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  • Author or Editor: Brian J. L. J. Joosten x
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Objective—To investigate whether training for show jumping that is commenced early after birth affects the characteristics of equine locomotory muscle.

Animals—19 Dutch Warmblood horses.

Procedures—Horses were assigned to a trained or not trained (control) group. After weaning, training (free jumping [2 d/wk] that was alternated with a 20-minute period of exercise in a mechanical rotating walker [3 d/wk]) was started and continued until horses were 3 years old. Fiber type composition (determined from myosin heavy chain [MyHC] content), fiber area, diffusion index (area supplied by 1 capillary), citrate synthase activity, and Na+,K+-ATPase content were assessed in gluteus medius muscle specimens collected at 0.5, 1, 2, and 3 years.

Results—Developmental changes included an increase in MyHC fiber type IIa and a decrease in type IIad; increases in fiber area, diffusion index, and citrate synthase activity; and a decrease in Na+,K+-ATPase content. The MyHC fiber type I and type IId were detected in high and low proportions, respectively. Training increased Na+,K+-ATPase content, but did not affect other variables.

Conclusions and Clinical Relevance—In horses, show jumping training at an early age resulted in increased Na+,K+-ATPase content of the deep portions of the gluteus medius muscle. The lack of training effects on the other muscle characteristics can partly be explained by the fact that an appropriate (aerobic) fiber type composition was already established at training commencement. These data also suggested that the developmental changes in equine muscle represent sufficient adaptation to meet the demands of this specific training.

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in American Journal of Veterinary Research


Objective—To investigate the effects of moderate short-term training on K+ regulation in plasma and erythrocytes during exercise and on skeletal muscle Na+,K+-ATPase concentration in young adult and middle-aged horses.

Animals—Four 4- to 6-year-old and four 10- to 16-yearold Dutch Warmblood horses.

Procedure—The horses underwent a 6-minute exercise trial before and after 12 days of training. Skeletal muscle Na+,K+-ATPase concentration was analyzed in gluteus medius and semitendinosus muscle specimens before and after the 12-day training period. Blood samples were collected before and immediately after the trials and at 3, 5, 7, and 10 minutes after cessation of exercise for assessment of several hematologic variables and analysis of plasma and whole-blood K+ concentrations.

Results—After training, Na+,K+-ATPase concentration in the gluteus medius, but not semitendinosus, muscle of middle-aged horses increased (32%), compared with pretraining values; this did not affect the degree of hyperkalemia that developed during exercise. The development of hyperkalemia during exercise in young adult horses was blunted (albeit not significantly) without any change in the concentration of Na+,K+-ATPase in either of the muscles. After training, the erythrocyte K+ concentration increased (7% to 10%) significantly in both groups of horses but did not change during the exercise trials.

Conclusions and Clinical Relevance—In horses, the activation of skeletal muscle Na+,K+-ATPase during exercise is likely to decrease with age. Training appears to result in an increase in Na+,K+-ATPase activity in skeletal muscle with subsequent upregulation of Na+,K+-ATPase concentration if the existing Na+,K+-ATPase capacity cannot meet requirements. (Am J Vet Res 2005;66:1252–1258)

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in American Journal of Veterinary Research