Histochemical and morphometric evaluation of skeletal muscle from horses with exertional rhabdomyolysis (tying-up).

The study investigates the effects of exertional rhabdomyolysis, a muscle degenerative condition, on horses. Researchers triggered the condition in 13 horses and compared skeletal muscle samples from affected and control groups to determine the disease’s histochemical and morphometric impacts.

Research Procedures

• The experiment began by deliberately inducing exertional rhabdomyolysis in thirteen horses that had prior histories of the condition. Researchers triggered by the disease by having the animals exercise for 20 minutes.
• The induced exertional rhabdomyolysis exhibited clinical symptoms such as lameness, trembling, increased respiration rates, and hampered limb movement. Affected horses’ serum also showed elevated levels of creatine kinase (CK) and aspartate aminotransferase (AST), and evidence of morphologic muscle lesions.
• CK levels peaked between 4 and 8 hours post-exercise while AST levels peaked between 24 and 48 hours. Histological analysis of the skeletal muscles revealed segmental muscle fiber degeneration 24 hours after the exercise period.

Findings from Muscle Analysis

• Damaged muscle fibers repaired through myoblastic regeneration. Horses with moderate to severe CK activity elevation (greater than 1,500 U/liter to greater than 5,000 U/liter) and clinical muscle soreness evidence demonstrated visible muscle fiber degeneration under microscopy.
• Biopsy samples from both affected and control horse groups were processed to indicate myofibrillar ATPase activity, which was used to determine fiber types, area percentages, and mean cross-sectional fiber sizes.
• Horses in the affected group had significantly larger type I, II, and intermediate fibers compared to the control group. Additionally, the affected horses displayed a greater percentage of intermediate fibers and a higher area occupied by these intermediate fibers.
• Type II fibers were primarily, though not exclusively, impacted in muscle samples with acute exertional rhabdomyolysis lesions.

Other Observations and Conclusion

• In one horse necropsied 24 hours after exercise, lesions were found in several postural muscles, the masseter muscle, and the heart. This finding suggests a widespread impact of the disease within the horse’s system.
• The researchers concluded that horses affected by exertional rhabdomyolysis have larger cross-sectional gluteus medius muscle fibers than horses without the condition. It was also found that acute exertional rhabdomyolysis lesions favorably deteriorate type II fibers.

This research furthers the understanding of exertional rhabdomyolysis in horses, particularly regarding how the disease affects the histochemical and morphometric aspects of skeletal muscles. The findings might support future investigations and therapies targeting this muscle condition in horses.