Relationships between myonuclear domain size and fibre properties in the muscles of Thoroughbred horses.

The research examined the relationship between myonuclear domain size, muscle fibre type, and metabolic properties in the muscles of Thoroughbred horses. The study found that the size of the myonuclear domain, which is the area of a muscle fibre that a single nucleus can regulate, depends more on the type of muscle fibre than on the activities of mitochondrial enzymes.

Study Methodology

• The study was conducted on the muscles of 3 Thoroughbred horses that were 3-5 years old.
• Muscle samples were taken post-mortem from 20 different areas.
• The muscle fibre type population was determined through staining with monoclonal antibodies to each myosin heavy chain isoform. The staining allowed for the identification of type I, IIa, and IIx muscle fibres.
• The activities of two enzymes, succinic dehydrogenase (SDH) and phosphofructokinase (PFK), were measured in each muscle sample. SDH is associated with oxidative metabolism while PFK is associated with glycolytic metabolism.
• 30 single fibres were isolated from each muscle sample and analysed under a microscope to measure fibre volume and the number of myonuclei for a given length of fibre.
• The size of the myonuclear domain in each single fibre was determined after standardising the sarcomere length.

Study Results

• The soleus, vastus lateralis, and gluteus medius muscles had the highest percentage of type I, IIa, and IIx muscle fibres, respectively.
• The muscles with the highest enzymatic activities were the diaphragm (highest SDH activity) and the longissimus lumborum (highest PFK activity).
• The myonuclear domains were largest in the splenius muscle and smallest in the soleus and masseter muscles.
• The size of the myonuclear domain had a significant relationship with the type I muscle fibre population, but not with the SDH activity of the muscles.

Conclusions

• The results suggest that the size of a muscle fibre’s myonuclear domain is more dependent on the type of muscle fibre than the levels of mitochondrial enzyme activity.

The research provides important insights into muscle plasticity in horses, which could have implications for training programs, injury prevention, and the understanding of muscle structure and function in these animals.