Caffeine contractures, twitch characteristics and the threshold for Ca(2+)-induced Ca2+ release in skeletal muscle from horses with chronic intermittent rhabdomyolysis.

The research explores the connection between chronic intermittent rhabdomyolysis in horses with abnormal calcium regulation. The study finds that horses with exercise-related rhabdomyolysis not only have an altered response to caffeine-induced muscle contractions, but they also have different twitch characteristics and a lower threshold for calcium-induced calcium release.

Methodology

• The study focused on examining muscle from horses with intermittent exercise-associated rhabdomyolysis, particularly seeking abnormalities in calcium regulation. The semimembranosus muscle fibre bundles were studied in vitro.
• The study also investigated the response of these horses to caffeine-induced contractures, changes in the duration of electrically elicited twitches, and the effects of substituting strontium for calcium.
• Examinations were made across different horse breeds including Thoroughbreds and Standardbreds.
• In addition to these, the threshold for calcium-induced calcium release in terminal cisternae-containing fractions of muscles from horses with rhabdomyolysis was also studied.

Findings

• Findings from the study reveal that the time to reach a 50% relaxation of caffeine-induced contractures was shorter in horses with exercise-related rhabdomyolysis. Additionally, these horses experienced a longer duration for electrically elicited twitches.
• Replacing calcium with strontium eliminated the difference in caffeine contracture between the normal horses and those with rhabdomyolysis. This indicates that calcium is a significant factor in the physical abnormalities presented by horses with rhabdomyolysis.
• In muscle segments containing terminal cisternae, the threshold at which calcium induces calcium release was found to be lower than normal in horses with rhabdomyolysis, suggesting a higher sensitivity or reactivity to calcium.
• Thoroughbreds with rhabdomyolysis had a shorter time to reach peak twitch tension compared to healthy Standardbred horses. This points to breed-specific differences in muscle response.
• Fibre typing – a molecular technique used to differentiate various types of muscle fibres – did not yield any notable differences between the breeds or groups studied.
• No significant histological changes were observed in the muscle from horses with rhabdomyolysis. Low grade to moderate degenerative myopathy – a type of muscular disease – was reported in a few cases.

Conclusion

• The results of the research suggest that horses with intermittent exercise-associated rhabdomyolysis may have abnormalities in calcium regulation.
• This can lead to differences in muscle contractions and twitch characteristic as well as a reduced threshold for calcium-induced calcium release.
• The use of different horse breeds in the study indicates that findings may be applicable across various breeds, potentially guiding future preventative and treatment strategies for rhabdomyolysis in the wider horse population.