• horse;
  • muscle;
  • lactate;
  • ATP;
  • IMP;
  • potassium


Three horses performed exercise on a treadmill under normoxic conditions (F1O2–0.21) and during exposure to acute hypoxia (F1O2–0.17) and hyperoxia (F1O2–0.35) achieved by addition of nitrogen and oxygen to inspired air by means by an open flow system. The gases were added after the horses had walked and trotted at 2 submaximal speeds (2 and 4 m/s for 7–15 min). Thereafter, the treadmill speed was increased in 3 steps to attain near maximal oxygen uptake within 5–8 min (8–10.5 m/s for normoxia and hyperoxia and 7–10 m/s for hypoxia). Heart rate and oxygen uptake were recorded continuously. By use of indwelling catheters in the pulmonary and carotid arteries, blood samples were drawn at rest and immediately after exercise. Muscle biopsies (m. gluteus) were taken before and immediately after exercise. In all horses, at the end of exercise, heart rates were lower and oxygen uptake, arterial and venous PO2, haemoglobin saturation and oxygen content were higher following hyperoxia than following hypoxia. At the end of exercise, all horses had lower concentrations of lactate, ammonia and potassium in plasma and lower lactate in muscle after hyperoxia than after hypoxia and normoxia. Concentrations of ATP were higher and of G-6-P lower after hyperoxia than after normoxia. Significant correlations were observed between post exercise concentrations of lactate and ammonia (r=0.80) and between lactate and potassium (r=0.96) in plasma. There was also a correlation between muscle and plasma lactate (r=0.92). Post exercise ATP concentrations were correlated with IMP (r=0.85), ammonia (r=0.94) and potassium (r=0.84) concentrations in plasma. The results indicate that acute hypoxia and hyperoxia during intense exercise influence the production and/or removal of lactate.