Proglycogen, macroglycogen, glucose, and glucose-6-phosphate concentrations in skeletal muscles of horses with polysaccharide storage myopathy performing light exercise.

This research investigates the concentrations of certain molecules related to glucose storage and metabolism in horses with polysaccharide storage myopathy (PSSM), an inherited muscle disease, before and after light exercise. The findings suggest that glucose uptake in muscle tissue is heightened in PSSM-affected horses post mild exercise, and that these horses excessively store forms of glycogen, which may interfere with the functionality of enzymes linked to glycogen synthesis and degradation.

Research Methodology

• The study involved six horses diagnosed with PSSM and four healthy control horses. The exercise regime for both groups involved intervals of 2 minutes of walking followed by 2 minutes of trotting on a treadmill. For the PSSM-affected horses, this was continued until they developed muscle cramping. The control group carried out a similar exercise test for a maximum of 20 minutes.
• The activity of creatine kinase (CK) in serum, a commonly used biomarker for muscle damage, was measured before and 4 hours after the exercise. Additionally, biopsy specimens of gluteal muscle were obtained pre and post-exercise and analyzed for concentrations of total glycogen (G(t)), proglycogen (PG), macroglycogen (MG), glucose-6-phosphate (G-6-P), free glucose, and lactate.

Research Findings

• Post-exercise, the PSSM horses had a mean CK activity that was 26 times higher than the control horses, indicating greater muscle damage.
• Before exercise, the concentrations of muscle glycogen in the form of PG, MG, and G(t) were notably higher in the PSSM horses compared to the control horses (1.5, 2.2, and 1.7 times respectively).
• No significant changes in concentrations of G(t), PG, MG, G-6-P, and lactate were observed in PSSM horses post-exercise. However, the concentration of free glucose in their muscle tissue increased significantly.

Conclusion and Implications

• The lack of notable change in glycogen and G-6-P levels, but significant increase in free glucose levels in PSSM horses post exercise, indicates elevated glucose uptake in the muscle tissue of affected horses.
• This, in combination with the pre-exercise findings of high glycogen concentration in PSSM horses, suggests over-storage of glycogen in its PG and MG forms. This excessive storage may disturb the interactions between enzymes involved in the synthesis and degradation of glycogen (glycosomes) and thus may be a factor in the pathogenesis of PSSM.