Blood chemistry and skeletal muscle metabolic responses to exercise in horses with recurrent exertional rhabdomyolysis.

This study analyses the effect of exercise on blood chemistry and muscle metabolism in horses suffering from recurrent exertional rhabdomyolysis (RER), finding no clear correlation between certain blood variables and the development of rhabdomyolysis, despite higher levels of cortisol and blood glucose in RER horses.

Research Methodology

• Thirteen horses were used in the study, six of which had a history of RER.
• The horses were subjected to two different exercise tests on a treadmill: a 55-minute submaximal exercise and a 10-minute near-maximal exercise.
• Blood samples and muscle biopsies from the gluteus medius muscle were taken before and after each exercise test, and again 24 hours after the submaximal test.

Findings

• Out of the six RER horses, three developed rhabdomyolysis during the submaximal exercise, and one during the near-maximal exercise.
• The concentrations of several blood variables – potassium, glucose, free fatty acids, ammonia, lactate, cortisol, adrenaline, and noradrenaline – were measured. If consistent patterns among these variables could have been identified in the horses that developed rhabdomyolysis, they might have signaled a way to predict RER flare-ups, but no such patterns were found.
• The horses with RER that developed rhabdomyolysis (known as RERa horses) had higher cortisol and blood glucose concentrations. However, in most other respects, their blood and muscle metabolic responses were similar to those of RER horses that didn’t develop rhabdomyolysis (known as RERb horses) and the control group.
• At rest, both RERa and RERb horses had significantly higher muscle glycogen concentrations (> 650 mmol/kg dry weight) compared to the control horses. These higher levels might have been considered a risk for rhabdomyolysis, as the rapid breakdown of glycogen can cause muscle damage.
• After both submaximal and near-maximal exercises, the lactate concentrations in RERa horses’ muscles were similar or lower than the control group’s, suggesting that in these horses, rhabdomyolysis wasn’t caused by an excessively fast anaerobic glycolysis (the process that breaks down glucose for energy, producing lactate).

Conclusion

• The results suggested that the higher resting glycogen levels in RER horses did not lead to any causative overly-fast glycolysis and lactate accumulation, typically associated with muscle damage and therefore rhabdomyolysis. However, these findings alone do not isolate a definitive cause of RER flare-ups in exercising horses.