Plasma hypoxanthine and ammonia in humans during prolonged exercise

In this study we examined the time course of changes in the plasma concentr ation of oxypurines [hypoxanthine (Hx), xanthine and urate] during prolonge d cycling to fatigue. Ten subjects with an estimated maximum oxygen uptake ((V) over dot O-2max) of 54 (range 47-67) ml . kg(-1) . min(-1) cycled at [ mean(SEM)] 74(2)% of (V) over dot O-2max until fatigue [79(8) mini. Plasma levels of oxypurines increased during exercise, but the magnitude and the t ime course varied considerably between subjects. The plasma concentration o f Hx ([Hx]) was 1.3 (0.3) mu mol/l at rest and increased eight fold at fati gue. After 60 min of exercise plasma [Hx] was >10 mu mol/l in four subjects , whereas in the remaining five subjects it was <5 mu mol/l. The muscle con tents of total adenine nucleotides (TAN = ATP + ADP + AMP) and inosine mono phosphate (IMP) were measured before and after exercise in five subjects. S ubjects with a high plasma [Hx] at fatigue also demonstrated a pronounced d ecrease in muscle TAN and increase in IMP. Plasma [Hx] after 60 min of exer cise correlated significantly with plasma concentration of ammonia ([NH3], r = 0.90) and blood lactate (r = 0.66). Endurance, measured as time to fati gue, was inversely correlated to plasma [Hx] at 60 min(r = -0.68, P < 0.05) but not to either plasma [NH3] or blood lactate. It is concluded that duri ng moderate-intensity exercise, plasma [Hx] increases, but to a variable ex tent between subjects. The present data suggest that plasma [Hx] is a marke r of adenine nucleotide degradation and energetic stress during exercise. T he potential use of plasma [Hx] to assess training status and to identify o vertraining deserves further attention.