Muscle histopathology and plasma aspartate aminotransferase, creatine kinase and myoglobin changes with exercise in horses with recurrent exertional rhabdomyolysis

Six horses with a history of recurrent exertional rhabdomyolysis (RER) (Horses A‐F) and 7 control horses performed a submaximal and later a near‐maximal treadmill exercise test. Blood samples were obtained before, during and after exercise and muscle biopsies were taken before and after exercise. At rest, plasma aspartate aminotransferase (AST) activities in horses with RER were above 95% confidence intervals for control horses. During submaximal exercise, 3 horses with RER (A, B and C) had much greater increases in plasma AST, creatine kinase (CK) and myoglobin concentrations than did Horses D, E and F and control horses. Clinical signs of muscle stiffness and pain were only obvious in Horse A. During near‐maximal exercise, only Horse C showed a substantial increase in CK activity and myoglobin concentrations without any associated clinical signs of rhabdomyolysis. Muscle biopsies from Horses A, B and C contained necrotic type II fibres which, on electron microscopic examination, contained disrupted myofibrils and swollen mitochondria. These results suggest that, in RER, subclinical episodes of muscle fibre necrosis and associated increases in plasma AST, CK and myoglobin occur with exercise more frequently than could be detected clinically. Furthermore, the pattern of increase in muscle enzymes and myoglobin concentrations in the 6 horses with RER suggested that the high plasma AST and CK activities commonly observed at rest in symptom‐free Standardbred horses are probably a result of repeated subclinical episodes of rhabdomyolysis after exercise, rather than leakage due to abnormal sarcolemmal permeability.