Skeletal Muscle Fiber Type Composition and Citrate Synthase Activity in Fit and Unfit Warmbloods and Quarter Horses.

The research article investigates the skeletal muscle adaptations in Warmbloods (WB) in comparison to Quarter Horses (QH), with a particular focus on the muscle fiber types and citrate synthase (CS) activity. The purpose of the study is to understand how selective breeding and discipline-specific training has affected muscle composition in these breeds.

Objective and Methodology

• The researchers aimed to analyze skeletal muscle fiber composition and CS activity (a marker for mitochondrial volume density) in both fit and unfit Warmbloods and Quarter Horses.
• This analysis involved taking gluteus medius muscle biopsies from fit and unfit horses in both breeds. These samples were then analyzed via ATPase activity to determine the types of muscle fibers present.
• The researchers compared the data collected from both these breeds using one-way ANOVA.

Findings

• The study found significant differences in the muscle fiber profile between the breeds. Both fit and unfit WBs had a significantly higher proportion of type 1 fibers and lower proportion of type 2X fibers compared to QHs.
• The CS activity differed between the breeds and reflected fitness level to some extent. Unfit WBs demonstrated significantly higher CS activity than unfit QHs. However, there was no significant difference in CS activity between the fit WBs and fit QHs.
• CS activity was notably higher in fit QHs compared to unfit ones. This increase was not found between the fit and unfit WBs, suggesting that fitness training has different impacts on mitochondrial dynamics across breeds.

Conclusions

• The research concluded that WBs inherently have a high percentage of type 1 (slow-twitch) fibers and low percentage of type 2X (fast-twitch) fibers regardless of their fitness level. They also have high mitochondrial content; this contrasts with QHs that are inherently low in type 1 fibers, high in type 2X fibers, and that have low mitochondrial content.
• The research also suggested that the mitochondrial volume density (as indicated by citrate synthase activity) can’t accurately be predicted from the contractile fiber type composition alone. This was concluded based on the similar CS activity observed in fit WBs and QHs, even though there was a significant difference in type 2X fibers between the breeds.