Skeletal muscle mitochondrial myopathy as a cause of exercise intolerance in a horse.

This research article outlines a previously unidentified cause of exercise intolerance in horses – a deficiency in a specific mitochondrial enzyme in their skeletal muscle. The case study of an Arabian mare, which had severe intolerance and stiffness during exercise, led to this novel finding.

Detailed Examination

• The study begins by detailing the physical examination of the Arabian mare. The researchers performed routine blood tests as well as specific examinations like endoscopy, and ultrasonograms of the heart and iliac arteries.
• No abnormalities were found in these tests, indicating that the cause of the exercise intolerance wasn’t linked to common physical or cardiorespiratory issues.

Exercise Analysis

• The mare was then made to exercise at varying speeds (1.5-7 m/s), which unearthed marked lactic acidosis, or the accumulation of lactic acid in the bloodstream faster than it can be removed.
• An increase in mixed venous PVO2, which is the partial pressure of oxygen in mixed venous blood, was observed. However, the oxygen consumption did not change significantly.

Muscle Biopsy and Analysis

• Muscle biopsies from the mare were then examined. Here, they found a large accumulation of mitochondria with odd shapes for their inner membrane folds (called cristae), which are usually efficient and regular for optimal function.
• Deeper biochemical analysis revealed a very low activity of NADH CoQ reductase, the first enzyme complex in the mitochondrial respiratory chain. This enzyme essentially helps in the production of ATP, the body’s energy currency.

Conclusion

• The mitochondria are the “powerhouses” of cells and are responsible for energy production. If an essential component of their function is impaired, then it can lead to decreased energy output.
• In this case, the lack of functional NADH CoQ reductase most likely led to impaired energy metabolism during exercise, thereby producing high amounts of lactic acid and causing muscle pain and stiffness.
• This research identifies mitochondrial dysfunction as a potential cause of exercise intolerance in horses, opening up new avenues for treatment and prevention strategies.