ACCUMULATION OF ALLANTOIN AND URIC-ACID IN PLASMA OF EXERCISING TROTTERS

Plasma concentrations of hypoxanthine, uric acid, and allantoin, which are breakdown products of adenine nucleotides, were measured in Stand ardbred and Finnhorse trotters during and after an exercise test on a high-speed treadmill, after an incremental exercise test performed on a racetrack, and after a racing competition. Fiber-type composition of the middle gluteal muscle and the muscle concentrations of adenine nu cleotides and inosine monophosphate were measured after the racetrack test. Changes in the concentration of hypoxanthine were not observed i n any of the tests. Peak concentration of uric acid was measured betwe en 5 and 30 minutes after exercise, and it was three- to tenfold highe r than the value at rest. The variability can be explained by intensit y of the exercise test and variation among horses. The concentration o f allantoin after exercise was 2 to 3 times as high as that at rest, d epending on the intensity of the exercise, although the absolute incre ase was about 10 times as high as the increase in the concentration of uric acid. Peak values of allantoin for the treadmill and the racetra ck tests were obtained 4 to 6 minutes after exercise and < 30 minutes after the races. Peak concentration of allantoin correlated positively with the percentage of type-II (IIA + IIB) fibers in the middle glute al muscle. Significant correlations were not observed between plasma c oncentration of uric acid or allantoin and muscle concentrations of ad enosine triphosphate (ATP) or inosine monophosphate. It can be conclud ed that in horses, breakdown of ATP during and after exercise continue s until allantoin is produced. The peak concentration of allantoin inc reases with the intensity of exercise, is reached rapidly after exerci se, and the variation in the time to the peak value is small among hor ses. It is suggested that the main source of allantoin is the fast-twi tch, type-II fibers and that the mixed muscle concentrations of adenin e nucleotides are of limited value when estimating the effects of exer cise on ATP content of the muscle tissue.