Effect of exercise on development of capillary supply and oxidative capacity in skeletal muscle of horses.

OBJECTIVE: To determine whether postnatal development of oxidative capacity and capillary supply of skeletal muscle is affected by various movement regimens in horses. ANIMALS: 35 foals. PROCEDURES: Dutch Warmblood foals were allocated into 3 groups (box stall rest, box stall rest with training, and free pasture exercise). Training comprised an increasing number of gallop sprints from 1 week after birth to 22 weeks of age. From 22 to 48 weeks, the 3 groups were combined and allowed to exercise freely. Capillary supply (diffusion index [ie, area supplied by 1 capillary]), citrate synthase (CS) activity, and succinate dehydrogenase (SDH) activity were measured in biopsy specimens of deep gluteus medius muscle. RESULTS: During the first 22 weeks, diffusion index increased in all 3 groups (the training and pasture groups had a smaller increase, compared with the box stall rest group), total SDH activity increased in the training and pasture groups and decreased in the box stall rest group, and CS activity decreased in all groups. The effect of the various movement regimens on the diffusion index remained after the groups were combined. CONCLUSIONS AND CLINICAL RELEVANCE: Withholding of exercise had a negative effect on the capillary supply (ie, diffusion index increased) that remained after box stall rest was discontinued and on oxidative capacity. Box stall rest with training prevented the negative effects and eventually had the same positive effect as pasture exercise.

Effect of show jumping training on the development of locomotory muscle in young horses.

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.

Myosin heavy chain isoforms in equine gluteus medius muscle: comparison of mRNA and protein expression profiles.

The major structural protein in skeletal muscle, myosin heavy chain (MyHC), is primarily transcriptionally controlled. We compared the expression of MyHC isoforms on the mRNA and protein level in biopsies from the m. gluteus medius from adult untrained horses. In transverse sections, the majority of fibers showed qualitatively identical mRNA and protein expression patterns. However, coexpression of 2a and 2d/x MyHCs was substantially more common at the protein than at the mRNA level, suggesting a fine-tuning of these two genes in normal muscle not subjected to any training protocol. Because transverse sections give a limited sampling of mRNA expression in the case of uneven distribution of transcripts in a muscle fiber, we also analyzed longitudinal sections. We present, for the first time, evidence that expression of MyHC mRNA and protein was equal along the length of the fiber. Hence, mRNA expression is not regulated by differential expression of isoforms by separate myonuclei. It is concluded that the number of protein hybrid fibers in equine gluteus medius muscle is controlled by alteration of the transcription pattern uniformly along the fiber, rather than by simultaneous transcription of genes. The differences with the results in muscle of small animals and humans are discussed.

Differential expression of equine myosin heavy-chain mRNA and protein isoforms in a limb muscle.

The horse is one of the few animals kept and bred for its athletic performance and is therefore an interesting model for human sports performance. The regulation of the development of equine locomotion in the first year of life, and the influence of early training on later performance, are largely unknown. The major structural protein in skeletal muscle, myosin heavy-chain (MyHC), is believed to be primarily transcriptionally controlled. To investigate the expression of the MyHC genes at the transcriptional level, we isolated cDNAs encoding the equine MyHC isoforms type 1 (slow), type 2a (fast oxidative), and type 2d/x (fast glycolytic). cDNAs encoding the 2b gene were not identified. The mRNA expression was compared to the protein expression on a fiber-to-fiber basis using in situ hybridization (non-radioactive) and immunohistochemistry. Marked differences were detected between the expression of MyHC transcripts and MyHC protein isoforms in adult equine gluteus medius muscle. Mismatches were primarily due to the presence of hybrid fibers expressing two fast (2ad) MyHC protein isoforms, but only one fast (mainly 2a) MyHC RNA isoform. This discrepancy was most likely not due to differential mRNA expression of myonuclei.