title: Plasma hypoxanthine and ammonia in humans during prolonged exercise.
abstract: In this study we examined the time course of changes in the plasma concentration of oxypurines [hypoxanthine (Hx), xanthine and urate] during prolonged cycling to fatigue. Ten subjects with an estimated maximum oxygen uptake (VO2(max)) of 54 (range 47-67) ml x kg(-1) x min(-1) cycled at [mean (SEM)] 74 (2)% of VO2(max) until fatigue [79 (8) min]. Plasma levels of oxypurines increased during exercise, but the magnitude and the time course varied considerably between subjects. The plasma concentration of Hx ([Hx]) was 1.3 (0.3) micromol/l at rest and increased eight fold at fatigue. After 60 min of exercise plasma [Hx] was >10 micromol/l in four subjects, whereas in the remaining five subjects it was <5 micromol/l. The muscle contents of total adenine nucleotides (TAN = ATP+ADP+AMP) and inosine monophosphate (IMP) were measured before and after exercise in five subjects. Subjects with a high plasma [Hx] at fatigue also demonstrated a pronounced decrease in muscle TAN and increase in IMP. Plasma [Hx] after 60 min of exercise correlated significantly with plasma concentration of ammonia ([NH(3)], r = 0.90) and blood lactate (r = 0.66). Endurance, measured as time to fatigue, was inversely correlated to plasma [Hx] at 60 min (r = -0.68, P < 0.05) but not to either plasma [NH(3)] or blood lactate. It is concluded that during moderate-intensity exercise, plasma [Hx] increases, but to a variable extent between subjects. The present data suggest that plasma [Hx] is a marker of adenine nucleotide degradation and energetic stress during exercise. The potential use of plasma [Hx] to assess training status and to identify overtraining deserves further attention.
authors.affiliation: Department of Physiology and Pharmacology, Box 5626, Karolinska Institute, Stockholm University College of Physical Education and Sports, S-11486 Stockholm, Sweden. kent.sahlin@fyfa.ki.se
authors.forename: K
authors.initials: K
authors.lastname: Sahlin
authors.forename: M
authors.initials: M
authors.lastname: Tonkonogi
authors.forename: K
authors.initials: K
authors.lastname: Söderlund
mesh_terms: D000227:Adenine Nucleotides / Q000378:metabolism; D000328:Adult; D000641:Ammonia / Q000097:blood*; D015444:Exercise / Q000502:physiology*; D006339:Heart Rate; D006801:Humans; D019271:Hypoxanthine / Q000097:blood*; D007291:Inosine Monophosphate / Q000378:metabolism; D007700:Kinetics; D019344:Lactic Acid / Q000097:blood; D008297:Male; D009132:Muscles / Q000378:metabolism; D010101:Oxygen Consumption; D010807:Physical Endurance; D014527:Uric Acid / Q000097:blood; D019820:Xanthine / Q000097:blood
chemical_list: D000227:Adenine Nucleotides; D007291:Inosine Monophosphate; D019820:Xanthine; D014527:Uric Acid; D019271:Hypoxanthine; D019344:Lactic Acid; D000641:Ammonia