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

This study investigates the changes in muscle histopathology and plasma biomarkers with exercise in horses suffering from recurrent exertional rhabdomyolysis (RER). Findings suggest that subclinical manifestations of muscle fiber necrosis and related biochemical changes occur more frequently in exercising RER horses than can be detected clinically. Moreover, high plasma levels of certain muscle enzymes in symptom-free horses may be due to repeated subclinical rhabdomyolysis episodes post-exercise, not abnormal sarcolemmal permeability.

Methodology

• The researchers conducted this study on six horses with RER history (A, B, C, D, E, F) and seven control horses.
• Both the groups of horses performed submaximal and then, later, near-maximal exercise on a treadmill.
• Blood samples were collected at various stages: before, during, and after the exercise.
• Muscle biopsies were taken before and after the exercise.

Key Findings

• At rest, horses with RER had plasma aspartate aminotransferase (AST) activities higher than the control horses beyond the 95% confidence interval.
• During submaximal exercise, three RER horses (A, B, and C) had significantly higher 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 apparent in Horse A.

Near-Maximal Exercise Observations

• During near-maximal exercise, it was only Horse C that showed a considerable increase in CK activity and myoglobin concentrations, with no associated clinical signs of rhabdomyolysis.

Biopsy Results

• Muscle biopsies from Horses A, B, and C revealed necrotic type II fibers that contained disrupted myofibrils and swollen mitochondria on electron microscopic examination.

Conclusion

• New findings propose that subclinical bouts of muscle fiber necrosis, along with associated increases in plasma AST, CK, and myoglobin, happen more frequently with exercise in horses with RER than can be detected clinically.
• The pattern of increased muscle enzymes and myoglobin concentrations in the six horses with RER suggests that the high plasma AST and CK activities commonly seen in symptom-free Standardbred horses are likely due to frequent subclinical rhabdomyolysis episodes after exercise, rather than abnormalities in sarcolemmal permeability.