In vitro contractile responses and contracture testing of skeletal muscle from Quarter Horses with exertional rhabdomyolysis

Objective - To determine whether increased sensitivity to pharmacologic agents was a general property of equine exertional myopathies, including polysaccharide storage myopathy (PSSM) in Quarter Horses. Animals - 5 adult Quarter Horses with exertional rhabdomyolysis and abnormal polysaccharide accumulation in skeletal muscle and 4 clinically normal adult Quarter or Quarter-type horses. Procedures - Twitch time course measurements and contracture responses to various concentrations of caffeine and halothane for small bundles of intact external intercostal muscle fibers were measured in all horses. Results - Caffeine contracture threshold of muscles from Quarter Horses with PSSM was not different from that of clinically normal horses (5 mM in both groups). Muscles from horses with PSSM and from clinically normal horses did not have contracture in response to up to 2% halothane. Conclusions and Clinical Relevance - Results were in contrast to the increased sensitivity to caffeine and halothane for muscles from Thoroughbreds with recurrent exertional rhabdomyolysis (RER). Although clinical signs of muscular stiffness after exercise are similar between Quarter Horses with PSSM and Thoroughbreds with RER, these breeds appear to have 2 distinct myopathies with different pathophysiologic bases. Unlike RER in Thoroughbreds, PSSM in Quarter Horses does not appear to be accompanied by a defect in regulation of muscle contraction.