Abnormal regulation of muscle contraction in horses with recurrent exertional rhabdomyolysis.

OBJECTIVE: 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 recurrent exertional rhabdomyolysis (RER). ANIMALS: 5 adult Thoroughbred horses with RER and 7 clinically normal adult Thoroughbred or mixed-breed horses. PROCEDURES: 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 clinically normal horses and horses with RER. RESULTS: 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. CONCLUSIONS AND CLINICAL RELEVANCE: Regulation of muscle contraction is abnormal in Thoroughbreds with RER. The specific defect may be attributable to abnormal intracellular calcium regulation. Knowledge of the specific defect involved in RER may lead to improved prevention and treatment of RER-affected horses.     

Exclusion of linkage of the RYR1, CACNA1S, and ATP2A1 genes to recurrent exertional rhabdomyolysis in Thoroughbreds

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.     

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. ANIMALS: 4 Thoroughbred mares with RER and 4 clinically normal (control) Thoroughbreds. PROCEDURES: 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 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.     

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.     

Myoplasmic calcium regulation in myotubes from horses with recurrent exertional rhabdomyolysis

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 horses. 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 > or = 2mM caffeine; however, there was no difference in their response to 4-CMC (> or = 1 mM). 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 (500 microM) 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.     

Inheritance of recurrent exertional rhabdomyolysis in thoroughbreds

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. DESIGN: Clinical trial. 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 of inheritance. 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 auto-. somal 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.     

Malignant hyperthermia susceptibility: biochemical basis for pathogenesis and diagnosis.

Malignant hyperthermia (MH) is a hypermetabolic and hypercontractile syndrome triggered by anesthesia or various stressors that cause a sustained increase in sarcoplasmic ionized Ca. Susceptibility is apparently inherited in an autosomal dominant pattern. The primary molecular defect results in hypersensitive ligand-gating of the Ca-release channel of sarcoplasmic reticulum (SR) in skeletal muscle: channel opening is stimulated by abnormally low concentrations of agonist. We attribute MH to a mutation in the gene for the fast twitch muscle isoform of the Ca-channel, resulting in the expression of a cardiac-like isoform in fast muscle. Syndromes with some resemblance to MH can occur due to other genetic or acquired imbalances in Ca-flux across SR that favor net release of Ca. Either defective uptake or release can be detected as increased sensitivity of muscle to the contracture-producing effects of caffeine and halothane. Thus, caffeine and/or halothane contracture tests for MH-susceptibility may give false positives when there is decreased Ca-uptake, such as in muscular dystrophies. Ca-channel hypersensitivity and decreased Ca-uptake activity can be detected by assays using isolated SR. Functional assays using lymphocytes are being assessed as potential replacements for muscle contracture tests. Polymorphism analysis of proteins or nucleic acids for the MH or closely-linked genes has been used to trace the inheritance of MH-susceptibility.     

Effect of ration and exercise on plasma creatine kinase activity and lactate concentration in Thoroughbred horses with recurrent exertional rhabdomyolysis

OBJECTIVE: To determine the effects of 3 rations (low grain, fat, high grain) on plasma creatine kinase (CK) activity and lactate concentration in Thoroughbred horses with recurrent exertional rhabdomyolysis (RER). ANIMALS: 5 Thoroughbreds with RER and 3 healthy Thoroughbreds (control horses). PROCEDURES: Rations were formulated to meet (low-grain and fat rations) or exceed (high-grain ration) daily energy requirements. Each ration was fed to horses in a crossover design for 3 weeks. Horses were exercised on a treadmill Monday through Friday; maximum speed on Monday and Friday was 11 m/s (6% slope), on Tuesday and Thursday was 9 m/s, and on Wednesday was 4.5 m/s. Plasma CK activity and lactate concentration were determined before and after exercise. RESULTS: Horses with RER fed the high-grain ration had significantly greater CK activity and change in CK activity 4 hours after exercise, compared with those fed the low-grain ration. Horses with RER exercised at the trot or canter had significantly greater increases in CK activity, compared with those exercised at the gallop. Plasma lactate concentrations after exercise were similar in control and affected horses. Lactate concentration and CK activity were not correlated in horses with RER. CONCLUSIONS AND CLINICAL RELEVANCE: Rations high in grain and formulated to exceed daily energy requirements may increase episodes of rhabdomyolysis in thoroughbred horses susceptible to RER.     

Effect of oral administration of dantrolene sodium on serum creatine kinase activity after exercise in horses with recurrent exertional rhabdomyolysis

OBJECTIVE: To determine the effect of oral administration of dantrolene sodium on serum creatine kinase (CK) activity after exercise in horses with recurrent exertional rhabdomyolysis (RER). ANIMALS: 2 healthy horses and 5 Thoroughbreds with RER. PROCEDURE: 3 horses received 2 doses of dantrolene (4, 6, or 8 mg/kg, p.o., with and without withdrawal of food) 2 days apart; 90 minutes after dosing, plasma dantrolene concentration was measured spectrofluorometrically. On the basis of these results, 5 Thoroughbreds with RER from which food was withheld received dantrolene (4 mg/kg) or an inert treatment (water [20 mL]) orally 90 minutes before treadmill exercise (30 minutes, 5 d/wk) during two 3-week periods. Serum CK activity was determined 4 hours after exercise. Plasma dantrolene concentration was measured before and 90 minutes after dosing on the first and last days of dantrolene treatment and before dosing on the first day of the inert treatment period, RESULTS: 90 minutes after dosing, mean +/- SEM plasma dantrolene concentration was 0.62 +/- 0.13 and 0 microg/mL in the dantrolene and inert treatment groups, respectively. Serum CK activity was lower in dantrolene-treated horses (264 +/- 13 U/L), compared with activity in water-treated horses (1,088 +/- 264 U/L). Two horses displayed marked muscle stiffness on the inert treatment. CONCLUSIONS AND CLINICAL RELEVANCE: In 5 horses with RER from which food had been withheld, 4 mg of dantrolene/kg administered orally provided measurable, though variable, plasma concentrations and significantly decreased serum CK activity after exercise in 4 of those horses.     

Plasma and urine electrolyte and mineral concentrations in Thoroughbred horses with recurrent exertional rhabdomyolysis after consumption of diets varying in cation-anion balance

OBJECTIVE: To determine whether plasma, urine, and fecal electrolyte and mineral concentrations differ between clinically normal horses and Thoroughbreds with recurrent exertional rhabdomyolysis (RER) after consumption of diets varying in cation-anion balance. ANIMALS: 5 Thoroughbred mares with RER and 6 clinically normal mixed-breed mares. PROCEDURE: Each of 3 isocaloric diets designated as low, medium, and high on the basis of dietary cation-anion balance (DCAB) values of 85, 190, and 380, respectively, were fed to horses for 14 days. During the last 72 hours, 3 horses with RER and 3 control horses had daily urine and fecal samples obtained by total 24-hour collection. Remaining horses had urine samples collected daily by single catheterization. RESULTS: For each diet, no differences existed between horses with RER and control horses in plasma pH, electrolyte concentrations, and creatine kinase activity or in urine pH and renal fractional excretion (FE) values. Plasma pH, strong ion difference, bicarbonate and total carbon dioxide concentrations, and base excess decreased and plasma chloride and ionized calcium concentrations increased with decreasing DCAB. Urine pH decreased with decreasing DCAB. The FE of chloride and phosphorus were greatest for horses fed the low diet. The FE values for all electrolytes exept magnesium did not differ between urine samples obtained by single catheterization and total 24-hour collection. Daily balance of calcium, phosphorus, sodium, chloride, and potassium did not differ significantly among horses fed the various diets. CONCLUSIONS: In clinically normal horses and in horses with RER, the DCAB strongly affects plasma and urine pH and the FE of sodium, potassium, chloride, and phosphorus.     

Canine stress syndrome/malignant hyperthermia susceptibility: calcium-homeostasis defect in muscle and lymphocytes

This study provides the first comprehensive characterisation of the calcium (Ca) homeostasis defects found in muscle and lymphocytes of a malignant hyperthermia (MH)-susceptible dog. Novel findings regarding this dog are reported, compared to controls. First, a canine stress syndrome occurs, analogous to the porcine stress syndrome; susceptibility can be identified by exercise challenge testing. Secondly, caffeine causes Ca release from muscle sarcoplasmic reticulum in a greater amount and at a greater rate. Thirdly, there is a compensatory increase in Ca sequestration by sarcoplasmic reticulum. Fourthly, lymphocytes have lower cytosolic-free Ca and a greater ability to prevent Ca increase. Halothane increases Ca by a greater amount and rate. Fifthly, muscle is more resistant to the contracture-producing effects of caffeine, as occurs in the non-rigid variant of MH susceptibility in man. This resistance, despite increased caffeine-induced release through the Ca channel, may be attributable to increased Ca sequestration by sarcoplasmic reticulum. Finally, erythrocyte osmotic fragility and creatine kinase tests fail to distinguish between the MH-susceptible dog and controls.     

Sarcoplasmic Reticulum Ca2+-ATPase Activity and Glycogen Content in Various Fiber Types after Intensive Exercise in Thoroughbred Horses

To find a new parameter indicating muscle fitness in Thoroughbred horses, we examined time-dependent recovery of glycogen content and sarcoplasmic reticulum (SR) Ca²⁺-ATPase activity of skeletal muscle after intensive treadmill running. Two repeated 50-sec running sessions (13 m/sec) were performed on a flat treadmill (approximately 90%VO₂max). Muscle samples of the middle gluteal muscle were taken before exercise (pre) and 1 min, 20 min, 60 min, and 24 hr after exercise. Muscle fiber type composition was determined in the pre muscle samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. SR Ca²⁺-ATPase activity of the muscle and glycogen content of each muscle fiber type were determined with biochemical analysis and quantitative histochemical staining, respectively. As compared to the pre value, the glycogen content of each muscle fiber type was reduced by 15–27% at 1 min, 20 min, and 60 min after the exercise and recovered to the pre value at 24 hr after exercise test. These results indicate that 24 hr is enough time to recover glycogen content after short-term intensive exercise. The mean value of the SR Ca²⁺-ATPase activity showed a slight decrease (not significant) immediately after exercise, and complete recovery at 60 min after exercise. There were no significant relationship between the changes in glycogen content of each muscle fiber type and SR Ca²⁺-ATPase. Although further studies are needed, SR Ca²⁺-ATPase is not a useful parameter to detect muscle fitness, at least in Thoroughbred horses.     

Epidemiologic analysis of factors influencing exertional rhabdomyolysis in Thoroughbreds

OBJECTIVE: To determine incidence, effect on performance, and management practices associated with exertional rhabdomyolysis (ER) in Thoroughbreds. SAMPLE POPULATION: Medical records for 984 Thoroughbreds and a survey of trainers of horses with and without ER. PROCEDURES: Medical records for 984 Thoroughbreds stabled at a midwestern racetrack were examined to determine the incidence of ER during the 1995 racing season. A retrospective questionnaire was administered to trainers to determine management practices associated with ER in 59 Thoroughbreds with ER and 47 control Thoroughbreds in training. Multiple logistic regression was used to determine management factors associated with ER. RESULTS: ER affected 48 of 984 (4.9%) Thoroughbreds. Two-year-old females were most frequently affected, and 36 of 96 (37.5%) trainers had > or = 1 horse with ER. Horses with ER were more likely not to race during the racing season, compared with control horses. For horses that raced, differences were not found with respect to racing performance between ER and control horses. Exertional rhabdomyolysis developed frequently in susceptible horses that had > or = 1 day of rest prior to exercise and that galloped during exercise. Horses with ER were commonly fed > 4.5 kg of grain daily. Nervous and extremely nervous horses were 5.4 times more likely, and horses with some form of lameness were 4.2 times more likely, to have ER. CONCLUSIONS AND CLINICAL RELEVANCE: Exertional rhabdomyolysis is common in Thoroughbreds, and ER can be affected by temperament, sex, age, diet, exercise routines, and lameness. Management that minimizes excitability, particularly when tailored to each horse, may be most effective for controlling ER.     

Effect of dietary starch, fat, and bicarbonate content on exercise responses and serum creatine kinase activity in equine recurrent exertional rhabdomyolysis

To determine the effect of dietary starch, bicarbonate, and fat content on metabolic responses and serum creatine kinase (CK) activity in exercising Thoroughbreds with recurrent exertional rhabdomyolysis (RER), 5 RER horses were fed 3 isocaloric diets (28.8 Mcal/d [120.5 MJ/d]) for 3 weeks in a crossover design and exercised for 30 minutes on a treadmill 5 days/wk. On the last day of each diet, an incremental standardized exercise test (SET) was performed. The starch diet contained 40% digestible energy (DE) as starch and 5% as fat: the bicarbonate-starch diet was identical but was supplemented with sodium bicarbonate (4.2% of the pellet): and the fat diet provided 7% DE as starch and 20% as fat. Serum CK activity before the SET was similar among the diets. Serum CK activity (log transformed) after submaximal exercise differed dramatically among the diets and was greatest on the bicarbonate-starch diet (6.51 +/- 1.5) and lowest on the fat diet (5.71 +/- 0.6). Appreciable differences were observed in the severity of RER among individual horses. Postexercise plasma pH, bicarbonate concentration, and lactate concentration did not differ among the diets. Resting heart rates before the SET were markedly lower on the fat diet than on the starch diet. Muscle lactate and glycogen concentrations before and after the SET did not differ markedly among the diets. A high-fat, low-starch diet results in dramatically lower postexercise CK activity in severely affected RER horses than does a low-fat, high-starch diet without measurably altering muscle lactate and glycogen concentrations. Dietary bicarbonate supplementation at the concentration administered in this study did not prevent increased serum CK activity on a high-starch diet.     

A glycogen synthase 1 mutation associated with equine polysaccharide storage myopathy and exertional rhabdomyolysis occurs in a variety of UK breeds

Reasons for performing study: A glycogen synthase (GYS1) mutation has been described in horses with histopathological evidence of polysaccharide storage myopathy (PSSM) in the USA. It is unknown whether the same mutation is present in horses from the UK. Objectives: To determine whether the GYS1 mutation occurs in UK horses with histopathological evidence of PSSM and exertional rhabdomyolysis. Hypothesis: The R309H GYS1 mutation is present in a variety of UK horse breeds and that the mutation is commonly associated with exertional rhabdomyolysis. Methods: DNA was extracted from 47 muscle or blood samples from UK horses with histories of exertional rhabdomyolysis in which muscle biopsy diagnosis had been pursued. The proportions of GYS1 mutation positive cases were compared among histopathologically defined groups. In addition, breeds that carried the GYS1 mutation were identified from a total of 37 grade 2 (amylase‐resistant) PSSM cases. Results: Of 47 horses with exertional rhabdomyolysis in which a muscle biopsy diagnosis was pursued, 10 (21%) carried the GYS1 mutation. The mutation was only found in horses with grade 2 PSSM (i.e. not in horses with normal, idiopathic myopathy or grade 1 PSSM biopsy samples). In total, the GYS1 mutation was found in 24/37 (65%) of grade 2 PSSM cases. A variety of breeds, including Quarter Horse, Appaloosa, Warmblood, Connemara‐cross, Cob, Polo Pony and Thoroughbred cross carried the mutation. Conclusions: The GYS1 mutation is an important cause of exertional rhabdomyolysis of UK horse breeds but does not account for all forms of PSSM. Potential relevance: Genotyping is recommended in cases of exertional rhabdomyolysis, prior to or in combination with, muscle biopsy. However a significant proportion of horses with histopathological evidence of PSSM and/or exertional rhabdomyolysis have different diseases.     

An epidemiological study of myopathies in Warmblood horses

REASON FOR PERFORMING STUDY: There are few detailed reports describing muscular disorders in Warmblood horses. OBJECTIVES: To determine the types of muscular disorders that occur in Warmblood horses, along with presenting clinical signs, associated risk factors and response to diet and exercise recommendations, and to compare these characteristics between horses diagnosed with polysaccharide storage myopathy (PSSM), those diagnosed with a neuromuscular disorder other than PSSM (non-PSSM) and control horses. METHODS: Subject details, muscle biopsy diagnosis and clinical history were compiled for Warmblood horses identified from records of biopsy submissions to the University of Minnesota Neuromuscular Diagnostic Laboratory. A standardised questionnaire was answered by owners at least 6 months after receiving the muscle biopsy report for an affected and a control horse. RESULTS: Polysaccharide storage myopathy (72/132 horses) was the most common myopathy identified followed by recurrent exertional rhabdomyolysis (RER) (7/132), neurogenic or myogenic atrophy (7/132), and nonspecific myopathic changes (14/132). Thirty-two biopsies were normal. Gait abnormality, 'tying-up', Shivers, muscle fasciculations and atrophy were common presenting clinical signs. Forty-five owners completed questionnaires. There were no differences in sex, age, breed, history or management between control, PSSM and non-PSSM horses. Owners that provided the recommended low starch fat supplemented diet and regular daily exercise reported improvement in clinical signs in 68% (19/28) of horses with a biopsy submission and 71% of horses diagnosed with PSSM (15/21). CONCLUSIONS: Muscle biopsy evaluation was a valuable tool to identify a variety of myopathies in Warmblood breeds including PSSM and RER. These myopathies often presented as gait abnormalities or overt exertional rhabdomyolysis and both a low starch fat supplemented diet and regular exercise appeared to be important in their successful management. POTENTIAL RELEVANCE: Warmbloods are affected by a variety of muscle disorders, which, following muscle biopsy diagnosis can be improved through changes in diet and exercise regimes.     

Heritability of recurrent exertional rhabdomyolysis in Thoroughbred racehorses

OBJECTIVE: To determine the likely mode of inheritance and identify probable foundation horses for recurrent exertional rhabdomyolysis (RER) in Thoroughbred (TB) racehorses. ANIMALS: 4 families of TB racehorses with a high prevalence of RER, consisting of 3 to 53 horses/family, were used to determine mode of inheritance. Sixty-two TB horses with RER and 34 control TB racehorses without RER were used to identify probable foundation horses for the RER trait. PROCEDURE: RER was diagnosed by a veterinarian and verified by detecting high serum creatine kinase activity. Pedigrees dating from 1930 for all horses were entered into a database. Pedigrees of horses in 4 families were visually inspected for a pattern of inheritance and used for calculation of foundation horse contributions and inbreeding coefficients. The Markov chain Monte Carlo technique was used to analyze pedigrees of the 62 affected and 34 control horses for the conditional probability of foundation genotypes. A dominant mode of inheritance with variable expression model was used. RESULTS: Pedigree analysis supported an autosomal dominant mode of inheritance with variable expression. All affected horses from the 4 families shared a common ancestor. This ancestor and 5 other stallions had a conditional probability of 1.00 for being affected. All 6 stallions shared a common male ancestor within 3 to 5 generations. CONCLUSIONS AND CLINICAL RELEVANCE: On the basis of this study, the RER trait has been in TB racehorses for more than 70 years and may be inherited as an autosomal dominant trait with variable expression.     

Mechanisms responsible for reduced cardiotoxicity of mitoxantrone compared to doxorubicin examined in isolated guinea-pig heart preparations

We reported previously that doxorubicin, an anticancer agent that has an anthracycline structure, alters Ca2+ releasing and uptake mechanisms in the sarcoplasmic reticulum of myocardial cells. These effects of doxorubicin are apparently related to its cardiotoxicity. Mitoxantrone is a similar anticancer agent with an anthracenedion structure that has been shown to be significantly less cardiotoxic. In the present study, the effects of mitoxantrone on the functions of the sarcoplasmic reticulum were examined in isolated muscle preparations obtained from the guinea-pig heart. In electrically-stimulated left atrial muscle preparations, incubation in vitro for 4 hr with 30 or 100 microM mitoxantrone significantly prolonged the time to the peak of twitch tension, markedly increased the developed tension observed at lower stimulation frequencies, thereby attenuating the slope of positive force-frequency relationships, and increased the postrest contraction observed after a 60-sec quiescent period. In myocytes isolated from ventricular muscles, 30 microM mitoxantrone increased the peak and the size of intracellular Ca2+ concentrations ([Ca2+] i), and prolonged the time to peak [Ca2+]i. In skinned muscle fiber preparations obtained from the left ventricular muscle, 30 muM mitoxantrone significantly increased the caffeine-induced contraction without affecting the Ca2+ sensitivity of contractile proteins. These results suggest that mitoxantrone enhances Ca2+ release from the sarcoplasmic reticulum in isolated atrial muscle preparations obtained from the guinea-pig heart. Apparent enhancement of the sarcoplasmic reticulum functions, in contrast to anthracyclines that has been shown to suppress these functions, seems to explain the relative lack of marked cardiotoxicity of mitoxantrone.     

Etiopathogenetic defect of malignant hyperthermia: Hypersensitive calcium-release channel of skeletal muscle sarcoplasmic reticulum

Summary and conclusions Overwhelming clinical, physiological, pharmacological, and biochemical evidence indicates that MH occurs due to a genetic defect of intracellular calcium homeostasis in skeletal muscle. Specifically, the balance of data clearly and plausibly implicates the channel through which calcium is released from the terminal cisternae of sarcoplasmic reticulum as the site of the underlying molecular lesion of MH. This putative channel is commonly believed to be ligand-gated, that is, opened by the binding of specific agonists (Meisner, 1984). Apparently, the gating of the MH channel is abnormally sensitized, that is more tightly coupled than normal, to the binding of agonists such as calcium (and ATP). Consequently, the MH channel requires less trigger calcium or ATP for activation.Caffeine may act as weak agonist of the putative, purinergic (ATP) receptor of the ligand-gated calcium channel of the sarcoplasmic reticulum (Meisner, 1984).Persistent elevations of myoplasmic free calcium in the vicinity of the channel, which would be subliminal for normal muscle, may trigger massive and regenerative calcium-induced calcium-release in MHS muscle. Uncontrollable and sustained activation of the contractile apparatus and metabolic machinery follows. The source of the trigger calcium may be the lumen of the transverse tubules via their voltage-gated calcium-channels (Ikemoto et al., 1984; Curtis & Catteral, 1984; Graf & Schatzmann, 1984), dislodged calcium from binding sites on the inner leaflet of the transverse tubular membrane (Frank, 1979; Curtis & Eisenberg, 1985), or the lumen of the terminal cisternae of sarcoplasmic reticulum via their ligand-gated calcium channels.Such persistent, but normally subliminal, elevations of trigger calcium at the site of the calcium-release channel presumably occurs in MHS muscle with administration of potent volatile anesthetics, especially halothane, and depolarizing muscle relaxants, especially succinylcholine. The gaseous anesthetics apparently act indirectly on the calcium channel by virtue of their being potent but non-specific membrane perturbing agents (Ohnishi et al., 1986) which increase the passive permeability of the sarcoplasmic reticulum, and possibly surface membrane, to calcium.In individuals which are homozygous for the genetic defect the less potent muscle stimuli of anoxia, heat, and exertion may be sufficient to increase the local trigger calcium concentration to a level resulting in uncontrollable calcium-release. Such homozygosity occurs rarely in people but commonly in swine due to inbreeding and the high frequency of the MH gene. Homozygosity for the MH gene may explain the occurrence of porcine (Topel, 1968), human (Wingard, 1974), and canine (O'Brien & Rand, 1985) stress syndromes: a double dosage of the genetic defect may render homozygotes hypersensitive to stimuli less potent than volatile anesthetics.Dantrolene, the MH-preventive and reversing (if administered early enough in the syndrome) drug, apparently acts by a different mechanism than drugs such as ruthenium red and the amide local anesthetics which appear to physically block the pore of the ligand-gated calcium channel (Ohnishi et al., 1986). Dantrolene's effect on the calcium channel is apparently indirect. It blocks the membrane-perturbing effects of halothane (Nelson & Denborough, 1977; Ohnishi et al., 1986) perhaps by stabilizing muscle membranes and therefore their calcium channels (voltage-gated in transverse tubules and ligand-gated in terminal cisternae). However, this muscle relaxant is less effective at inhibiting the calcium-releasing effects of calcium or caffeine probably because these act directly on the calcium channel (Nelson & Denborough, 1977; Nelson, 1984; Britt et al., 1984; Araki et al., 1985; Ohnishi et al., 1986).Modelling the MH defect as a hypersensitive calcium-release channel also provides a plausible explanation for observations of gradations of susceptibility to triggering of MH for individuals and within populations (Gronert, 1980; Ellis & Heffron, 1985). This model predicts gradations due to gene dosage (homozygous versus heterozygous) as well as due to environmental, pharmacologic, and genetic factors which would compromise or challenge calcium homeostatic potential, such as: heat, anoxia, exertion, drugs which stimulate muscle, and other myopathies.Thus, MH is initiated due to a hypersensitive calcium-release mechanism of skeletal muscle sarcoplasmic reticulum. Malignant hyperthermia may be reversed by dantrolene and symptomatic treatment up until a critical irreversible point in the development of the syndrome. This point of irreversibility probably corresponds to the occurrence of significant degenerative structural and functional changes within the muscle fiber. Loss of calcium-sequestration capabilities by sarcoplasmic reticulum is likely the most important factor which contributes to the irreversibility of MH.     

Malignant Hyperthermia Associated with Ryanodine Receptor 1 (C7360G) Mutation in Quarter Horses

Background: Anesthetic‐induced malignant hyperthermia (MH) has been documented in Quarter Horses with a single point mutation in the ryanodine receptor 1 gene (RyR1) at nucleotide C7360G, generating a R2454G amino acid substitution. However, there have been no reports of nonanesthetic manifestations of MH in horses with the C7360G mutation. Objective: To describe clinical manifestations of Quarter Horses with the C7360G mutation. Animals: Eleven Quarter Horses with the RyR1 C7360G mutation. Methods: This prospective study included horses with suspected MH, undetermined etiology of sudden death, death within hours of onset of rhabdomyolysis, muscle rigidity, stiffness, intermittent sweating, and persistent increases in serum muscle enzyme activities. Whole blood in EDTA and skeletal muscle were processed for genetic and histochemical analysis. Medical records and pedigrees were collected when available. Results: Both anesthetic‐ and non–anesthetic‐associated myopathic manifestations of MH occurred in halter Quarter Horses with mutation of RyR1. The disease is inherited as an autosomal dominant trait. Clinical and laboratory abnormalities were similar in both forms. Rhabdomyolysis was a common finding in both groups of horses. Skeletal muscle histochemical findings were nonspecific and compatible with a noninflammatory myopathic process. Conclusions and Clinical Importance: MH is a potentially fatal disease of Quarter Horses that could be triggered by halogenated anesthetics and other nonanesthetic factors that may include exercise, stress, breeding, illnesses, and concurrent myopathies.