Calcium sensitivity of force production and myofibrillar ATPase activity in muscles from Thoroughbreds with recurrent exertional rhabdomyolysis.

The research article delves into whether the recurrent muscle disorder observed in Thoroughbred horses, namely recurrent exertional rhabdomyolysis (RER), has anything to do with changes in the way the contraction process of muscle fibres is activated and regulated by calcium ions. It concludes that the sensitivity of muscle fibres to calcium does not contribute to the pathological muscle contraction related to RER. Consequently, the authors propose that RER may be due to a genetic mutation affecting muscle excitation and contraction, or the concentration of calcium within muscle cells.

Methods of the Study

• The researchers chose eight Thoroughbred horses for their study, four of whom had RER (the experimental group), while the remaining four were clinically healthy and served as the control group.
• They obtained single chemically-skinned (stripped of cell membranes) slow-twitch (Type-I) and fast-twitch (Type-II) muscle fibres from the horses, using a procedure called a punch biopsy.
• These individual muscle fibres were attached to a device capable of measuring the forces the fibres produced in response to different concentrations of calcium. Additionally, the researchers examined the maximum activity of myofibrillar ATPase (an enzyme pivotal in muscle contraction), and its sensitivity to calcium, using muscle sample preparations.

Findings of the Research

• The results showed that Type-I muscle fibres were more readily activated by calcium than Type-II muscle fibres, as expected from previous research.
• However, there was no difference in calcium sensitivity and force production between the Type-II muscle fibres from RER-affected horses and those from healthy controls.
• Similarly, there was no difference in calcium sensitivity or ATPase activity between the myofibril preparations from both the afflicted and control groups.

Conclusions

• The study concluded that alterations in myofibrillar calcium sensitivity do not contribute to the pathological muscle contractions associated with RER.
• Instead, they proposed that RER in Thoroughbreds could be the result of a novel, inheritable defect affecting the way muscle excitation and contraction are coordinated, or the concentration of calcium within muscle cells.