Glycogen depletion patterns in horses competing in day 2 of a three day event.

The study investigates how strenuous physical activity during a three-day equestrian event affects glycogen levels in specific muscle fibers of horses. Six Thoroughbred horses were subject to muscle biopsies before and after the second day’s speed and endurance test, and significant changes in glycogen levels were detected.

Methodology

• Muscle biopsy samples were taken from the left middle gluteal muscle of six Thoroughbred horses both the day before and within 30 minutes of completion of the speed and endurance test during the second day of a three-day event.
• These samples underwent a histochemical reaction for myosin adenosine triphosphatase activity after acid pre-incubation to show Type I, IIA, and IIB fibers. These are muscle fibers that differ in their speed of contraction and level of endurance.
• The glycogen content of these individual fibers was then examined through the periodic acid Schiff (PAS) reaction, a method used for staining and measuring the level of glycogen in tissues.
• Total glycogen in the muscle was also measured fluorometrically after hydrolysis to glucose.

Findings

• The total muscle glycogen decreased significantly (P less than 0.001) after the exercise compared to before, displaying a reduction of 306.0 +/- 34.6 mmoles glucose units per kilogram of dry weight.
• There was a significant decrease (P less than 0.05) in the percentage of Type I fibers that had medium PAS staining intensity, indicating a reduction in glycogen levels in these fibers post-exercise as compared to pre-exercise.
• Similarly, there were significant decreases (P less than 0.01 and P less than 0.001, respectively) in the percentage of Type IIA and IIB fibers presenting high PAS staining intensity post-exercise compared to pre-exercise. This implies that these fibers also experienced a considerable depletion in their glycogen storage after the exercise.
• Interestingly, there were significant increases (P less than 0.01 and P less than 0.001, respectively) in the percentage of Type IIA and IIB fibers that were categorized as having medium PAS staining intensity post-exercise as compared to pre-exercise.

Conclusion

• Overall, these findings suggest that moderate to arduous physical activity, like that in a three-day equestrian event, can result in considerable glycogen depletion in various muscle fibers of horses.