocitrate remained constant. The constancy of the citrate/isocitrate ratio indicates that the concentration of intra‐cellular free Mg2+ is not affected by exercise. All metabolite levels returned to resting values during recovery. Comparison of these data with the rate of operation of the purine nucleotide cycle leads us to conclude that the increase in citric acid cycle intermediates can be accounted for by the operation of the purine nucleotide cycle.