Elevated myoplasmic calcium in exercise-induced equine rhabdomyolysis

Exertional rhabdomyolysis is a myopathy of unknown pathophysiology. We measured intracellular resting calcium concentration ([Ca²⁺]_i) by means of Ca²⁺-selective microelectrodes in intercostal muscle fibers from horses suffering from rhabdomyolysis, and from horses with no evidence of neuromuscular disorder. [Ca²⁺]_i was several-fold higher in muscle fibers from horses suffering from rhabdomyolysis when compared to controls. Treatment of rhabdomyolytic horses with dantrolene, an agent that prevents Ca²⁺ release from the sarcoplasmic reticulum, reduced [Ca²⁺]_i toward control values, and accelerated the recovery from this myopathy. These results indicate that an acute episode of rhabdomyolysis is associated with elevation in [Ca²⁺]_i in skeletal muscles, and that dantrolene might be of benefit in treating this disease.