Abnormal regulation of muscle contraction in horses with recurrent exertional rhabdomyolysis
Lentz, L.R.; Valberg, S.J.; Balog, E.M.; Mickelson, J.R.; Gallant, E.M.
American Journal of Veterinary Research 60(8): 992-999
1999
ISSN/ISBN: 0002-9645 PMID: 10451211 Document Number: 503863
Five adult Thoroughbred horses with recurrent exertional rhabdomyolysis (RER) and 7 healthy adult Thoroughbred or mixed-breed horses were used to determine whether abnormal regulation of muscle contraction similar to that associated with malignant hyperthermia (MH) was evident in intact external intercostal muscle cells from Thoroughbreds with RER. Twitch time course variables and contracture responses to various concentrations of potassium, caffeine and halothane were measured in small bundles of intact external intercostal muscle cells from healthy horses and horses with RER. Threshold for significant contracture induced by potassium depolarization was lower for RER-affected muscles compared with normal muscles, although the relationship between potassium concentration and membrane potential were not different. Thresholds for contracture induced by caffeine and halothane were also lower for RER-affected muscles, compared with normal muscles. Lower thresholds for caffeine- and halothane-induced contractures, as well as depolarization-elicited contractures, in RER-affected muscles suggest a defect in myoplasmic calcium regulation. It is concluded that the regulation of muscle contraction in Thoroughbreds with RER is irregular. The specific defect may be attributable to abnormal intracellular calcium regulation.