Changes in plasma hypoxanthine and free radical markers during exercise in man.

Eight men cycled for about 6 minutes at workloads corresponding to 44 and 72% of maximal oxygen uptake and to fatigue at 98% maximal oxygen uptake. Blood samples from a brachial artery and a femoral vein were taken at rest and during exercise. Hypoxanthine, xanthine and urate in plasma were significantly elevated at fatigue and after 10 minutes of recovery. Only hypoxanthine showed a significant arterio-femoral venous difference. The release of hypoxanthine from the legs increased during the recovery period and was three-fold higher 10 minutes post exercise than at the end of exercise. It is concluded that the marked increase in plasma hypoxanthine which occurs during intensive exercise originates from the working muscle whereas the transformation to xanthine and urate may occur in other tissues. Glutathione, methemoglobin and malondialdehyd (MDA) were used as plasma markers of free radicals. Total glutathione (glutathione + glutathionedisulfide) in blood and plasma increased during intensive exercise and may be indicative of free radical formation. However, MDA was not detectable in plasma during any conditions (less than 0.1 mumol x l-1 plasma) and methemoglobin decreased slightly during exercise. Further studies using more specific techniques are required to determine whether the formation of free radicals is increased after brief intensive exercise.