• horse;
  • exercise;
  • sarcolemmal permeability;
  • myoglobin;
  • purine nucleotides;
  • xanthine oxidase


Xanthine oxidase has been claimed to produce tissue damaging oxygen free radicals during purine nucleotide degradation. The aim of this study was to evaluate whether xanthine oxidase (XO) activity is present in horse plasma and whether it is associated with the post exercise release of muscular proteins into plasma. Purine nucleotide degradation was measured as the accumulation of reaction end products, uric acid and allantoin, and activities of aspartate aminotransferase (AST), creatine kinase (CK), carbonic anhydrase (CA) and the concentration of myoglobin in plasma were measured as indicators of increased sarcolemmal permeability after a standardised submaximal exercise test (SET) on a high speed treadmill and after maximal exercise (trotting races). The activity of XO in plasma was significantly elevated up to 5 min after the SET and correlated positively with the activity of CK measured 1 h after exercise, but not with the concentrations of uric acid or allantoin. After the trotting races, concentration of myoglobin and activity of CK peaked within 4 h after exercise. No correlation was found between CK and the concentrations of uric acid and allantoin. CA activity did not change significantly in any of the samples. These findings suggest that XO, both in plasma and in endothelial cells, may contribute to the formation of free radicals and therefore to the genesis of exercise induced increase in the sarcolemmal permeability.