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- Author or Editor: Seiko Yamano x
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Objective—To determine the growth-related changes in metabolic and anatomic properties in equine muscle fiber type, including hybrid fibers identified with immunohistochemical analysis.
Animals—24 2-, 6-, 12-, and 24-month-old female Thoroughbreds.
Procedure—Samples were obtained from the gluteus medius muscle of all horses. Expression of myosin heavy chain (MHC) isoforms MHC-I, -IIa, -IIb, and -IIx in each muscle fiber was detected by use of 4 primary monoclonal antibodies: BA-D5, SC-71, BFF3, and BF-35, respectively. Five muscle fiber types (types I, I/IIA, IIA, IIA/IIX, and IIX) were immunohistochemically identified. The area and activity of succinic dehydrogenase (SDH) in each fiber type were determined by use of quantitative histochemical staining and image analysis.
Results—Although the proportion of type I and IIX fibers did not change with age, the proportion of type IIA and IIA/IIX fibers significantly increased and decreased, respectively, from 2 months to 24 months of age. The increase in proportion of type IIA fibers with growth may have been attributable to muscle fiber-type transition from type IIA/IIX fibers but not from type IIX fibers. Values for SDH activity and fiber area in hybrid fiber types were intermediate to those for their respective pure phenotypes.
Conclusions and Clinical Relevance—Hybrid fibers have an important role for determining the proportion of muscle fiber type in horses < 24 months old, and the metabolic and anatomic properties of the hybrid fibers are well coordinated, as in mature horses. (Am J Vet Res 2005;66:401–405)
Objective—To determine the effect of growth and training on metabolic properties in muscle fibers of the gluteus medius muscle in adolescent Thoroughbred horses.
Animals—Twenty 2-year-old Thoroughbreds.
Procedure—Horses were randomly assigned to 2 groups. Horses in the training group were trained for 16 weeks, and control horses were kept on pasture without training. Samples were obtained by use of a needle-biopsy technique from the middle gluteus muscle of each horse before and after the training period. Composition and oxidative enzyme (succinic dehydrogenase [SDH]) activity of each fiber type were determined by use of quantitative histochemical staining procedures. Whole-muscle activity of SDH and glycolytic enzyme (phosphofructokinase) as well as myosin heavy-chain isoforms were analyzed biochemically and electrophoretically, respectively.
Results—The SDH activity of type-I and -IIA fibers increased during growth, whereas whole-muscle activity was unchanged. Percentage of type-IIX/B muscle fibers decreased during training, whereas that of myosin heavy-chain IIa increased. The SDH activity of each fiber type as well as whole-muscle SDH activity increased during training. An especially noticeable increase in SDH activity was found in type-IIX/B fibers.
Conclusions and Clinical Relevance—Changes in muscle fibers of adolescent Thoroughbreds are caused by training and not by growth. The most noticeable change was for the SDH activity of type-IIX/B fibers. These changes in the gluteus medius muscle of adolescent Thoroughbreds were considered to be appropriate adaptations to running middle distances at high speeds. (Am J Vet Res 2002;63:1408–1412)