Objective—To determine whether an alteration in calcium
regulation by skeletal muscle sarcoplasmic reticulum,
similar to known defects that cause malignant
hyperthermia (MH), could be identified in membrane
vesicles isolated from the muscles of Thoroughbreds
with recurrent exertional rhabdomyolysis (RER).
Sample Population—Muscle biopsy specimens
from 6 Thoroughbreds with RER and 6 healthy (control)
Procedures—RER was diagnosed on the basis of a history
of > 3 episodes of exertional rhabdomyolysis confirmed
by increases in serum creatine kinase (CK) activity.
Skeletal muscle membrane vesicles, prepared by
differential centrifugation of muscle tissue
homogenates obtained from the horses, were characterized
for sarcoplasmic reticulum (SR) activities, including
the Ca2+ release rate for the ryanodine receptor-Ca2+
release channel, [3H]ryanodine binding activities, and
rate of SR Ca2+-ATPase activity and its activation by Ca2+.
Results—Time course of SR Ca2+-induced Ca2+
release and [3H]ryanodine binding to the ryanodine
receptor after incubation with varying concentrations
of ryanodine, caffeine, and ionized calcium did not differ
between muscle membranes obtained from control
and RER horses. Furthermore, the maximal rate
of SR Ca2+-ATPase activity and its affinity for Ca2+ did
not differ between muscle membranes from control
horses and horses with RER.
Conclusions and Clinical Relevance—Despite clinical
and physiologic similarities between RER and MH,
we concluded that RER in Thoroughbreds does not
resemble the SR ryanodine receptor defect responsible
for MH and may represent a novel defect in muscle
excitation-contraction coupling, calcium regulation,
or contractility. (Am J Vet Res 2000;61:242–247)
Objective—To determine whether there was genetic linkage between the recurrent exertional rhabdomyolysis (RER) trait in Thoroughbred horse pedigrees and DNA markers in genes (the sarcoplasmic reticulum calcium release channel [RYR1] gene, the sarcoplasmic reticulum calcium ATPase [ATP2A1] gene, and the transverse tubule dihydropyridine receptor-voltage sensor [CACNA1S] gene) that are important in myoplasmic calcium regulation.
Animals—34 horses in the University of Minnesota RER resource herd and 62 Thoroughbreds from 3 families of Thoroughbreds outside of the university in which RER-affected status was assigned after 2 or more episodes of ER had been observed.
Procedures—Microsatellite DNA markers from the RYR1, ATP2A1, and CACNA1S gene loci on equine chromosomes 10, 13, and 30 were identified. Genotypes were obtained for all horses in the 4 families affected by RER, and data were used to test for linkage of these 3 loci to the RER phenotype.
Results—Analysis of the RYR1, CACNA1S, and ATP2A1 microsatellites excluded a link between those markers and the RER trait.
Conclusions and Clinical Relevance—It is likely that the heritable alterations in muscle contractility that are characteristic of RER are caused by a gene that is not yet known to cause related muscle disease in other species.
Objective—To determine whether alterations in
myoplasmic calcium regulation can be identified in
muscle cell cultures (myotubes) and intact muscle
fiber bundles derived from Thoroughbreds affected
with recurrent exertional rhabdomyolysis (RER).
Animals—6 related Thoroughbreds with RER and 8
clinically normal (control) Thoroughbred or crossbred
Procedures—Myotube cell cultures were grown from
satellite cells obtained from muscle biopsy specimens
of RER-affected and control horses. Fura-2 fluorescence
was used to measure resting myoplasmic
calcium concentration as well as caffeine- and 4-chloro-m-cresol (4-CMC)-induced increases in
myoplasmic calcium. In addition, intact intercostal
muscle fiber bundles were prepared from both types
of horses, and their sensitivities to caffeine- and 4-CMC-induced contractures were determined.
Results—Myotubes of RER-affected and control
horses had identical resting myoplasmic calcium concentrations.
Myotubes from RER-affected horses had
significantly higher myoplasmic calcium concentrations
than myotubes from control horses following
the addition of ≥ 2mM caffeine; however, there was
no difference in their response to 4-CMC (≥ 1mM).
Caffeine contracture thresholds for RER and control
intact muscle cell bundles (2 vs 10mM, respectively)
were significantly different, but 4-CMC contracture
thresholds of muscle bundles from RER-affected and
control horses (500µM) did not differ.
Conclusions and Clinical Relevance—An increase
in caffeine sensitivity of muscle cells derived from a
family of related RER-affected horses was detected in
vitro by use of cell culture with calcium imaging and
by use of fiber bundle contractility techniques. An
alteration in muscle cell calcium regulation is a primary
factor in the cause of this heritable myopathy.
(Am J Vet Res 2002;63:1724–1731)
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
Animals—4 Thoroughbred mares with RER and 4
clinically normal (control) Thoroughbreds.
Procedure—Single chemically-skinned type-I (slowtwitch)
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
Conclusion and Clinical Relevance—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.
(Am J Vet Res 2001;62:1647–1652)
Objective—To develop a diagnostic test for recurrent
exertional rhabdomyolysis (RER) in Thoroughbreds that
relied on in vitro contracture of muscle biopsy specimens
and determine whether the inheritance pattern
of RER diagnosed on the basis of this contracture test
was consistent with an autosomal dominant trait.
Animals—8 adult horses with RER and 16 control
adult horses for development of the contracture test;
23 foals for inheritance of RER.
Procedure—External intercostal muscle biopsy specimens
from the 24 adult horses were tested for contracture
in response to halothane and caffeine, and
criteria for a positive test result were determined.
These criteria were then applied to results for the 23
foals to determine whether they had RER. Simple
segregation analysis was performed to determine
whether results were consistent with a dominant pattern
Results—Results of the contracture test were positive
for 5 of the 12 colts and 4 of the 11 fillies. Results
of segregation analysis were consistent with an autosomal
dominant pattern of inheritance. Two sires with
RER produced colts with RER, supporting the hypothesis
that RER had an autosomal, rather than an
X-linked, inheritance pattern. In addition, in 1
instance, an unaffected colt was produced by 2 affected
parents, which was not consistent with a recessive
mode of inheritance.
Conclusions and Clinical Relevance—Although the
expression of the RER trait is influenced by sex, temperament,
and diet, among other factors, results from
the in vitro muscle contracture test and this breeding
trial suggest that RER in Thoroughbreds can be modeled
as a genetic trait with an autosomal dominant pattern of
inheritance. ( J Am Vet Med Assoc 2005;227:762–767)