Skeletal muscle metabolic response to exercise in horses with ‘tying-up’ due to polysaccharide storage myopathy.

The research studies the metabolic response of horses’ muscles to exercise, particularly in horses with ‘tying-up’ due to Polysaccharide storage myopathy (PSSM). It finds that the storage of excessive glycogen in PSSM horses is due to an increase in glycogen synthesis, not a decrease in usage, and that horses with this condition could benefit from a specific diet and exercise regimen.

Objective and Methodology

• This research aimed to understand whether the excessive accumulation of glycogen in horses with Polysaccharide Storage Myopathy (PSSM), a condition causing muscle cramping and stiffness, is due to a metabolic defect obstructing the proper use of glycogen during exercise.
• The study analyzed muscle biopsies, blood lactate levels, and serum CK values taken from resting and exercising Quarter Horses (both normal and those with PSSM) and Thoroughbreds with recurrent exertional rhabdomyolysis (a muscle disorder).

Results of the Study

• At rest, muscle glycogen concentrations in PSSM horses were found to be significantly higher than in normal Quarter Horses or Thoroughbreds with recurrent exertional rhabdomyolysis.
• Muscle lactate levels were similar between PSSM and normal Quarter Horses, but significantly higher in PSSM horses compared to Thoroughbreds.
• During a maximal exercise test, muscle glycogen concentrations in PSSM horses declined significantly more, and lactate levels increased more than in normal Quarter Horses, indicating that the glycolytic pathway (a metabolic pathway that coverts glucose to lactate) was functional in these horses.
• Unlike normal Quarter Horses, PSSM horses did not show a significant change in blood lactate levels after exercise, indicating that their oxidative metabolism was unimpeded during submaximal exercise.
• Additionally, serum CK activity, an indicator of muscle damage, showed a significant increase post-exercise in PSSM horses as compared to normal Quarter Horses.

Conclusions and Implications

• Despite the common belief that impaired utilization of glycogen could be a reason for its excessive accumulation in PSSM horses, this study found rather that this was due to increased synthesis of glycogen. As such, the PSSM horses were not found to be incapable of effectively utilizing glycogen during exercise.
• Given these findings, it was suggested that PSSM horses might benefit from a diet low in soluble carbohydrates with added fat and gradually increased daily exercise. This could help in reducing excessive glycogen accumulation and improving its utilization, thereby managing the symptoms of PSSM better.