Calcium sensitivity of force production and myofibrillar ATPase activity in muscles from Thoroughbreds with recurrent exertional rhabdomyolysis.
OBJECTIVE
To determine whether the basis for recurrent exertional rhabdomyolysis (RER) in Thoroughbreds lies in an alteration in the activation and regulation of the myofibrillar contractile apparatus by ionized calcium.


METHODS
4 Thoroughbred mares with RER and 4 clinically normal (control) Thoroughbreds.


METHODS
Single chemically-skinned type-I (slow-twitch) and type-II (fast-twitch) muscle fibers were obtained from punch biopsy specimens, mounted to a force transducer, and the tensions that developed in response to a series of calcium concentrations were measured. In addition, myofibril preparations were isolated from muscle biopsy specimens and the maximal myofibrillar ATPase activity, as well as its sensitivity to ionized calcium, were measured.


RESULTS
Equine type-I muscle fibers were more readily activated by calcium than were type-II muscle fibers. However, there was no difference between the type-II fibers of RER-affected and control horses in terms of calcium sensitivity of force production. There was also no difference between muscle myofibril preparations from RER-affected and control horses in calcium sensitivity of myofibrillar ATPase activity.


CONCLUSIONS
An alteration in myofibrillar calcium sensitivity is not a basis for pathologic contracture development in muscles from RER-affected horses. Recurrent exertional rhabdomyolysis in Thoroughbreds may represent a novel heritable defect in the regulation of muscle excitation-contraction coupling or myoplasmic calcium concentration.
Mlekoday
J A
JA
Department of Clinical and Population Sciences, College of Veterinary Medicine, University of Minnesota, St. Paul 55108, USA.
Mickelson
J R
JR
Valberg
S J
SJ
Horton
J H
JH
Gallant
E M
EM
Thompson
L V
LV
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000818:Animals; D001706:Biopsy / Q000662:veterinary; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D005260:Female; D006734:Horse Diseases / Q000201:enzymology / Q000235:genetics / Q000503:physiopathology*; D006736:Horses; D009119:Muscle Contraction / Q000187:drug effects; D018656:Muscle Fibers, Fast-Twitch / Q000201:enzymology / Q000502:physiology; D018657:Muscle Fibers, Slow-Twitch / Q000201:enzymology / Q000502:physiology; D018482:Muscle, Skeletal / Q000201:enzymology / Q000503:physiopathology*; D009210:Myofibrils / Q000201:enzymology*; D018995:Myosin Heavy Chains / Q000378:metabolism; D012206:Rhabdomyolysis / Q000201:enzymology / Q000235:genetics / Q000503:physiopathology / Q000662:veterinary*; D014159:Transducers / Q000662:veterinary
D000251:Adenosine Triphosphatases; D018995:Myosin Heavy Chains; D002118:Calcium