Some factors influencing plasma AST/CK activities in thoroughbred racehorses

Sixty-six Thoroughbred horses in training (2 or 3 years of age) were studied during one flat racing season, to investigate the relationship between age, sex, time of year, dietary electrolyte imbalances and plasma aspartate aminotransferase/creatine kinase (AST/CK) activities. Management and training variations were minimised. Between February to October, monthly 24 h post exercise samples were collected and analysed for AST and CK activities. Sex and/or age were shown to have a significant effect on the occurrence of high CK (> 100 iu/litre) and AST (> 300 iu/litre) activities. Fillies were more likely to have elevated CK and AST than colts. Two-year-olds tended to have higher AST activities than three-year-olds. Time of year had no significant effect on the number of animals with high or low activities. Fourteen animals had consistently raised plasma AST activities (median > 300 iu/litre). Eight of these raced and seven won at least one race. The average daily intake of calcium, sodium, phosphorus and potassium was determined at each sampling time. In eight of the horses, the fractional electrolyte excretion (FE) of phosphate (PO4) and sodium (Na) was determined monthly. FE values were also determined in three horses which had suffered repeated attacks of the equine rhabdomyolysis syndrome. The FE, PO4 and Na values reflected dietary intake of calcium, phosphorus and sodium respectively. It was not possible to determine any effect of training or a predisposition to equine rhabdomyolysis. No obvious differences were found between the resting thyroxine values of three animals with consistently low AST activities, six of those with consistently raised AST and three animals which had suffered repeated attacks of rhabdomyolysis.