انباشت اوریک اسید در پلاسما پس از مسابقه و تمرین مکرر اسب سواری Accumulation of Uric Acid in Plasma after Repeated Bouts of Exercise in the Horse

INTRODUCTION During intense exercise, muscle phosphocreatine (PCr) and glycogen are used for the rephosphorylation of ADP. When muscle PCr stores are failing ADP accumulation starts. Increased ADP level triggers the myokinase reaction, where two ADP molecules form one molecule of ATP and one of AMP. The latter is further deaminated to IMP and metabolized via inosine, hypoxanthine, xanthine to uric acid in humans and finally to allantoin in horses. High-intensity exercise has been shown to cause a significant decrease in the ATP content and a corresponding increase in the IMP content in the muscle of the Thoroughbred horse (8,22). The decline in muscle ATP is mirrored by the postexercise appearance of the end products of the pathway, uric acid and allantoin into plasma of the horse (8,19). During the degradation of purine nucleotides, oxidation of hypoxanthine takes place in the capillary endothelial cells of muscle, liver and other tissues where xanthine dehydrogenase/oxidase (XDH/XO) oxidizes it to xanthine and to uric acid (10,18). Uric acid is transported to kidneys or to hepatocytes where uricase converts it to allantoin in nonprimate mammals (16). Uric acid can also be formed in plasma that contains XO even after moderate exercise (20). The XO form of the enzyme produces reactive oxygen species that have been claimed to initiate exercise-induced muscle injury by attracting neutrophils and by modifying their adhesion to endothelium (6,26). Antioxidants (vitamin E, ascorbic acid, uric acid, etc.) act as scavengers of free radicals and may thus have a role in preventing the tissue injury. Regular training in rats is known to increase the amount of endogenous antioxidants in plasma (17) and antioxidant enzymes in liver and in skeletal muscle (13). It has been shown that there is a threshold for both adenine nucleotide degradation, as indicated by muscle IMP and plasma NH3 accumulation, in equine muscle (22) and in human plasma (21), and for the plasma accumulation of hypoxanthine but not for uric acid (11 ) in human athletes. Studies on the possible threshold in the plasma accumulation of uric acid in trotters have not been performed, and the aim of this study was to investigate whether such a threshold exists. Furthermore, it was investigated whether the plasma activity of XO plays a role in the accumulation of uric acid and whether the exercise-induced production of reactive oxygen species is accompanied by a change in the plasma antioxidant capacity. MATERIALS AND METHODS Horses and Experimental Design The experimental design was approved by the Ethical Committee for Animal Experiments of Agricultural Research Centre. Six Standardbred trotters, aged 3-10 years, were used in an experiment lasting for 4 days. Three of the horses were geldings, two stallions and one was a mare. On days 1 and 4, the horses performed at 60-rain intervals three exercise bouts with increasing intensity. On day 1, horses twice trotted 3000 m and once 2000 m in that order and on day 4 twice 2100 m and once 1600 m on a 1000-m racecourse. The speed of the horses was adjusted according to their individual condition so that they ran faster heat by heat, and the third heat on day 4 was at maximal speed. The average speeds -+ SD were 9.6 + 0.3, 10.1 + 0.3 and 10.6 + 0.3 m • s 1 on day 1 and 10.4 -+ 0.8, 10.8 + 0.3 and 11.7 -+ 0.6 m • s -1 on day 4, respectively. A pacecar was used to keep the speeds correct and steady. After each heat, horses were walked for 15 min and they then stood in a shelter. On day 2, horses did a light 30-rain workout and spent 2 hr in a paddock where they were allowed to move freely. On day 3, horses spent 4 hr in the paddock. Blood samples from the jugular vein were taken before the first heat, immediately, 5, 10, 15, 30 and 60 min after each heat and 2, 4, 6 hr after the third heat. Samples (20 ml) were drawn to a syringe either through a catheter (Intraflon 12G, Vycon, Belgium) placed into the jugular vein before the test or by venepuncture. Blood was transferred to lithium heparin tubes and for the lactate analysis to tubes containing heparin, fluoride, nitrite and a hemotyzing agent (Analzo GMRD-047, Analox Instruments Ltd., London, U.K.). Li-heparin tubes were kept on ice until the plasma was separated by centrifugation and stored frozen (-70°C) until analysed.