Skeletal myosin heavy chain composition and carriage training.

This research article investigates whether intensive carriage training alters the composition of myosin heavy chain (MHC) in the muscles of horses, specifically in the gluteus medius muscle, along with the influence on muscle fibre types, fibre oxdiative capacity and capillaries. The study demonstrated that heavy carriage exercise led to changes in the MHC composition resulting in enhanced muscle endurance but decreased shortening velocity.

Research Context

• The research was conducted on a specific type of protein found in horse muscle called the Myosin Heavy Chain (MHC). MHC exists in three different isoforms (versions); the slow (MHC-I) and the two fast isoforms (MHC-IIA and MHC-IIX), leading to the creation of five fiber types in the muscle (type I, IIA, IIX, and two hybrid types – I+IIA and IIA+IIX).
• The goal of the study was to ascertain if heavy carriage training changes the composition of MHC in the muscles of horses, as well as the prevalence of particular fiber types.

Research Methodology

• Fourteen Andalusian mares aged between 42 and 46 months were selected for the study. Seven horses served as a control group to determine the impact of growth on muscle, while the remaining seven underwent an eight-month carriage training program.
• Gluteus medius muscle biopsies were taken and analyzed biochemically for MHC composition. Additionally, they were examined immunohistochemically for fiber types with specific antibodies, and histochemically for fiber areas, fiber oxidative capacity, and capillaries.

Findings and Interpretation

• After the heavy carriage training, a decrease in MHC-IIX and an increase in MHC-I was observed, suggesting a shift in MHC isoforms from IIX to IIA to I. This implies a decrease in the shortening of the muscle velocity, yet an increase in fatigue resistance.
• The percentages of type IIX and IIAX fibres decreased, whereas the percentage of type I fibres increased. MHC-IIA composition and IIA fibre percentage remained unchanged with training.
• The training had no significant effect on fibre areas and capillaries, but the percentage of fibres with high oxidative capacity increased suggesting an increase in endurance.
• No changes in muscle variables were observed in the control group after the 8 month training period, indicating that the observed changes in trained horses were indeed due to the training effect.

Conclusion

• The results of the study thus imply that heavy carriage training does alter MHC composition in horse skeletal muscle. This shift in MHC composition in muscle, therefore, suggests a response to exercise-induced muscle remodeling.